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<?xml version='1.0' encoding='utf-8'?>
<rfc xmlns:xi="http://www.w3.org/2001/XInclude" version="3" category="std" consensus="true" docName="draft-ietf-quic-tls-34" indexInclude="true" ipr="trust200902" number="9001" prepTime="2021-05-27T14:13:42" scripts="Common,Latin" sortRefs="true" submissionType="IETF" symRefs="true" tocDepth="3" tocInclude="true" xml:lang="en">
  <link href="https://dx.doi.org/10.17487/rfc9001" rel="alternate"/>
  <link href="urn:issn:2070-1721" rel="alternate"/>
  <link href="https://datatracker.ietf.org/doc/draft-ietf-quic-tls-34" rel="prev"/>
  <front>
    <title>Using TLS to Secure QUIC</title>
    <seriesInfo name="RFC" value="9001" stream="IETF"/>
    <author initials="M." surname="Thomson" fullname="Martin Thomson" role="editor">
      <organization showOnFrontPage="true">Mozilla</organization>
      <address>
        <email>mt@lowentropy.net</email>
      </address>
    </author>
    <author initials="S." surname="Turner" fullname="Sean Turner" role="editor">
      <organization showOnFrontPage="true">sn3rd</organization>
      <address>
        <email>sean@sn3rd.com</email>
      </address>
    </author>
    <date month="05" year="2021"/>
    <area>Transport</area>
    <workgroup>QUIC</workgroup>
    <keyword>crypto</keyword>
    <keyword>opportunistic encryption</keyword>
    <keyword>plaintext quic</keyword>
    <abstract pn="section-abstract">
      <t indent="0" pn="section-abstract-1">This document describes how Transport Layer Security (TLS) is used to secure
QUIC.</t>
    </abstract>
    <boilerplate>
      <section anchor="status-of-memo" numbered="false" removeInRFC="false" toc="exclude" pn="section-boilerplate.1">
        <name slugifiedName="name-status-of-this-memo">Status of This Memo</name>
        <t indent="0" pn="section-boilerplate.1-1">
            This is an Internet Standards Track document.
        </t>
        <t indent="0" pn="section-boilerplate.1-2">
            This document is a product of the Internet Engineering Task Force
            (IETF).  It represents the consensus of the IETF community.  It has
            received public review and has been approved for publication by
            the Internet Engineering Steering Group (IESG).  Further
            information on Internet Standards is available in Section 2 of 
            RFC 7841.
        </t>
        <t indent="0" pn="section-boilerplate.1-3">
            Information about the current status of this document, any
            errata, and how to provide feedback on it may be obtained at
            <eref target="https://www.rfc-editor.org/info/rfc9001" brackets="none"/>.
        </t>
      </section>
      <section anchor="copyright" numbered="false" removeInRFC="false" toc="exclude" pn="section-boilerplate.2">
        <name slugifiedName="name-copyright-notice">Copyright Notice</name>
        <t indent="0" pn="section-boilerplate.2-1">
            Copyright (c) 2021 IETF Trust and the persons identified as the
            document authors. All rights reserved.
        </t>
        <t indent="0" pn="section-boilerplate.2-2">
            This document is subject to BCP 78 and the IETF Trust's Legal
            Provisions Relating to IETF Documents
            (<eref target="https://trustee.ietf.org/license-info" brackets="none"/>) in effect on the date of
            publication of this document. Please review these documents
            carefully, as they describe your rights and restrictions with
            respect to this document. Code Components extracted from this
            document must include Simplified BSD License text as described in
            Section 4.e of the Trust Legal Provisions and are provided without
            warranty as described in the Simplified BSD License.
        </t>
      </section>
    </boilerplate>
    <toc>
      <section anchor="toc" numbered="false" removeInRFC="false" toc="exclude" pn="section-toc.1">
        <name slugifiedName="name-table-of-contents">Table of Contents</name>
        <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1">
          <li pn="section-toc.1-1.1">
            <t indent="0" keepWithNext="true" pn="section-toc.1-1.1.1"><xref derivedContent="1" format="counter" sectionFormat="of" target="section-1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-introduction">Introduction</xref></t>
          </li>
          <li pn="section-toc.1-1.2">
            <t indent="0" keepWithNext="true" pn="section-toc.1-1.2.1"><xref derivedContent="2" format="counter" sectionFormat="of" target="section-2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-notational-conventions">Notational Conventions</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.2.2">
              <li pn="section-toc.1-1.2.2.1">
                <t indent="0" keepWithNext="true" pn="section-toc.1-1.2.2.1.1"><xref derivedContent="2.1" format="counter" sectionFormat="of" target="section-2.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-tls-overview">TLS Overview</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.3">
            <t indent="0" pn="section-toc.1-1.3.1"><xref derivedContent="3" format="counter" sectionFormat="of" target="section-3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-protocol-overview">Protocol Overview</xref></t>
          </li>
          <li pn="section-toc.1-1.4">
            <t indent="0" pn="section-toc.1-1.4.1"><xref derivedContent="4" format="counter" sectionFormat="of" target="section-4"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-carrying-tls-messages">Carrying TLS Messages</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.4.2">
              <li pn="section-toc.1-1.4.2.1">
                <t indent="0" pn="section-toc.1-1.4.2.1.1"><xref derivedContent="4.1" format="counter" sectionFormat="of" target="section-4.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-interface-to-tls">Interface to TLS</xref></t>
                <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.4.2.1.2">
                  <li pn="section-toc.1-1.4.2.1.2.1">
                    <t indent="0" pn="section-toc.1-1.4.2.1.2.1.1"><xref derivedContent="4.1.1" format="counter" sectionFormat="of" target="section-4.1.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-handshake-complete">Handshake Complete</xref></t>
                  </li>
                  <li pn="section-toc.1-1.4.2.1.2.2">
                    <t indent="0" pn="section-toc.1-1.4.2.1.2.2.1"><xref derivedContent="4.1.2" format="counter" sectionFormat="of" target="section-4.1.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-handshake-confirmed">Handshake Confirmed</xref></t>
                  </li>
                  <li pn="section-toc.1-1.4.2.1.2.3">
                    <t indent="0" pn="section-toc.1-1.4.2.1.2.3.1"><xref derivedContent="4.1.3" format="counter" sectionFormat="of" target="section-4.1.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-sending-and-receiving-hands">Sending and Receiving Handshake Messages</xref></t>
                  </li>
                  <li pn="section-toc.1-1.4.2.1.2.4">
                    <t indent="0" pn="section-toc.1-1.4.2.1.2.4.1"><xref derivedContent="4.1.4" format="counter" sectionFormat="of" target="section-4.1.4"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-encryption-level-changes">Encryption Level Changes</xref></t>
                  </li>
                  <li pn="section-toc.1-1.4.2.1.2.5">
                    <t indent="0" pn="section-toc.1-1.4.2.1.2.5.1"><xref derivedContent="4.1.5" format="counter" sectionFormat="of" target="section-4.1.5"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-tls-interface-summary">TLS Interface Summary</xref></t>
                  </li>
                </ul>
              </li>
              <li pn="section-toc.1-1.4.2.2">
                <t indent="0" pn="section-toc.1-1.4.2.2.1"><xref derivedContent="4.2" format="counter" sectionFormat="of" target="section-4.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-tls-version">TLS Version</xref></t>
              </li>
              <li pn="section-toc.1-1.4.2.3">
                <t indent="0" pn="section-toc.1-1.4.2.3.1"><xref derivedContent="4.3" format="counter" sectionFormat="of" target="section-4.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-clienthello-size">ClientHello Size</xref></t>
              </li>
              <li pn="section-toc.1-1.4.2.4">
                <t indent="0" pn="section-toc.1-1.4.2.4.1"><xref derivedContent="4.4" format="counter" sectionFormat="of" target="section-4.4"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-peer-authentication">Peer Authentication</xref></t>
              </li>
              <li pn="section-toc.1-1.4.2.5">
                <t indent="0" pn="section-toc.1-1.4.2.5.1"><xref derivedContent="4.5" format="counter" sectionFormat="of" target="section-4.5"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-session-resumption">Session Resumption</xref></t>
              </li>
              <li pn="section-toc.1-1.4.2.6">
                <t indent="0" pn="section-toc.1-1.4.2.6.1"><xref derivedContent="4.6" format="counter" sectionFormat="of" target="section-4.6"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-0-rtt">0-RTT</xref></t>
                <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.4.2.6.2">
                  <li pn="section-toc.1-1.4.2.6.2.1">
                    <t indent="0" pn="section-toc.1-1.4.2.6.2.1.1"><xref derivedContent="4.6.1" format="counter" sectionFormat="of" target="section-4.6.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-enabling-0-rtt">Enabling 0-RTT</xref></t>
                  </li>
                  <li pn="section-toc.1-1.4.2.6.2.2">
                    <t indent="0" pn="section-toc.1-1.4.2.6.2.2.1"><xref derivedContent="4.6.2" format="counter" sectionFormat="of" target="section-4.6.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-accepting-and-rejecting-0-r">Accepting and Rejecting 0-RTT</xref></t>
                  </li>
                  <li pn="section-toc.1-1.4.2.6.2.3">
                    <t indent="0" pn="section-toc.1-1.4.2.6.2.3.1"><xref derivedContent="4.6.3" format="counter" sectionFormat="of" target="section-4.6.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-validating-0-rtt-configurat">Validating 0-RTT Configuration</xref></t>
                  </li>
                </ul>
              </li>
              <li pn="section-toc.1-1.4.2.7">
                <t indent="0" pn="section-toc.1-1.4.2.7.1"><xref derivedContent="4.7" format="counter" sectionFormat="of" target="section-4.7"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-helloretryrequest">HelloRetryRequest</xref></t>
              </li>
              <li pn="section-toc.1-1.4.2.8">
                <t indent="0" pn="section-toc.1-1.4.2.8.1"><xref derivedContent="4.8" format="counter" sectionFormat="of" target="section-4.8"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-tls-errors">TLS Errors</xref></t>
              </li>
              <li pn="section-toc.1-1.4.2.9">
                <t indent="0" pn="section-toc.1-1.4.2.9.1"><xref derivedContent="4.9" format="counter" sectionFormat="of" target="section-4.9"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-discarding-unused-keys">Discarding Unused Keys</xref></t>
                <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.4.2.9.2">
                  <li pn="section-toc.1-1.4.2.9.2.1">
                    <t indent="0" pn="section-toc.1-1.4.2.9.2.1.1"><xref derivedContent="4.9.1" format="counter" sectionFormat="of" target="section-4.9.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-discarding-initial-keys">Discarding Initial Keys</xref></t>
                  </li>
                  <li pn="section-toc.1-1.4.2.9.2.2">
                    <t indent="0" pn="section-toc.1-1.4.2.9.2.2.1"><xref derivedContent="4.9.2" format="counter" sectionFormat="of" target="section-4.9.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-discarding-handshake-keys">Discarding Handshake Keys</xref></t>
                  </li>
                  <li pn="section-toc.1-1.4.2.9.2.3">
                    <t indent="0" pn="section-toc.1-1.4.2.9.2.3.1"><xref derivedContent="4.9.3" format="counter" sectionFormat="of" target="section-4.9.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-discarding-0-rtt-keys">Discarding 0-RTT Keys</xref></t>
                  </li>
                </ul>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.5">
            <t indent="0" pn="section-toc.1-1.5.1"><xref derivedContent="5" format="counter" sectionFormat="of" target="section-5"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-packet-protection">Packet Protection</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.5.2">
              <li pn="section-toc.1-1.5.2.1">
                <t indent="0" pn="section-toc.1-1.5.2.1.1"><xref derivedContent="5.1" format="counter" sectionFormat="of" target="section-5.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-packet-protection-keys">Packet Protection Keys</xref></t>
              </li>
              <li pn="section-toc.1-1.5.2.2">
                <t indent="0" pn="section-toc.1-1.5.2.2.1"><xref derivedContent="5.2" format="counter" sectionFormat="of" target="section-5.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-initial-secrets">Initial Secrets</xref></t>
              </li>
              <li pn="section-toc.1-1.5.2.3">
                <t indent="0" pn="section-toc.1-1.5.2.3.1"><xref derivedContent="5.3" format="counter" sectionFormat="of" target="section-5.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-aead-usage">AEAD Usage</xref></t>
              </li>
              <li pn="section-toc.1-1.5.2.4">
                <t indent="0" pn="section-toc.1-1.5.2.4.1"><xref derivedContent="5.4" format="counter" sectionFormat="of" target="section-5.4"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-header-protection">Header Protection</xref></t>
                <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.5.2.4.2">
                  <li pn="section-toc.1-1.5.2.4.2.1">
                    <t indent="0" pn="section-toc.1-1.5.2.4.2.1.1"><xref derivedContent="5.4.1" format="counter" sectionFormat="of" target="section-5.4.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-header-protection-applicati">Header Protection Application</xref></t>
                  </li>
                  <li pn="section-toc.1-1.5.2.4.2.2">
                    <t indent="0" pn="section-toc.1-1.5.2.4.2.2.1"><xref derivedContent="5.4.2" format="counter" sectionFormat="of" target="section-5.4.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-header-protection-sample">Header Protection Sample</xref></t>
                  </li>
                  <li pn="section-toc.1-1.5.2.4.2.3">
                    <t indent="0" pn="section-toc.1-1.5.2.4.2.3.1"><xref derivedContent="5.4.3" format="counter" sectionFormat="of" target="section-5.4.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-aes-based-header-protection">AES-Based Header Protection</xref></t>
                  </li>
                  <li pn="section-toc.1-1.5.2.4.2.4">
                    <t indent="0" pn="section-toc.1-1.5.2.4.2.4.1"><xref derivedContent="5.4.4" format="counter" sectionFormat="of" target="section-5.4.4"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-chacha20-based-header-prote">ChaCha20-Based Header Protection</xref></t>
                  </li>
                </ul>
              </li>
              <li pn="section-toc.1-1.5.2.5">
                <t indent="0" pn="section-toc.1-1.5.2.5.1"><xref derivedContent="5.5" format="counter" sectionFormat="of" target="section-5.5"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-receiving-protected-packets">Receiving Protected Packets</xref></t>
              </li>
              <li pn="section-toc.1-1.5.2.6">
                <t indent="0" pn="section-toc.1-1.5.2.6.1"><xref derivedContent="5.6" format="counter" sectionFormat="of" target="section-5.6"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-use-of-0-rtt-keys">Use of 0-RTT Keys</xref></t>
              </li>
              <li pn="section-toc.1-1.5.2.7">
                <t indent="0" pn="section-toc.1-1.5.2.7.1"><xref derivedContent="5.7" format="counter" sectionFormat="of" target="section-5.7"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-receiving-out-of-order-prot">Receiving Out-of-Order Protected Packets</xref></t>
              </li>
              <li pn="section-toc.1-1.5.2.8">
                <t indent="0" pn="section-toc.1-1.5.2.8.1"><xref derivedContent="5.8" format="counter" sectionFormat="of" target="section-5.8"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-retry-packet-integrity">Retry Packet Integrity</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.6">
            <t indent="0" pn="section-toc.1-1.6.1"><xref derivedContent="6" format="counter" sectionFormat="of" target="section-6"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-key-update">Key Update</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.6.2">
              <li pn="section-toc.1-1.6.2.1">
                <t indent="0" pn="section-toc.1-1.6.2.1.1"><xref derivedContent="6.1" format="counter" sectionFormat="of" target="section-6.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-initiating-a-key-update">Initiating a Key Update</xref></t>
              </li>
              <li pn="section-toc.1-1.6.2.2">
                <t indent="0" pn="section-toc.1-1.6.2.2.1"><xref derivedContent="6.2" format="counter" sectionFormat="of" target="section-6.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-responding-to-a-key-update">Responding to a Key Update</xref></t>
              </li>
              <li pn="section-toc.1-1.6.2.3">
                <t indent="0" pn="section-toc.1-1.6.2.3.1"><xref derivedContent="6.3" format="counter" sectionFormat="of" target="section-6.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-timing-of-receive-key-gener">Timing of Receive Key Generation</xref></t>
              </li>
              <li pn="section-toc.1-1.6.2.4">
                <t indent="0" pn="section-toc.1-1.6.2.4.1"><xref derivedContent="6.4" format="counter" sectionFormat="of" target="section-6.4"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-sending-with-updated-keys">Sending with Updated Keys</xref></t>
              </li>
              <li pn="section-toc.1-1.6.2.5">
                <t indent="0" pn="section-toc.1-1.6.2.5.1"><xref derivedContent="6.5" format="counter" sectionFormat="of" target="section-6.5"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-receiving-with-different-ke">Receiving with Different Keys</xref></t>
              </li>
              <li pn="section-toc.1-1.6.2.6">
                <t indent="0" pn="section-toc.1-1.6.2.6.1"><xref derivedContent="6.6" format="counter" sectionFormat="of" target="section-6.6"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-limits-on-aead-usage">Limits on AEAD Usage</xref></t>
              </li>
              <li pn="section-toc.1-1.6.2.7">
                <t indent="0" pn="section-toc.1-1.6.2.7.1"><xref derivedContent="6.7" format="counter" sectionFormat="of" target="section-6.7"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-key-update-error-code">Key Update Error Code</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.7">
            <t indent="0" pn="section-toc.1-1.7.1"><xref derivedContent="7" format="counter" sectionFormat="of" target="section-7"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-security-of-initial-message">Security of Initial Messages</xref></t>
          </li>
          <li pn="section-toc.1-1.8">
            <t indent="0" pn="section-toc.1-1.8.1"><xref derivedContent="8" format="counter" sectionFormat="of" target="section-8"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-quic-specific-adjustments-t">QUIC-Specific Adjustments to the TLS Handshake</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.8.2">
              <li pn="section-toc.1-1.8.2.1">
                <t indent="0" pn="section-toc.1-1.8.2.1.1"><xref derivedContent="8.1" format="counter" sectionFormat="of" target="section-8.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-protocol-negotiation">Protocol Negotiation</xref></t>
              </li>
              <li pn="section-toc.1-1.8.2.2">
                <t indent="0" pn="section-toc.1-1.8.2.2.1"><xref derivedContent="8.2" format="counter" sectionFormat="of" target="section-8.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-quic-transport-parameters-e">QUIC Transport Parameters Extension</xref></t>
              </li>
              <li pn="section-toc.1-1.8.2.3">
                <t indent="0" pn="section-toc.1-1.8.2.3.1"><xref derivedContent="8.3" format="counter" sectionFormat="of" target="section-8.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-removing-the-endofearlydata">Removing the EndOfEarlyData Message</xref></t>
              </li>
              <li pn="section-toc.1-1.8.2.4">
                <t indent="0" pn="section-toc.1-1.8.2.4.1"><xref derivedContent="8.4" format="counter" sectionFormat="of" target="section-8.4"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-prohibit-tls-middlebox-comp">Prohibit TLS Middlebox Compatibility Mode</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.9">
            <t indent="0" pn="section-toc.1-1.9.1"><xref derivedContent="9" format="counter" sectionFormat="of" target="section-9"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-security-considerations">Security Considerations</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.9.2">
              <li pn="section-toc.1-1.9.2.1">
                <t indent="0" pn="section-toc.1-1.9.2.1.1"><xref derivedContent="9.1" format="counter" sectionFormat="of" target="section-9.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-session-linkability">Session Linkability</xref></t>
              </li>
              <li pn="section-toc.1-1.9.2.2">
                <t indent="0" pn="section-toc.1-1.9.2.2.1"><xref derivedContent="9.2" format="counter" sectionFormat="of" target="section-9.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-replay-attacks-with-0-rtt">Replay Attacks with 0-RTT</xref></t>
              </li>
              <li pn="section-toc.1-1.9.2.3">
                <t indent="0" pn="section-toc.1-1.9.2.3.1"><xref derivedContent="9.3" format="counter" sectionFormat="of" target="section-9.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-packet-reflection-attack-mi">Packet Reflection Attack Mitigation</xref></t>
              </li>
              <li pn="section-toc.1-1.9.2.4">
                <t indent="0" pn="section-toc.1-1.9.2.4.1"><xref derivedContent="9.4" format="counter" sectionFormat="of" target="section-9.4"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-header-protection-analysis">Header Protection Analysis</xref></t>
              </li>
              <li pn="section-toc.1-1.9.2.5">
                <t indent="0" pn="section-toc.1-1.9.2.5.1"><xref derivedContent="9.5" format="counter" sectionFormat="of" target="section-9.5"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-header-protection-timing-si">Header Protection Timing Side Channels</xref></t>
              </li>
              <li pn="section-toc.1-1.9.2.6">
                <t indent="0" pn="section-toc.1-1.9.2.6.1"><xref derivedContent="9.6" format="counter" sectionFormat="of" target="section-9.6"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-key-diversity">Key Diversity</xref></t>
              </li>
              <li pn="section-toc.1-1.9.2.7">
                <t indent="0" pn="section-toc.1-1.9.2.7.1"><xref derivedContent="9.7" format="counter" sectionFormat="of" target="section-9.7"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-randomness">Randomness</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.10">
            <t indent="0" pn="section-toc.1-1.10.1"><xref derivedContent="10" format="counter" sectionFormat="of" target="section-10"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-iana-considerations">IANA Considerations</xref></t>
          </li>
          <li pn="section-toc.1-1.11">
            <t indent="0" pn="section-toc.1-1.11.1"><xref derivedContent="11" format="counter" sectionFormat="of" target="section-11"/>. <xref derivedContent="" format="title" sectionFormat="of" target="name-references">References</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.11.2">
              <li pn="section-toc.1-1.11.2.1">
                <t indent="0" pn="section-toc.1-1.11.2.1.1"><xref derivedContent="11.1" format="counter" sectionFormat="of" target="section-11.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-normative-references">Normative References</xref></t>
              </li>
              <li pn="section-toc.1-1.11.2.2">
                <t indent="0" pn="section-toc.1-1.11.2.2.1"><xref derivedContent="11.2" format="counter" sectionFormat="of" target="section-11.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-informative-references">Informative References</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.12">
            <t indent="0" pn="section-toc.1-1.12.1"><xref derivedContent="Appendix A" format="default" sectionFormat="of" target="section-appendix.a"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-sample-packet-protection">Sample Packet Protection</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.12.2">
              <li pn="section-toc.1-1.12.2.1">
                <t indent="0" pn="section-toc.1-1.12.2.1.1"><xref derivedContent="A.1" format="counter" sectionFormat="of" target="section-a.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-keys">Keys</xref></t>
              </li>
              <li pn="section-toc.1-1.12.2.2">
                <t indent="0" pn="section-toc.1-1.12.2.2.1"><xref derivedContent="A.2" format="counter" sectionFormat="of" target="section-a.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-client-initial">Client Initial</xref></t>
              </li>
              <li pn="section-toc.1-1.12.2.3">
                <t indent="0" pn="section-toc.1-1.12.2.3.1"><xref derivedContent="A.3" format="counter" sectionFormat="of" target="section-a.3"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-server-initial">Server Initial</xref></t>
              </li>
              <li pn="section-toc.1-1.12.2.4">
                <t indent="0" pn="section-toc.1-1.12.2.4.1"><xref derivedContent="A.4" format="counter" sectionFormat="of" target="section-a.4"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-retry">Retry</xref></t>
              </li>
              <li pn="section-toc.1-1.12.2.5">
                <t indent="0" pn="section-toc.1-1.12.2.5.1"><xref derivedContent="A.5" format="counter" sectionFormat="of" target="section-a.5"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-chacha20-poly1305-short-hea">ChaCha20-Poly1305 Short Header Packet</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.13">
            <t indent="0" pn="section-toc.1-1.13.1"><xref derivedContent="Appendix B" format="default" sectionFormat="of" target="section-appendix.b"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-aead-algorithm-analysis">AEAD Algorithm Analysis</xref></t>
            <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.13.2">
              <li pn="section-toc.1-1.13.2.1">
                <t indent="0" pn="section-toc.1-1.13.2.1.1"><xref derivedContent="B.1" format="counter" sectionFormat="of" target="section-b.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-analysis-of-aead_aes_128_gc">Analysis of AEAD_AES_128_GCM and AEAD_AES_256_GCM Usage Limits</xref></t>
                <ul bare="true" empty="true" indent="2" spacing="compact" pn="section-toc.1-1.13.2.1.2">
                  <li pn="section-toc.1-1.13.2.1.2.1">
                    <t indent="0" pn="section-toc.1-1.13.2.1.2.1.1"><xref derivedContent="B.1.1" format="counter" sectionFormat="of" target="section-b.1.1"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-confidentiality-limit">Confidentiality Limit</xref></t>
                  </li>
                  <li pn="section-toc.1-1.13.2.1.2.2">
                    <t indent="0" pn="section-toc.1-1.13.2.1.2.2.1"><xref derivedContent="B.1.2" format="counter" sectionFormat="of" target="section-b.1.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-integrity-limit">Integrity Limit</xref></t>
                  </li>
                </ul>
              </li>
              <li pn="section-toc.1-1.13.2.2">
                <t indent="0" pn="section-toc.1-1.13.2.2.1"><xref derivedContent="B.2" format="counter" sectionFormat="of" target="section-b.2"/>.  <xref derivedContent="" format="title" sectionFormat="of" target="name-analysis-of-aead_aes_128_cc">Analysis of AEAD_AES_128_CCM Usage Limits</xref></t>
              </li>
            </ul>
          </li>
          <li pn="section-toc.1-1.14">
            <t indent="0" pn="section-toc.1-1.14.1"><xref derivedContent="" format="none" sectionFormat="of" target="section-appendix.c"/><xref derivedContent="" format="title" sectionFormat="of" target="name-contributors">Contributors</xref></t>
          </li>
          <li pn="section-toc.1-1.15">
            <t indent="0" pn="section-toc.1-1.15.1"><xref derivedContent="" format="none" sectionFormat="of" target="section-appendix.d"/><xref derivedContent="" format="title" sectionFormat="of" target="name-authors-addresses">Authors' Addresses</xref></t>
          </li>
        </ul>
      </section>
    </toc>
  </front>
  <middle>
    <section anchor="introduction" numbered="true" toc="include" removeInRFC="false" pn="section-1">
      <name slugifiedName="name-introduction">Introduction</name>
      <t indent="0" pn="section-1-1">This document describes how QUIC <xref target="QUIC-TRANSPORT" format="default" sectionFormat="of" derivedContent="QUIC-TRANSPORT"/> is secured using TLS
<xref target="TLS13" format="default" sectionFormat="of" derivedContent="TLS13"/>.</t>
      <t indent="0" pn="section-1-2">TLS 1.3 provides critical latency improvements for connection establishment over
previous versions.  Absent packet loss, most new connections can be established
and secured within a single round trip; on subsequent connections between the
same client and server, the client can often send application data immediately,
that is, using a zero round-trip setup.</t>
      <t indent="0" pn="section-1-3">This document describes how TLS acts as a security component of QUIC.</t>
    </section>
    <section anchor="notational-conventions" numbered="true" toc="include" removeInRFC="false" pn="section-2">
      <name slugifiedName="name-notational-conventions">Notational Conventions</name>
      <t indent="0" pn="section-2-1">The key words "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>", "<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL NOT</bcp14>", "<bcp14>SHOULD</bcp14>",
"<bcp14>SHOULD NOT</bcp14>", "<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>", "<bcp14>MAY</bcp14>", and "<bcp14>OPTIONAL</bcp14>" in this
document are to be interpreted as described in BCP 14 <xref target="RFC2119" format="default" sectionFormat="of" derivedContent="RFC2119"/> <xref target="RFC8174" format="default" sectionFormat="of" derivedContent="RFC8174"/>
when, and only when, they appear in all capitals, as shown here.</t>
      <t indent="0" pn="section-2-2">This document uses the terminology established in <xref target="QUIC-TRANSPORT" format="default" sectionFormat="of" derivedContent="QUIC-TRANSPORT"/>.</t>
      <t indent="0" pn="section-2-3">For brevity, the acronym TLS is used to refer to TLS 1.3, though a newer version
could be used; see <xref target="tls-version" format="default" sectionFormat="of" derivedContent="Section 4.2"/>.</t>
      <section anchor="tls-overview" numbered="true" toc="include" removeInRFC="false" pn="section-2.1">
        <name slugifiedName="name-tls-overview">TLS Overview</name>
        <t indent="0" pn="section-2.1-1">TLS provides two endpoints with a way to establish a means of communication over
an untrusted medium (for example, the Internet). TLS enables authentication of
peers and provides confidentiality and integrity protection for messages that
endpoints exchange.</t>
        <t indent="0" pn="section-2.1-2">Internally, TLS is a layered protocol, with the structure shown in
<xref target="tls-layers" format="default" sectionFormat="of" derivedContent="Figure 1"/>.</t>
        <figure anchor="tls-layers" align="left" suppress-title="false" pn="figure-1">
          <name slugifiedName="name-tls-layers">TLS Layers</name>
          <artwork name="" type="" align="left" alt="" pn="section-2.1-3.1">
          +-------------+------------+--------------+---------+
Content   |             |            |  Application |         |
Layer     |  Handshake  |   Alerts   |     Data     |   ...   |
          |             |            |              |         |
          +-------------+------------+--------------+---------+
Record    |                                                   |
Layer     |                      Records                      |
          |                                                   |
          +---------------------------------------------------+
</artwork>
        </figure>
        <t indent="0" pn="section-2.1-4">Each content-layer message (e.g., handshake, alerts, and application data) is
carried as a series of typed TLS records by the record layer.  Records are
individually cryptographically protected and then transmitted over a reliable
transport (typically TCP), which provides sequencing and guaranteed delivery.</t>
        <t indent="0" pn="section-2.1-5">The TLS authenticated key exchange occurs between two endpoints: client and
server.  The client initiates the exchange and the server responds.  If the key
exchange completes successfully, both client and server will agree on a secret.
TLS supports both pre-shared key (PSK) and Diffie-Hellman over either finite
fields or elliptic curves ((EC)DHE) key exchanges.  PSK is the basis for Early
Data (0-RTT); the latter provides forward secrecy (FS) when the (EC)DHE
keys are destroyed.  The two modes can also be combined to provide forward
secrecy while using the PSK for authentication.</t>
        <t indent="0" pn="section-2.1-6">After completing the TLS handshake, the client will have learned and
authenticated an identity for the server, and the server is optionally able to
learn and authenticate an identity for the client.  TLS supports X.509
<xref target="RFC5280" format="default" sectionFormat="of" derivedContent="RFC5280"/> certificate-based authentication for both server and client.
When PSK key exchange is used (as in resumption), knowledge of the PSK
serves to authenticate the peer.</t>
        <t indent="0" pn="section-2.1-7">The TLS key exchange is resistant to tampering by attackers, and it produces
shared secrets that cannot be controlled by either participating peer.</t>
        <t indent="0" pn="section-2.1-8">TLS provides two basic handshake modes of interest to QUIC:</t>
        <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-2.1-9">
          <li pn="section-2.1-9.1">A full 1-RTT handshake, in which the client is able to send application data
after one round trip and the server immediately responds after receiving the
first handshake message from the client.</li>
          <li pn="section-2.1-9.2">A 0-RTT handshake, in which the client uses information it has previously
learned about the server to send application data immediately.  This
application data can be replayed by an attacker, so 0-RTT is not suitable for
carrying instructions that might initiate any action that could cause
unwanted effects if replayed.</li>
        </ul>
        <t indent="0" pn="section-2.1-10">A simplified TLS handshake with 0-RTT application data is shown in <xref target="tls-full" format="default" sectionFormat="of" derivedContent="Figure 2"/>.</t>
        <figure anchor="tls-full" align="left" suppress-title="false" pn="figure-2">
          <name slugifiedName="name-tls-handshake-with-0-rtt">TLS Handshake with 0-RTT</name>
          <artwork name="" type="" align="left" alt="" pn="section-2.1-11.1">
    Client                                             Server

    ClientHello
   (0-RTT Application Data)  --------&gt;
                                                  ServerHello
                                         {EncryptedExtensions}
                                                    {Finished}
                             &lt;--------      [Application Data]
   {Finished}                --------&gt;

   [Application Data]        &lt;-------&gt;      [Application Data]

    () Indicates messages protected by Early Data (0-RTT) Keys
    {} Indicates messages protected using Handshake Keys
    [] Indicates messages protected using Application Data
       (1-RTT) Keys
</artwork>
        </figure>
        <t indent="0" pn="section-2.1-12"><xref target="tls-full" format="default" sectionFormat="of" derivedContent="Figure 2"/> omits the EndOfEarlyData message, which is not used in QUIC; see
<xref target="remove-eoed" format="default" sectionFormat="of" derivedContent="Section 8.3"/>. Likewise, neither ChangeCipherSpec nor KeyUpdate messages are
used by QUIC. ChangeCipherSpec is redundant in TLS 1.3; see <xref target="compat-mode" format="default" sectionFormat="of" derivedContent="Section 8.4"/>.
QUIC has its own key update mechanism; see <xref target="key-update" format="default" sectionFormat="of" derivedContent="Section 6"/>.</t>
        <t indent="0" pn="section-2.1-13">Data is protected using a number of encryption levels:</t>
        <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-2.1-14">
          <li pn="section-2.1-14.1">Initial keys</li>
          <li pn="section-2.1-14.2">Early data (0-RTT) keys</li>
          <li pn="section-2.1-14.3">Handshake keys</li>
          <li pn="section-2.1-14.4">Application data (1-RTT) keys</li>
        </ul>
        <t indent="0" pn="section-2.1-15">Application data can only appear in the early data and application data
levels. Handshake and alert messages may appear in any level.</t>
        <t indent="0" pn="section-2.1-16">The 0-RTT handshake can be used if the client and server have previously
communicated.  In the 1-RTT handshake, the client is unable to send protected
application data until it has received all of the handshake messages sent by the
server.</t>
      </section>
    </section>
    <section anchor="protocol-overview" numbered="true" toc="include" removeInRFC="false" pn="section-3">
      <name slugifiedName="name-protocol-overview">Protocol Overview</name>
      <t indent="0" pn="section-3-1">QUIC <xref target="QUIC-TRANSPORT" format="default" sectionFormat="of" derivedContent="QUIC-TRANSPORT"/> assumes responsibility for the confidentiality and
integrity protection of packets.  For this it uses keys derived from a TLS
handshake <xref target="TLS13" format="default" sectionFormat="of" derivedContent="TLS13"/>, but instead of carrying TLS records over QUIC (as with
TCP), TLS handshake and alert messages are carried directly over the QUIC
transport, which takes over the responsibilities of the TLS record layer, as
shown in <xref target="quic-layers" format="default" sectionFormat="of" derivedContent="Figure 3"/>.</t>
      <figure anchor="quic-layers" align="left" suppress-title="false" pn="figure-3">
        <name slugifiedName="name-quic-layers">QUIC Layers</name>
        <artwork name="" type="" align="left" alt="" pn="section-3-2.1">
+--------------+--------------+ +-------------+
|     TLS      |     TLS      | |    QUIC     |
|  Handshake   |    Alerts    | | Applications|
|              |              | |  (h3, etc.) |
+--------------+--------------+-+-------------+
|                                             |
|                QUIC Transport               |
|   (streams, reliability, congestion, etc.)  |
|                                             |
+---------------------------------------------+
|                                             |
|            QUIC Packet Protection           |
|                                             |
+---------------------------------------------+
</artwork>
      </figure>
      <t indent="0" pn="section-3-3">QUIC also relies on TLS for authentication and negotiation of parameters that
are critical to security and performance.</t>
      <t indent="0" pn="section-3-4">Rather than a strict layering, these two protocols cooperate: QUIC uses the TLS
handshake; TLS uses the reliability, ordered delivery, and record layer provided
by QUIC.</t>
      <t indent="0" pn="section-3-5">At a high level, there are two main interactions between the TLS and QUIC
components:</t>
      <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-3-6">
        <li pn="section-3-6.1">The TLS component sends and receives messages via the QUIC component, with
QUIC providing a reliable stream abstraction to TLS.</li>
        <li pn="section-3-6.2">The TLS component provides a series of updates to the QUIC component,
including (a) new packet protection keys to install and (b) state changes such
as handshake completion, the server certificate, etc.</li>
      </ul>
      <t indent="0" pn="section-3-7"><xref target="schematic" format="default" sectionFormat="of" derivedContent="Figure 4"/> shows these interactions in more detail, with the QUIC packet
protection being called out specially.</t>
      <figure anchor="schematic" align="left" suppress-title="false" pn="figure-4">
        <name slugifiedName="name-quic-and-tls-interactions">QUIC and TLS Interactions</name>
        <artwork name="" type="" align="left" alt="" pn="section-3-8.1">
+------------+                               +------------+
|            |&lt;---- Handshake Messages -----&gt;|            |
|            |&lt;- Validate 0-RTT Parameters -&gt;|            |
|            |&lt;--------- 0-RTT Keys ---------|            |
|    QUIC    |&lt;------- Handshake Keys -------|    TLS     |
|            |&lt;--------- 1-RTT Keys ---------|            |
|            |&lt;------- Handshake Done -------|            |
+------------+                               +------------+
 |         ^
 | Protect | Protected
 v         | Packet
+------------+
|   QUIC     |
|  Packet    |
| Protection |
+------------+
</artwork>
      </figure>
      <t indent="0" pn="section-3-9">Unlike TLS over TCP, QUIC applications that want to send data do not send it
using TLS Application Data records. Rather, they send it as QUIC STREAM
frames or other frame types, which are then carried in QUIC packets.</t>
    </section>
    <section anchor="carrying-tls" numbered="true" toc="include" removeInRFC="false" pn="section-4">
      <name slugifiedName="name-carrying-tls-messages">Carrying TLS Messages</name>
      <t indent="0" pn="section-4-1">QUIC carries TLS handshake data in CRYPTO frames, each of which consists of a
contiguous block of handshake data identified by an offset and length. Those
frames are packaged into QUIC packets and encrypted under the current
encryption level.  As with TLS over TCP, once TLS handshake data has been
delivered to QUIC, it is QUIC's responsibility to deliver it reliably. Each
chunk of data that is produced by TLS is associated with the set of keys that
TLS is currently using.  If QUIC needs to retransmit that data, it <bcp14>MUST</bcp14> use the
same keys even if TLS has already updated to newer keys.</t>
      <t indent="0" pn="section-4-2">Each encryption level corresponds to a packet number space. The packet number
space that is used determines the semantics of frames. Some frames are
prohibited in different packet number spaces; see <xref section="12.5" sectionFormat="of" target="QUIC-TRANSPORT" format="default" derivedLink="https://rfc-editor.org/rfc/rfc9000#section-12.5" derivedContent="QUIC-TRANSPORT"/>.</t>
      <t indent="0" pn="section-4-3">Because packets could be reordered on the wire, QUIC uses the packet type to
indicate which keys were used to protect a given packet, as shown in
<xref target="packet-types-keys" format="default" sectionFormat="of" derivedContent="Table 1"/>. When packets of different types need to be sent,
endpoints <bcp14>SHOULD</bcp14> use coalesced packets to send them in the same UDP datagram.</t>
      <table anchor="packet-types-keys" align="center" pn="table-1">
        <name slugifiedName="name-encryption-keys-by-packet-t">Encryption Keys by Packet Type</name>
        <thead>
          <tr>
            <th align="left" colspan="1" rowspan="1">Packet Type</th>
            <th align="left" colspan="1" rowspan="1">Encryption Keys</th>
            <th align="left" colspan="1" rowspan="1">PN Space</th>
          </tr>
        </thead>
        <tbody>
          <tr>
            <th align="left" colspan="1" rowspan="1">Initial</th>
            <td align="left" colspan="1" rowspan="1">Initial secrets</td>
            <td align="left" colspan="1" rowspan="1">Initial</td>
          </tr>
          <tr>
            <th align="left" colspan="1" rowspan="1">0-RTT Protected</th>
            <td align="left" colspan="1" rowspan="1">0-RTT</td>
            <td align="left" colspan="1" rowspan="1">Application data</td>
          </tr>
          <tr>
            <th align="left" colspan="1" rowspan="1">Handshake</th>
            <td align="left" colspan="1" rowspan="1">Handshake</td>
            <td align="left" colspan="1" rowspan="1">Handshake</td>
          </tr>
          <tr>
            <th align="left" colspan="1" rowspan="1">Retry</th>
            <td align="left" colspan="1" rowspan="1">Retry</td>
            <td align="left" colspan="1" rowspan="1">N/A</td>
          </tr>
          <tr>
            <th align="left" colspan="1" rowspan="1">Version Negotiation</th>
            <td align="left" colspan="1" rowspan="1">N/A</td>
            <td align="left" colspan="1" rowspan="1">N/A</td>
          </tr>
          <tr>
            <th align="left" colspan="1" rowspan="1">Short Header</th>
            <td align="left" colspan="1" rowspan="1">1-RTT</td>
            <td align="left" colspan="1" rowspan="1">Application data</td>
          </tr>
        </tbody>
      </table>
      <t indent="0" pn="section-4-5"><xref section="17" sectionFormat="of" target="QUIC-TRANSPORT" format="default" derivedLink="https://rfc-editor.org/rfc/rfc9000#section-17" derivedContent="QUIC-TRANSPORT"/> shows how packets at the various encryption
levels fit into the handshake process.</t>
      <section anchor="interface-to-tls" numbered="true" toc="include" removeInRFC="false" pn="section-4.1">
        <name slugifiedName="name-interface-to-tls">Interface to TLS</name>
        <t indent="0" pn="section-4.1-1">As shown in <xref target="schematic" format="default" sectionFormat="of" derivedContent="Figure 4"/>, the interface from QUIC to TLS consists of four
primary functions:</t>
        <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-4.1-2">
          <li pn="section-4.1-2.1">Sending and receiving handshake messages</li>
          <li pn="section-4.1-2.2">Processing stored transport and application state from a resumed session
and determining if it is valid to generate or accept 0-RTT data</li>
          <li pn="section-4.1-2.3">Rekeying (both transmit and receive)</li>
          <li pn="section-4.1-2.4">Updating handshake state</li>
        </ul>
        <t indent="0" pn="section-4.1-3">Additional functions might be needed to configure TLS.  In particular, QUIC and
TLS need to agree on which is responsible for validation of peer credentials,
such as certificate validation <xref target="RFC5280" format="default" sectionFormat="of" derivedContent="RFC5280"/>.</t>
        <section anchor="handshake-complete" numbered="true" toc="include" removeInRFC="false" pn="section-4.1.1">
          <name slugifiedName="name-handshake-complete">Handshake Complete</name>
          <t indent="0" pn="section-4.1.1-1">In this document, the TLS handshake is considered complete when the TLS stack
has reported that the handshake is complete.  This happens when the TLS stack
has both sent a Finished message and verified the peer's Finished message.
Verifying the peer's Finished message provides the endpoints with an assurance
that previous handshake messages have not been modified.  Note that the
handshake does not complete at both endpoints simultaneously.  Consequently, any
requirement that is based on the completion of the handshake depends on the
perspective of the endpoint in question.</t>
        </section>
        <section anchor="handshake-confirmed" numbered="true" toc="include" removeInRFC="false" pn="section-4.1.2">
          <name slugifiedName="name-handshake-confirmed">Handshake Confirmed</name>
          <t indent="0" pn="section-4.1.2-1">In this document, the TLS handshake is considered confirmed at the server when
the handshake completes.  The server <bcp14>MUST</bcp14> send a HANDSHAKE_DONE frame as soon as
the handshake is complete.  At the client, the handshake is considered confirmed
when a HANDSHAKE_DONE frame is received.</t>
          <t indent="0" pn="section-4.1.2-2">Additionally, a client <bcp14>MAY</bcp14> consider the handshake to be confirmed when it
receives an acknowledgment for a 1-RTT packet.  This can be implemented by
recording the lowest packet number sent with 1-RTT keys and comparing it to the
Largest Acknowledged field in any received 1-RTT ACK frame: once the latter is
greater than or equal to the former, the handshake is confirmed.</t>
        </section>
        <section anchor="sending-and-receiving-handshake-messages" numbered="true" toc="include" removeInRFC="false" pn="section-4.1.3">
          <name slugifiedName="name-sending-and-receiving-hands">Sending and Receiving Handshake Messages</name>
          <t indent="0" pn="section-4.1.3-1">In order to drive the handshake, TLS depends on being able to send and receive
handshake messages. There are two basic functions on this interface: one where
QUIC requests handshake messages and one where QUIC provides bytes that comprise
handshake messages.</t>
          <t indent="0" pn="section-4.1.3-2">Before starting the handshake, QUIC provides TLS with the transport parameters
(see <xref target="quic_parameters" format="default" sectionFormat="of" derivedContent="Section 8.2"/>) that it wishes to carry.</t>
          <t indent="0" pn="section-4.1.3-3">A QUIC client starts TLS by requesting TLS handshake bytes from TLS.  The client
acquires handshake bytes before sending its first packet.  A QUIC server starts
the process by providing TLS with the client's handshake bytes.</t>
          <t indent="0" pn="section-4.1.3-4">At any time, the TLS stack at an endpoint will have a current sending encryption
level and a receiving encryption level. TLS encryption levels determine the QUIC
packet type and keys that are used for protecting data.</t>
          <t indent="0" pn="section-4.1.3-5">Each encryption level is associated with a different sequence of bytes, which is
reliably transmitted to the peer in CRYPTO frames. When TLS provides handshake
bytes to be sent, they are appended to the handshake bytes for the current
encryption level. The encryption level then determines the type of packet that
the resulting CRYPTO frame is carried in; see <xref target="packet-types-keys" format="default" sectionFormat="of" derivedContent="Table 1"/>.</t>
          <t indent="0" pn="section-4.1.3-6">Four encryption levels are used, producing keys for Initial, 0-RTT, Handshake,
and 1-RTT packets. CRYPTO frames are carried in just three of these levels,
omitting the 0-RTT level. These four levels correspond to three packet number
spaces: Initial and Handshake encrypted packets use their own separate spaces;
0-RTT and 1-RTT packets use the application data packet number space.</t>
          <t indent="0" pn="section-4.1.3-7">QUIC takes the unprotected content of TLS handshake records as the content of
CRYPTO frames. TLS record protection is not used by QUIC. QUIC assembles
CRYPTO frames into QUIC packets, which are protected using QUIC packet
protection.</t>
          <t indent="0" pn="section-4.1.3-8">QUIC CRYPTO frames only carry TLS handshake messages.  TLS
alerts are turned into QUIC CONNECTION_CLOSE error codes; see <xref target="tls-errors" format="default" sectionFormat="of" derivedContent="Section 4.8"/>.
TLS application data and other content types cannot be carried by QUIC at any
encryption level; it is an error if they are received from the TLS stack.</t>
          <t indent="0" pn="section-4.1.3-9">When an endpoint receives a QUIC packet containing a CRYPTO frame from the
network, it proceeds as follows:</t>
          <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-4.1.3-10">
            <li pn="section-4.1.3-10.1">If the packet uses the current TLS receiving encryption level, sequence the
data into the input flow as usual. As with STREAM frames, the offset is used
to find the proper location in the data sequence.  If the result of this
process is that new data is available, then it is delivered to TLS in order.</li>
            <li pn="section-4.1.3-10.2">If the packet is from a previously installed encryption level, it <bcp14>MUST NOT</bcp14>
contain data that extends past the end of previously received data in that
flow. Implementations <bcp14>MUST</bcp14> treat any violations of this requirement as a
connection error of type PROTOCOL_VIOLATION.</li>
            <li pn="section-4.1.3-10.3">If the packet is from a new encryption level, it is saved for later processing
by TLS.  Once TLS moves to receiving from this encryption level, saved data
can be provided to TLS.  When TLS provides keys for a higher encryption level,
if there is data from a previous encryption level that TLS has not consumed,
this <bcp14>MUST</bcp14> be treated as a connection error of type PROTOCOL_VIOLATION.</li>
          </ul>
          <t indent="0" pn="section-4.1.3-11">Each time that TLS is provided with new data, new handshake bytes are requested
from TLS.  TLS might not provide any bytes if the handshake messages it has
received are incomplete or it has no data to send.</t>
          <t indent="0" pn="section-4.1.3-12">The content of CRYPTO frames might either be processed incrementally by TLS or
buffered until complete messages or flights are available.  TLS is responsible
for buffering handshake bytes that have arrived in order.  QUIC is responsible
for buffering handshake bytes that arrive out of order or for encryption levels
that are not yet ready.  QUIC does not provide any means of flow control for
CRYPTO frames; see <xref section="7.5" sectionFormat="of" target="QUIC-TRANSPORT" format="default" derivedLink="https://rfc-editor.org/rfc/rfc9000#section-7.5" derivedContent="QUIC-TRANSPORT"/>.</t>
          <t indent="0" pn="section-4.1.3-13">Once the TLS handshake is complete, this is indicated to QUIC along with any
final handshake bytes that TLS needs to send.  At this stage, the transport
parameters that the peer advertised during the handshake are authenticated;
see <xref target="quic_parameters" format="default" sectionFormat="of" derivedContent="Section 8.2"/>.</t>
          <t indent="0" pn="section-4.1.3-14">Once the handshake is complete, TLS becomes passive.  TLS can still receive data
from its peer and respond in kind, but it will not need to send more data unless
specifically requested -- either by an application or QUIC.  One reason to send
data is that the server might wish to provide additional or updated session
tickets to a client.</t>
          <t indent="0" pn="section-4.1.3-15">When the handshake is complete, QUIC only needs to provide TLS with any data
that arrives in CRYPTO streams.  In the same manner that is used during the
handshake, new data is requested from TLS after providing received data.</t>
        </section>
        <section anchor="encryption-level-changes" numbered="true" toc="include" removeInRFC="false" pn="section-4.1.4">
          <name slugifiedName="name-encryption-level-changes">Encryption Level Changes</name>
          <t indent="0" pn="section-4.1.4-1">As keys at a given encryption level become available to TLS, TLS indicates to
QUIC that reading or writing keys at that encryption level are available.</t>
          <t indent="0" pn="section-4.1.4-2">The availability of new keys is always a result of providing inputs to TLS.  TLS
only provides new keys after being initialized (by a client) or when provided
with new handshake data.</t>
          <t indent="0" pn="section-4.1.4-3">However, a TLS implementation could perform some of its processing
asynchronously. In particular, the process of validating a certificate can take
some time. While waiting for TLS processing to complete, an endpoint <bcp14>SHOULD</bcp14>
buffer received packets if they might be processed using keys that are not yet
available. These packets can be processed once keys are provided by TLS. An
endpoint <bcp14>SHOULD</bcp14> continue to respond to packets that can be processed during this
time.</t>
          <t indent="0" pn="section-4.1.4-4">After processing inputs, TLS might produce handshake bytes, keys for new
encryption levels, or both.</t>
          <t indent="0" pn="section-4.1.4-5">TLS provides QUIC with three items as a new encryption level becomes available:</t>
          <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-4.1.4-6">
            <li pn="section-4.1.4-6.1">A secret</li>
            <li pn="section-4.1.4-6.2">An Authenticated Encryption with Associated Data (AEAD) function</li>
            <li pn="section-4.1.4-6.3">A Key Derivation Function (KDF)</li>
          </ul>
          <t indent="0" pn="section-4.1.4-7">These values are based on the values that TLS negotiates and are used by QUIC to
generate packet and header protection keys; see <xref target="packet-protection" format="default" sectionFormat="of" derivedContent="Section 5"/> and
<xref target="header-protect" format="default" sectionFormat="of" derivedContent="Section 5.4"/>.</t>
          <t indent="0" pn="section-4.1.4-8">If 0-RTT is possible, it is ready after the client sends a TLS ClientHello
message or the server receives that message.  After providing a QUIC client with
the first handshake bytes, the TLS stack might signal the change to 0-RTT
keys. On the server, after receiving handshake bytes that contain a ClientHello
message, a TLS server might signal that 0-RTT keys are available.</t>
          <t indent="0" pn="section-4.1.4-9">Although TLS only uses one encryption level at a time, QUIC may use more than
one level. For instance, after sending its Finished message (using a CRYPTO
frame at the Handshake encryption level) an endpoint can send STREAM data (in
1-RTT encryption). If the Finished message is lost, the endpoint uses the
Handshake encryption level to retransmit the lost message.  Reordering or loss
of packets can mean that QUIC will need to handle packets at multiple encryption
levels.  During the handshake, this means potentially handling packets at higher
and lower encryption levels than the current encryption level used by TLS.</t>
          <t indent="0" pn="section-4.1.4-10">In particular, server implementations need to be able to read packets at the
Handshake encryption level at the same time as the 0-RTT encryption level.  A
client could interleave ACK frames that are protected with Handshake keys with
0-RTT data, and the server needs to process those acknowledgments in order to
detect lost Handshake packets.</t>
          <t indent="0" pn="section-4.1.4-11">QUIC also needs access to keys that might not ordinarily be available to a TLS
implementation.  For instance, a client might need to acknowledge Handshake
packets before it is ready to send CRYPTO frames at that encryption level.  TLS
therefore needs to provide keys to QUIC before it might produce them for its own
use.</t>
        </section>
        <section anchor="tls-interface-summary" numbered="true" toc="include" removeInRFC="false" pn="section-4.1.5">
          <name slugifiedName="name-tls-interface-summary">TLS Interface Summary</name>
          <t indent="0" pn="section-4.1.5-1"><xref target="exchange-summary" format="default" sectionFormat="of" derivedContent="Figure 5"/> summarizes the exchange between QUIC and TLS for both
client and server. Solid arrows indicate packets that carry handshake data;
dashed arrows show where application data can be sent.  Each arrow is tagged
with the encryption level used for that transmission.</t>
          <figure anchor="exchange-summary" align="left" suppress-title="false" pn="figure-5">
            <name slugifiedName="name-interaction-summary-between">Interaction Summary between QUIC and TLS</name>
            <artwork name="" type="" align="left" alt="" pn="section-4.1.5-2.1">
Client                                                    Server
======                                                    ======

Get Handshake
                     Initial -------------&gt;
Install tx 0-RTT keys
                     0-RTT - - - - - - - -&gt;

                                              Handshake Received
                                                   Get Handshake
                     &lt;------------- Initial
                                           Install rx 0-RTT keys
                                          Install Handshake keys
                                                   Get Handshake
                     &lt;----------- Handshake
                                           Install tx 1-RTT keys
                     &lt;- - - - - - - - 1-RTT

Handshake Received (Initial)
Install Handshake keys
Handshake Received (Handshake)
Get Handshake
                     Handshake -----------&gt;
Handshake Complete
Install 1-RTT keys
                     1-RTT - - - - - - - -&gt;

                                              Handshake Received
                                              Handshake Complete
                                             Handshake Confirmed
                                           Install rx 1-RTT keys
                     &lt;--------------- 1-RTT
                           (HANDSHAKE_DONE)
Handshake Confirmed
</artwork>
          </figure>
          <t indent="0" pn="section-4.1.5-3"><xref target="exchange-summary" format="default" sectionFormat="of" derivedContent="Figure 5"/> shows the multiple packets that form a single "flight" of
messages being processed individually, to show what incoming messages trigger
different actions. This shows multiple "Get Handshake" invocations to retrieve
handshake messages at different encryption levels. New handshake messages are
requested after incoming packets have been processed.</t>
          <t indent="0" pn="section-4.1.5-4"><xref target="exchange-summary" format="default" sectionFormat="of" derivedContent="Figure 5"/> shows one possible structure for a simple handshake
exchange. The exact process varies based on the structure of endpoint
implementations and the order in which packets arrive. Implementations could
use a different number of operations or execute them in other orders.</t>
        </section>
      </section>
      <section anchor="tls-version" numbered="true" toc="include" removeInRFC="false" pn="section-4.2">
        <name slugifiedName="name-tls-version">TLS Version</name>
        <t indent="0" pn="section-4.2-1">This document describes how TLS 1.3 <xref target="TLS13" format="default" sectionFormat="of" derivedContent="TLS13"/> is used with QUIC.</t>
        <t indent="0" pn="section-4.2-2">In practice, the TLS handshake will negotiate a version of TLS to use.  This
could result in a version of TLS newer than 1.3 being negotiated if both
endpoints support that version.  This is acceptable provided that the features
of TLS 1.3 that are used by QUIC are supported by the newer version.</t>
        <t indent="0" pn="section-4.2-3">Clients <bcp14>MUST NOT</bcp14> offer TLS versions older than 1.3.  A badly configured TLS
implementation could negotiate TLS 1.2 or another older version of TLS.  An
endpoint <bcp14>MUST</bcp14> terminate the connection if a version of TLS older than 1.3 is
negotiated.</t>
      </section>
      <section anchor="clienthello-size" numbered="true" toc="include" removeInRFC="false" pn="section-4.3">
        <name slugifiedName="name-clienthello-size">ClientHello Size</name>
        <t indent="0" pn="section-4.3-1">The first Initial packet from a client contains the start or all of its first
cryptographic handshake message, which for TLS is the ClientHello.  Servers
might need to parse the entire ClientHello (e.g., to access extensions such as
Server Name Identification (SNI) or Application-Layer Protocol Negotiation
(ALPN)) in order to decide whether to accept the new incoming QUIC connection.
If the ClientHello spans multiple Initial packets, such servers would need to
buffer the first received fragments, which could consume excessive resources if
the client's address has not yet been validated.  To avoid this, servers <bcp14>MAY</bcp14>
use the Retry feature (see <xref section="8.1" sectionFormat="of" target="QUIC-TRANSPORT" format="default" derivedLink="https://rfc-editor.org/rfc/rfc9000#section-8.1" derivedContent="QUIC-TRANSPORT"/>) to only buffer
partial ClientHello messages from clients with a validated address.</t>
        <t indent="0" pn="section-4.3-2">QUIC packet and framing add at least 36 bytes of overhead to the ClientHello
message.  That overhead increases if the client chooses a Source Connection ID
field longer than zero bytes.  Overheads also do not include the token or a
Destination Connection ID longer than 8 bytes, both of which might be required
if a server sends a Retry packet.</t>
        <t indent="0" pn="section-4.3-3">A typical TLS ClientHello can easily fit into a 1200-byte packet.  However, in
addition to the overheads added by QUIC, there are several variables that could
cause this limit to be exceeded.  Large session tickets, multiple or large key
shares, and long lists of supported ciphers, signature algorithms, versions,
QUIC transport parameters, and other negotiable parameters and extensions could
cause this message to grow.</t>
        <t indent="0" pn="section-4.3-4">For servers, in addition to connection IDs and tokens, the size of TLS session
tickets can have an effect on a client's ability to connect efficiently.
Minimizing the size of these values increases the probability that clients can
use them and still fit their entire ClientHello message in their first Initial
packet.</t>
        <t indent="0" pn="section-4.3-5">The TLS implementation does not need to ensure that the ClientHello is large
enough to meet QUIC's requirements for datagrams that carry Initial packets; see
<xref section="14.1" sectionFormat="of" target="QUIC-TRANSPORT" format="default" derivedLink="https://rfc-editor.org/rfc/rfc9000#section-14.1" derivedContent="QUIC-TRANSPORT"/>. QUIC implementations use PADDING frames or
packet coalescing to ensure that datagrams are large enough.</t>
      </section>
      <section anchor="peer-authentication" numbered="true" toc="include" removeInRFC="false" pn="section-4.4">
        <name slugifiedName="name-peer-authentication">Peer Authentication</name>
        <t indent="0" pn="section-4.4-1">The requirements for authentication depend on the application protocol that is
in use.  TLS provides server authentication and permits the server to request
client authentication.</t>
        <t indent="0" pn="section-4.4-2">A client <bcp14>MUST</bcp14> authenticate the identity of the server.  This typically involves
verification that the identity of the server is included in a certificate and
that the certificate is issued by a trusted entity (see for example
<xref target="RFC2818" format="default" sectionFormat="of" derivedContent="RFC2818"/>).</t>
        <aside pn="section-4.4-3">
          <t indent="0" pn="section-4.4-3.1">Note: Where servers provide certificates for authentication, the size of the
  certificate chain can consume a large number of bytes.  Controlling the size
  of certificate chains is critical to performance in QUIC as servers are
  limited to sending 3 bytes for every byte received prior to validating the
  client address; see <xref section="8.1" sectionFormat="of" target="QUIC-TRANSPORT" format="default" derivedLink="https://rfc-editor.org/rfc/rfc9000#section-8.1" derivedContent="QUIC-TRANSPORT"/>.  The size of a
  certificate chain can be managed by limiting the number of names or
  extensions; using keys with small public key representations, like ECDSA; or
  by using certificate compression <xref target="COMPRESS" format="default" sectionFormat="of" derivedContent="COMPRESS"/>.</t>
        </aside>
        <t indent="0" pn="section-4.4-4">A server <bcp14>MAY</bcp14> request that the client authenticate during the handshake. A server
<bcp14>MAY</bcp14> refuse a connection if the client is unable to authenticate when requested.
The requirements for client authentication vary based on application protocol
and deployment.</t>
        <t indent="0" pn="section-4.4-5">A server <bcp14>MUST NOT</bcp14> use post-handshake client authentication (as defined in
<xref section="4.6.2" sectionFormat="of" target="TLS13" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8446#section-4.6.2" derivedContent="TLS13"/>) because the multiplexing offered by QUIC prevents
clients from correlating the certificate request with the application-level
event that triggered it (see <xref target="HTTP2-TLS13" format="default" sectionFormat="of" derivedContent="HTTP2-TLS13"/>).  More specifically,
servers <bcp14>MUST NOT</bcp14> send post-handshake TLS CertificateRequest messages, and
clients <bcp14>MUST</bcp14> treat receipt of such messages as a connection error of type
PROTOCOL_VIOLATION.</t>
      </section>
      <section anchor="resumption" numbered="true" toc="include" removeInRFC="false" pn="section-4.5">
        <name slugifiedName="name-session-resumption">Session Resumption</name>
        <t indent="0" pn="section-4.5-1">QUIC can use the session resumption feature of TLS 1.3. It does this by
carrying NewSessionTicket messages in CRYPTO frames after the handshake is
complete. Session resumption can be used to provide 0-RTT and can also be
used when 0-RTT is disabled.</t>
        <t indent="0" pn="section-4.5-2">Endpoints that use session resumption might need to remember some information
about the current connection when creating a resumed connection. TLS requires
that some information be retained; see <xref section="4.6.1" sectionFormat="of" target="TLS13" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8446#section-4.6.1" derivedContent="TLS13"/>. QUIC itself
does not depend on any state being retained when resuming a connection unless
0-RTT is also used; see <xref section="7.4.1" sectionFormat="of" target="QUIC-TRANSPORT" format="default" derivedLink="https://rfc-editor.org/rfc/rfc9000#section-7.4.1" derivedContent="QUIC-TRANSPORT"/> and
<xref target="enable-0rtt" format="default" sectionFormat="of" derivedContent="Section 4.6.1"/>. Application protocols could depend on state that is retained
between resumed connections.</t>
        <t indent="0" pn="section-4.5-3">Clients can store any state required for resumption along with the session
ticket. Servers can use the session ticket to help carry state.</t>
        <t indent="0" pn="section-4.5-4">Session resumption allows servers to link activity on the original connection
with the resumed connection, which might be a privacy issue for clients.
Clients can choose not to enable resumption to avoid creating this correlation.
Clients <bcp14>SHOULD NOT</bcp14> reuse tickets as that allows entities other than the server
to correlate connections; see <xref section="C.4" sectionFormat="of" target="TLS13" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8446#appendix-C.4" derivedContent="TLS13"/>.</t>
      </section>
      <section anchor="rtt" numbered="true" toc="include" removeInRFC="false" pn="section-4.6">
        <name slugifiedName="name-0-rtt">0-RTT</name>
        <t indent="0" pn="section-4.6-1">The 0-RTT feature in QUIC allows a client to send application data before the
handshake is complete.  This is made possible by reusing negotiated parameters
from a previous connection.  To enable this, 0-RTT depends on the client
remembering critical parameters and providing the server with a TLS session
ticket that allows the server to recover the same information.</t>
        <t indent="0" pn="section-4.6-2">This information includes parameters that determine TLS state, as governed by
<xref target="TLS13" format="default" sectionFormat="of" derivedContent="TLS13"/>, QUIC transport parameters, the chosen application protocol, and any
information the application protocol might need; see <xref target="app-0rtt" format="default" sectionFormat="of" derivedContent="Section 4.6.3"/>.  This
information determines how 0-RTT packets and their contents are formed.</t>
        <t indent="0" pn="section-4.6-3">To ensure that the same information is available to both endpoints, all
information used to establish 0-RTT comes from the same connection.  Endpoints
cannot selectively disregard information that might alter the sending or
processing of 0-RTT.</t>
        <t indent="0" pn="section-4.6-4"><xref target="TLS13" format="default" sectionFormat="of" derivedContent="TLS13"/> sets a limit of seven days on the time between the original
connection and any attempt to use 0-RTT.  There are other constraints on 0-RTT
usage, notably those caused by the potential exposure to replay attack; see
<xref target="replay" format="default" sectionFormat="of" derivedContent="Section 9.2"/>.</t>
        <section anchor="enable-0rtt" numbered="true" toc="include" removeInRFC="false" pn="section-4.6.1">
          <name slugifiedName="name-enabling-0-rtt">Enabling 0-RTT</name>
          <t indent="0" pn="section-4.6.1-1">The TLS early_data extension in the NewSessionTicket message is defined to
convey (in the max_early_data_size parameter) the amount of TLS 0-RTT data the
server is willing to accept.  QUIC does not use TLS early data.  QUIC uses 0-RTT
packets to carry early data.  Accordingly, the max_early_data_size parameter is
repurposed to hold a sentinel value 0xffffffff to indicate that the server is
willing to accept QUIC 0-RTT data.  To indicate that the server does not accept
0-RTT data, the early_data extension is omitted from the NewSessionTicket.  The
amount of data that the client can send in QUIC 0-RTT is controlled by the
initial_max_data transport parameter supplied by the server.</t>
          <t indent="0" pn="section-4.6.1-2">Servers <bcp14>MUST NOT</bcp14> send the early_data extension with a max_early_data_size field
set to any value other than 0xffffffff.  A client <bcp14>MUST</bcp14> treat receipt of a
NewSessionTicket that contains an early_data extension with any other value as
a connection error of type PROTOCOL_VIOLATION.</t>
          <t indent="0" pn="section-4.6.1-3">A client that wishes to send 0-RTT packets uses the early_data extension in the
ClientHello message of a subsequent handshake; see <xref section="4.2.10" sectionFormat="of" target="TLS13" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8446#section-4.2.10" derivedContent="TLS13"/>.
It then sends application data in 0-RTT packets.</t>
          <t indent="0" pn="section-4.6.1-4">A client that attempts 0-RTT might also provide an address validation token if
the server has sent a NEW_TOKEN frame; see <xref section="8.1" sectionFormat="of" target="QUIC-TRANSPORT" format="default" derivedLink="https://rfc-editor.org/rfc/rfc9000#section-8.1" derivedContent="QUIC-TRANSPORT"/>.</t>
        </section>
        <section anchor="accepting-and-rejecting-0-rtt" numbered="true" toc="include" removeInRFC="false" pn="section-4.6.2">
          <name slugifiedName="name-accepting-and-rejecting-0-r">Accepting and Rejecting 0-RTT</name>
          <t indent="0" pn="section-4.6.2-1">A server accepts 0-RTT by sending an early_data extension in the
EncryptedExtensions; see <xref section="4.2.10" sectionFormat="of" target="TLS13" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8446#section-4.2.10" derivedContent="TLS13"/>.  The server then
processes and acknowledges the 0-RTT packets that it receives.</t>
          <t indent="0" pn="section-4.6.2-2">A server rejects 0-RTT by sending the EncryptedExtensions without an early_data
extension.  A server will always reject 0-RTT if it sends a TLS
HelloRetryRequest.  When rejecting 0-RTT, a server <bcp14>MUST NOT</bcp14> process any 0-RTT
packets, even if it could.  When 0-RTT was rejected, a client <bcp14>SHOULD</bcp14> treat
receipt of an acknowledgment for a 0-RTT packet as a connection error of type
PROTOCOL_VIOLATION, if it is able to detect the condition.</t>
          <t indent="0" pn="section-4.6.2-3">When 0-RTT is rejected, all connection characteristics that the client assumed
might be incorrect.  This includes the choice of application protocol, transport
parameters, and any application configuration.  The client therefore <bcp14>MUST</bcp14> reset
the state of all streams, including application state bound to those streams.</t>
          <t indent="0" pn="section-4.6.2-4">A client <bcp14>MAY</bcp14> reattempt 0-RTT if it receives a Retry or Version Negotiation
packet.  These packets do not signify rejection of 0-RTT.</t>
        </section>
        <section anchor="app-0rtt" numbered="true" toc="include" removeInRFC="false" pn="section-4.6.3">
          <name slugifiedName="name-validating-0-rtt-configurat">Validating 0-RTT Configuration</name>
          <t indent="0" pn="section-4.6.3-1">When a server receives a ClientHello with the early_data extension, it has to
decide whether to accept or reject 0-RTT data from the client. Some of this
decision is made by the TLS stack (e.g., checking that the cipher suite being
resumed was included in the ClientHello; see <xref section="4.2.10" sectionFormat="of" target="TLS13" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8446#section-4.2.10" derivedContent="TLS13"/>). Even
when the TLS stack has no reason to reject 0-RTT data, the QUIC stack or the
application protocol using QUIC might reject 0-RTT data because the
configuration of the transport or application associated with the resumed
session is not compatible with the server's current configuration.</t>
          <t indent="0" pn="section-4.6.3-2">QUIC requires additional transport state to be associated with a 0-RTT session
ticket. One common way to implement this is using stateless session tickets and
storing this state in the session ticket. Application protocols that use QUIC
might have similar requirements regarding associating or storing state. This
associated state is used for deciding whether 0-RTT data must be rejected. For
example, HTTP/3 settings <xref target="QUIC-HTTP" format="default" sectionFormat="of" derivedContent="QUIC-HTTP"/> determine how 0-RTT data from the
client is interpreted. Other applications using QUIC could have different
requirements for determining whether to accept or reject 0-RTT data.</t>
        </section>
      </section>
      <section anchor="helloretryrequest" numbered="true" toc="include" removeInRFC="false" pn="section-4.7">
        <name slugifiedName="name-helloretryrequest">HelloRetryRequest</name>
        <t indent="0" pn="section-4.7-1">The HelloRetryRequest message (see <xref section="4.1.4" sectionFormat="of" target="TLS13" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8446#section-4.1.4" derivedContent="TLS13"/>) can be used to
request that a client provide new information, such as a key share, or to
validate some characteristic of the client.  From the perspective of QUIC,
HelloRetryRequest is not differentiated from other cryptographic handshake
messages that are carried in Initial packets. Although it is in principle
possible to use this feature for address verification, QUIC implementations
<bcp14>SHOULD</bcp14> instead use the Retry feature; see <xref section="8.1" sectionFormat="of" target="QUIC-TRANSPORT" format="default" derivedLink="https://rfc-editor.org/rfc/rfc9000#section-8.1" derivedContent="QUIC-TRANSPORT"/>.</t>
      </section>
      <section anchor="tls-errors" numbered="true" toc="include" removeInRFC="false" pn="section-4.8">
        <name slugifiedName="name-tls-errors">TLS Errors</name>
        <t indent="0" pn="section-4.8-1">If TLS experiences an error, it generates an appropriate alert as defined in
<xref section="6" sectionFormat="of" target="TLS13" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8446#section-6" derivedContent="TLS13"/>.</t>
        <t indent="0" pn="section-4.8-2">A TLS alert is converted into a QUIC connection error. The AlertDescription
value is
added to 0x0100 to produce a QUIC error code from the range reserved for
CRYPTO_ERROR; see <xref section="20.1" sectionFormat="of" target="QUIC-TRANSPORT" format="default" derivedLink="https://rfc-editor.org/rfc/rfc9000#section-20.1" derivedContent="QUIC-TRANSPORT"/>. The resulting value is
sent in a QUIC CONNECTION_CLOSE frame of type 0x1c.</t>
        <t indent="0" pn="section-4.8-3">QUIC is only able to convey an alert level of "fatal". In TLS 1.3, the only
existing uses for the "warning" level are to signal connection close; see
<xref section="6.1" sectionFormat="of" target="TLS13" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8446#section-6.1" derivedContent="TLS13"/>. As QUIC provides alternative mechanisms for
connection termination and the TLS connection is only closed if an error is
encountered, a QUIC endpoint <bcp14>MUST</bcp14> treat any alert from TLS as if it were at the
"fatal" level.</t>
        <t indent="0" pn="section-4.8-4">QUIC permits the use of a generic code in place of a specific error code; see
<xref section="11" sectionFormat="of" target="QUIC-TRANSPORT" format="default" derivedLink="https://rfc-editor.org/rfc/rfc9000#section-11" derivedContent="QUIC-TRANSPORT"/>. For TLS alerts, this includes replacing any
alert with a generic alert, such as handshake_failure (0x0128 in QUIC).
Endpoints <bcp14>MAY</bcp14> use a generic error code to avoid possibly exposing confidential
information.</t>
      </section>
      <section anchor="discarding-unused-keys" numbered="true" toc="include" removeInRFC="false" pn="section-4.9">
        <name slugifiedName="name-discarding-unused-keys">Discarding Unused Keys</name>
        <t indent="0" pn="section-4.9-1">After QUIC has completed a move to a new encryption level, packet protection
keys for previous encryption levels can be discarded.  This occurs several times
during the handshake, as well as when keys are updated; see <xref target="key-update" format="default" sectionFormat="of" derivedContent="Section 6"/>.</t>
        <t indent="0" pn="section-4.9-2">Packet protection keys are not discarded immediately when new keys are
available.  If packets from a lower encryption level contain CRYPTO frames,
frames that retransmit that data <bcp14>MUST</bcp14> be sent at the same encryption level.
Similarly, an endpoint generates acknowledgments for packets at the same
encryption level as the packet being acknowledged.  Thus, it is possible that
keys for a lower encryption level are needed for a short time after keys for a
newer encryption level are available.</t>
        <t indent="0" pn="section-4.9-3">An endpoint cannot discard keys for a given encryption level unless it has
received all the cryptographic handshake messages from its peer at that
encryption level and its peer has done the same.  Different methods for
determining this are provided for Initial keys (<xref target="discard-initial" format="default" sectionFormat="of" derivedContent="Section 4.9.1"/>) and
Handshake keys (<xref target="discard-handshake" format="default" sectionFormat="of" derivedContent="Section 4.9.2"/>).  These methods do not prevent packets
from being received or sent at that encryption level because a peer might not
have received all the acknowledgments necessary.</t>
        <t indent="0" pn="section-4.9-4">Though an endpoint might retain older keys, new data <bcp14>MUST</bcp14> be sent at the highest
currently available encryption level.  Only ACK frames and retransmissions of
data in CRYPTO frames are sent at a previous encryption level.  These packets
<bcp14>MAY</bcp14> also include PADDING frames.</t>
        <section anchor="discard-initial" numbered="true" toc="include" removeInRFC="false" pn="section-4.9.1">
          <name slugifiedName="name-discarding-initial-keys">Discarding Initial Keys</name>
          <t indent="0" pn="section-4.9.1-1">Packets protected with Initial secrets (<xref target="initial-secrets" format="default" sectionFormat="of" derivedContent="Section 5.2"/>) are not
authenticated, meaning that an attacker could spoof packets with the intent to
disrupt a connection.  To limit these attacks, Initial packet protection keys
are discarded more aggressively than other keys.</t>
          <t indent="0" pn="section-4.9.1-2">The successful use of Handshake packets indicates that no more Initial packets
need to be exchanged, as these keys can only be produced after receiving all
CRYPTO frames from Initial packets.  Thus, a client <bcp14>MUST</bcp14> discard Initial keys
when it first sends a Handshake packet and a server <bcp14>MUST</bcp14> discard Initial keys
when it first successfully processes a Handshake packet.  Endpoints <bcp14>MUST NOT</bcp14>
send Initial packets after this point.</t>
          <t indent="0" pn="section-4.9.1-3">This results in abandoning loss recovery state for the Initial encryption level
and ignoring any outstanding Initial packets.</t>
        </section>
        <section anchor="discard-handshake" numbered="true" toc="include" removeInRFC="false" pn="section-4.9.2">
          <name slugifiedName="name-discarding-handshake-keys">Discarding Handshake Keys</name>
          <t indent="0" pn="section-4.9.2-1">An endpoint <bcp14>MUST</bcp14> discard its Handshake keys when the TLS handshake is confirmed
(<xref target="handshake-confirmed" format="default" sectionFormat="of" derivedContent="Section 4.1.2"/>).</t>
        </section>
        <section anchor="discarding-0-rtt-keys" numbered="true" toc="include" removeInRFC="false" pn="section-4.9.3">
          <name slugifiedName="name-discarding-0-rtt-keys">Discarding 0-RTT Keys</name>
          <t indent="0" pn="section-4.9.3-1">0-RTT and 1-RTT packets share the same packet number space, and clients do not
send 0-RTT packets after sending a 1-RTT packet (<xref target="using-early-data" format="default" sectionFormat="of" derivedContent="Section 5.6"/>).</t>
          <t indent="0" pn="section-4.9.3-2">Therefore, a client <bcp14>SHOULD</bcp14> discard 0-RTT keys as soon as it installs 1-RTT
keys as they have no use after that moment.</t>
          <t indent="0" pn="section-4.9.3-3">Additionally, a server <bcp14>MAY</bcp14> discard 0-RTT keys as soon as it receives a 1-RTT
packet.  However, due to packet reordering, a 0-RTT packet could arrive after
a 1-RTT packet.  Servers <bcp14>MAY</bcp14> temporarily retain 0-RTT keys to allow decrypting
reordered packets without requiring their contents to be retransmitted with
1-RTT keys.  After receiving a 1-RTT packet, servers <bcp14>MUST</bcp14> discard 0-RTT keys
within a short time; the <bcp14>RECOMMENDED</bcp14> time period is three times the Probe
Timeout (PTO, see <xref target="QUIC-RECOVERY" format="default" sectionFormat="of" derivedContent="QUIC-RECOVERY"/>).  A server <bcp14>MAY</bcp14> discard 0-RTT keys earlier
if it determines that it has received all 0-RTT packets, which can be done by
keeping track of missing packet numbers.</t>
        </section>
      </section>
    </section>
    <section anchor="packet-protection" numbered="true" toc="include" removeInRFC="false" pn="section-5">
      <name slugifiedName="name-packet-protection">Packet Protection</name>
      <t indent="0" pn="section-5-1">As with TLS over TCP, QUIC protects packets with keys derived from the TLS
handshake, using the AEAD algorithm <xref target="AEAD" format="default" sectionFormat="of" derivedContent="AEAD"/> negotiated by TLS.</t>
      <t indent="0" pn="section-5-2">QUIC packets have varying protections depending on their type:</t>
      <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5-3">
        <li pn="section-5-3.1">Version Negotiation packets have no cryptographic protection.</li>
        <li pn="section-5-3.2">Retry packets use AEAD_AES_128_GCM to provide protection against accidental
modification and to limit the entities that can produce a valid Retry;
see <xref target="retry-integrity" format="default" sectionFormat="of" derivedContent="Section 5.8"/>.</li>
        <li pn="section-5-3.3">Initial packets use AEAD_AES_128_GCM with keys derived from the Destination
Connection ID field of the first Initial packet sent by the client; see
<xref target="initial-secrets" format="default" sectionFormat="of" derivedContent="Section 5.2"/>.</li>
        <li pn="section-5-3.4">All other packets have strong cryptographic protections for confidentiality
and integrity, using keys and algorithms negotiated by TLS.</li>
      </ul>
      <t indent="0" pn="section-5-4">This section describes how packet protection is applied to Handshake packets,
0-RTT packets, and 1-RTT packets. The same packet protection process is applied
to Initial packets. However, as it is trivial to determine the keys used for
Initial packets, these packets are not considered to have confidentiality or
integrity protection. Retry packets use a fixed key and so similarly lack
confidentiality and integrity protection.</t>
      <section anchor="protection-keys" numbered="true" toc="include" removeInRFC="false" pn="section-5.1">
        <name slugifiedName="name-packet-protection-keys">Packet Protection Keys</name>
        <t indent="0" pn="section-5.1-1">QUIC derives packet protection keys in the same way that TLS derives record
protection keys.</t>
        <t indent="0" pn="section-5.1-2">Each encryption level has separate secret values for protection of packets sent
in each direction. These traffic secrets are derived by TLS (see <xref section="7.1" sectionFormat="of" target="TLS13" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8446#section-7.1" derivedContent="TLS13"/>) and are used by QUIC for all encryption levels except the Initial
encryption level. The secrets for the Initial encryption level are computed
based on the client's initial Destination Connection ID, as described in
<xref target="initial-secrets" format="default" sectionFormat="of" derivedContent="Section 5.2"/>.</t>
        <t indent="0" pn="section-5.1-3">The keys used for packet protection are computed from the TLS secrets using the
KDF provided by TLS.  In TLS 1.3, the HKDF-Expand-Label function described in
<xref section="7.1" sectionFormat="of" target="TLS13" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8446#section-7.1" derivedContent="TLS13"/> is used with the hash function from the negotiated
cipher suite.  All uses of HKDF-Expand-Label in QUIC use a zero-length Context.</t>
        <t indent="0" pn="section-5.1-4">Note that labels, which are described using strings, are encoded
as bytes using ASCII <xref target="ASCII" format="default" sectionFormat="of" derivedContent="ASCII"/> without quotes or any trailing NUL
byte.</t>
        <t indent="0" pn="section-5.1-5">Other versions of TLS <bcp14>MUST</bcp14> provide a similar function in order to be
used with QUIC.</t>
        <t indent="0" pn="section-5.1-6">The current encryption level secret and the label "quic key" are input to the
KDF to produce the AEAD key; the label "quic iv" is used to derive the
Initialization Vector (IV); see <xref target="aead" format="default" sectionFormat="of" derivedContent="Section 5.3"/>.  The header protection key uses the
"quic hp" label; see <xref target="header-protect" format="default" sectionFormat="of" derivedContent="Section 5.4"/>.  Using these labels provides key
separation between QUIC and TLS; see <xref target="key-diversity" format="default" sectionFormat="of" derivedContent="Section 9.6"/>.</t>
        <t indent="0" pn="section-5.1-7">Both "quic key" and "quic hp" are used to produce keys, so the Length provided
to HKDF-Expand-Label along with these labels is determined by the size of keys
in the AEAD or header protection algorithm. The Length provided with "quic iv"
is the minimum length of the AEAD nonce or 8 bytes if that is larger; see
<xref target="AEAD" format="default" sectionFormat="of" derivedContent="AEAD"/>.</t>
        <t indent="0" pn="section-5.1-8">The KDF used for initial secrets is always the HKDF-Expand-Label function from
TLS 1.3; see <xref target="initial-secrets" format="default" sectionFormat="of" derivedContent="Section 5.2"/>.</t>
      </section>
      <section anchor="initial-secrets" numbered="true" toc="include" removeInRFC="false" pn="section-5.2">
        <name slugifiedName="name-initial-secrets">Initial Secrets</name>
        <t indent="0" pn="section-5.2-1">Initial packets apply the packet protection process, but use a secret derived
from the Destination Connection ID field from the client's first Initial
packet.</t>
        <t indent="0" pn="section-5.2-2">This secret is determined by using HKDF-Extract (see <xref section="2.2" sectionFormat="of" target="HKDF" format="default" derivedLink="https://rfc-editor.org/rfc/rfc5869#section-2.2" derivedContent="HKDF"/>)
with a salt of 0x38762cf7f55934b34d179ae6a4c80cadccbb7f0a and the input keying
material (IKM) of the Destination Connection ID field. This produces an
intermediate pseudorandom key (PRK) that is used to derive two separate secrets
for sending and receiving.</t>
        <t indent="0" pn="section-5.2-3">The secret used by clients to construct Initial packets uses the PRK and the
label "client in" as input to the HKDF-Expand-Label function from TLS
<xref target="TLS13" format="default" sectionFormat="of" derivedContent="TLS13"/> to produce a 32-byte secret.  Packets constructed by the server use
the same process with the label "server in".  The hash function for HKDF when
deriving initial secrets and keys is SHA-256
<xref target="SHA" format="default" sectionFormat="of" derivedContent="SHA"/>.</t>
        <t indent="0" pn="section-5.2-4">This process in pseudocode is:</t>
        <artwork name="" type="" align="left" alt="" pn="section-5.2-5">
initial_salt = 0x38762cf7f55934b34d179ae6a4c80cadccbb7f0a
initial_secret = HKDF-Extract(initial_salt,
                              client_dst_connection_id)

client_initial_secret = HKDF-Expand-Label(initial_secret,
                                          "client in", "",
                                          Hash.length)
server_initial_secret = HKDF-Expand-Label(initial_secret,
                                          "server in", "",
                                          Hash.length)
</artwork>
        <t indent="0" pn="section-5.2-6">The connection ID used with HKDF-Expand-Label is the Destination Connection ID
in the Initial packet sent by the client.  This will be a randomly selected
value unless the client creates the Initial packet after receiving a Retry
packet, where the Destination Connection ID is selected by the server.</t>
        <t indent="0" pn="section-5.2-7">Future versions of QUIC <bcp14>SHOULD</bcp14> generate a new salt value, thus ensuring that
the keys are different for each version of QUIC.  This prevents a middlebox that
recognizes only one version of QUIC from seeing or modifying the contents of
packets from future versions.</t>
        <t indent="0" pn="section-5.2-8">The HKDF-Expand-Label function defined in TLS 1.3 <bcp14>MUST</bcp14> be used for Initial
packets even where the TLS versions offered do not include TLS 1.3.</t>
        <t indent="0" pn="section-5.2-9">The secrets used for constructing subsequent Initial packets change when a
server sends a Retry packet to use the connection ID value selected by the
server.  The secrets do not change when a client changes the Destination
Connection ID it uses in response to an Initial packet from the server.</t>
        <aside pn="section-5.2-10">
          <t indent="0" pn="section-5.2-10.1">Note: The Destination Connection ID field could be any length up to 20 bytes,
  including zero length if the server sends a Retry packet with a zero-length
  Source Connection ID field. After a Retry, the Initial keys provide the client
  no assurance that the server received its packet, so the client has to rely on
  the exchange that included the Retry packet to validate the server address;
  see <xref section="8.1" sectionFormat="of" target="QUIC-TRANSPORT" format="default" derivedLink="https://rfc-editor.org/rfc/rfc9000#section-8.1" derivedContent="QUIC-TRANSPORT"/>.</t>
        </aside>
        <t indent="0" pn="section-5.2-11"><xref target="test-vectors" format="default" sectionFormat="of" derivedContent="Appendix A"/> contains sample Initial packets.</t>
      </section>
      <section anchor="aead" numbered="true" toc="include" removeInRFC="false" pn="section-5.3">
        <name slugifiedName="name-aead-usage">AEAD Usage</name>
        <t indent="0" pn="section-5.3-1">The Authenticated Encryption with Associated Data (AEAD) function (see
<xref target="AEAD" format="default" sectionFormat="of" derivedContent="AEAD"/>) used for QUIC packet protection is the AEAD that is negotiated for
use with the TLS connection.  For example, if TLS is using the
TLS_AES_128_GCM_SHA256 cipher suite, the AEAD_AES_128_GCM function is used.</t>
        <t indent="0" pn="section-5.3-2">QUIC can use any of the cipher suites defined in <xref target="TLS13" format="default" sectionFormat="of" derivedContent="TLS13"/> with the exception
of TLS_AES_128_CCM_8_SHA256.  A cipher suite <bcp14>MUST NOT</bcp14> be negotiated unless a
header protection scheme is defined for the cipher suite.  This document defines
a header protection scheme for all cipher suites defined in <xref target="TLS13" format="default" sectionFormat="of" derivedContent="TLS13"/> aside
from TLS_AES_128_CCM_8_SHA256.  These cipher suites have a 16-byte
authentication tag and produce an output 16 bytes larger than their input.</t>
        <t indent="0" pn="section-5.3-3">An endpoint <bcp14>MUST NOT</bcp14> reject a ClientHello that offers a cipher suite that it
does not support, or it would be impossible to deploy a new cipher suite.  This
also applies to TLS_AES_128_CCM_8_SHA256.</t>
        <t indent="0" pn="section-5.3-4">When constructing packets, the AEAD function is applied prior to applying
header protection; see <xref target="header-protect" format="default" sectionFormat="of" derivedContent="Section 5.4"/>. The unprotected packet header is part
of the associated data (A). When processing packets, an endpoint first
removes the header protection.</t>
        <t indent="0" pn="section-5.3-5">The key and IV for the packet are computed as described in <xref target="protection-keys" format="default" sectionFormat="of" derivedContent="Section 5.1"/>.
The nonce, N, is formed by combining the packet protection IV with the packet
number.  The 62 bits of the reconstructed QUIC packet number in network byte
order are left-padded with zeros to the size of the IV.  The exclusive OR of the
padded packet number and the IV forms the AEAD nonce.</t>
        <t indent="0" pn="section-5.3-6">The associated data, A, for the AEAD is the contents of the QUIC header,
starting from the first byte of either the short or long header, up to and
including the unprotected packet number.</t>
        <t indent="0" pn="section-5.3-7">The input plaintext, P, for the AEAD is the payload of the QUIC packet, as
described in <xref target="QUIC-TRANSPORT" format="default" sectionFormat="of" derivedContent="QUIC-TRANSPORT"/>.</t>
        <t indent="0" pn="section-5.3-8">The output ciphertext, C, of the AEAD is transmitted in place of P.</t>
        <t indent="0" pn="section-5.3-9">Some AEAD functions have limits for how many packets can be encrypted under the
same key and IV; see <xref target="aead-limits" format="default" sectionFormat="of" derivedContent="Section 6.6"/>.  This might be lower than the packet
number limit.  An endpoint <bcp14>MUST</bcp14> initiate a key update (<xref target="key-update" format="default" sectionFormat="of" derivedContent="Section 6"/>) prior to
exceeding any limit set for the AEAD that is in use.</t>
      </section>
      <section anchor="header-protect" numbered="true" toc="include" removeInRFC="false" pn="section-5.4">
        <name slugifiedName="name-header-protection">Header Protection</name>
        <t indent="0" pn="section-5.4-1">Parts of QUIC packet headers, in particular the Packet Number field, are
protected using a key that is derived separately from the packet protection key
and IV.  The key derived using the "quic hp" label is used to provide
confidentiality protection for those fields that are not exposed to on-path
elements.</t>
        <t indent="0" pn="section-5.4-2">This protection applies to the least significant bits of the first byte, plus
the Packet Number field.  The four least significant bits of the first byte are
protected for packets with long headers; the five least significant bits of the
first byte are protected for packets with short headers.  For both header forms,
this covers the reserved bits and the Packet Number Length field; the Key Phase
bit is also protected for packets with a short header.</t>
        <t indent="0" pn="section-5.4-3">The same header protection key is used for the duration of the connection, with
the value not changing after a key update (see <xref target="key-update" format="default" sectionFormat="of" derivedContent="Section 6"/>).  This allows
header protection to be used to protect the key phase.</t>
        <t indent="0" pn="section-5.4-4">This process does not apply to Retry or Version Negotiation packets, which do
not contain a protected payload or any of the fields that are protected by this
process.</t>
        <section anchor="header-protection-application" numbered="true" toc="include" removeInRFC="false" pn="section-5.4.1">
          <name slugifiedName="name-header-protection-applicati">Header Protection Application</name>
          <t indent="0" pn="section-5.4.1-1">Header protection is applied after packet protection is applied (see <xref target="aead" format="default" sectionFormat="of" derivedContent="Section 5.3"/>).
The ciphertext of the packet is sampled and used as input to an encryption
algorithm.  The algorithm used depends on the negotiated AEAD.</t>
          <t indent="0" pn="section-5.4.1-2">The output of this algorithm is a 5-byte mask that is applied to the protected
header fields using exclusive OR.  The least significant bits of the first byte
of the packet are masked by the least significant bits of the first mask byte,
and the packet number is masked with the remaining bytes.  Any unused bytes of
mask that might result from a shorter packet number encoding are unused.</t>
          <t indent="0" pn="section-5.4.1-3"><xref target="pseudo-hp" format="default" sectionFormat="of" derivedContent="Figure 6"/> shows a sample algorithm for applying header protection. Removing
header protection only differs in the order in which the packet number length
(pn_length) is determined (here "^" is used to represent exclusive OR).</t>
          <figure anchor="pseudo-hp" align="left" suppress-title="false" pn="figure-6">
            <name slugifiedName="name-header-protection-pseudocod">Header Protection Pseudocode</name>
            <sourcecode type="pseudocode" markers="false" pn="section-5.4.1-4.1">
mask = header_protection(hp_key, sample)

pn_length = (packet[0] &amp; 0x03) + 1
if (packet[0] &amp; 0x80) == 0x80:
   # Long header: 4 bits masked
   packet[0] ^= mask[0] &amp; 0x0f
else:
   # Short header: 5 bits masked
   packet[0] ^= mask[0] &amp; 0x1f

# pn_offset is the start of the Packet Number field.
packet[pn_offset:pn_offset+pn_length] ^= mask[1:1+pn_length]
</sourcecode>
          </figure>
          <t indent="0" pn="section-5.4.1-5">Specific header protection functions are defined based on the selected cipher
suite; see <xref target="hp-aes" format="default" sectionFormat="of" derivedContent="Section 5.4.3"/> and <xref target="hp-chacha" format="default" sectionFormat="of" derivedContent="Section 5.4.4"/>.</t>
          <t indent="0" pn="section-5.4.1-6"><xref target="fig-sample" format="default" sectionFormat="of" derivedContent="Figure 7"/> shows an example long header packet (Initial) and a short header
packet (1-RTT). <xref target="fig-sample" format="default" sectionFormat="of" derivedContent="Figure 7"/> shows the fields in each header that are covered
by header protection and the portion of the protected packet payload that is
sampled.</t>
          <figure anchor="fig-sample" align="left" suppress-title="false" pn="figure-7">
            <name slugifiedName="name-header-protection-and-ciphe">Header Protection and Ciphertext Sample</name>
            <artwork name="" type="" align="left" alt="" pn="section-5.4.1-7.1">
Initial Packet {
  Header Form (1) = 1,
  Fixed Bit (1) = 1,
  Long Packet Type (2) = 0,
  Reserved Bits (2),         # Protected
  Packet Number Length (2),  # Protected
  Version (32),
  DCID Len (8),
  Destination Connection ID (0..160),
  SCID Len (8),
  Source Connection ID (0..160),
  Token Length (i),
  Token (..),
  Length (i),
  Packet Number (8..32),     # Protected
  Protected Payload (0..24), # Skipped Part
  Protected Payload (128),   # Sampled Part
  Protected Payload (..)     # Remainder
}

1-RTT Packet {
  Header Form (1) = 0,
  Fixed Bit (1) = 1,
  Spin Bit (1),
  Reserved Bits (2),         # Protected
  Key Phase (1),             # Protected
  Packet Number Length (2),  # Protected
  Destination Connection ID (0..160),
  Packet Number (8..32),     # Protected
  Protected Payload (0..24), # Skipped Part
  Protected Payload (128),   # Sampled Part
  Protected Payload (..),    # Remainder
}
</artwork>
          </figure>
          <t indent="0" pn="section-5.4.1-8">Before a TLS cipher suite can be used with QUIC, a header protection algorithm
<bcp14>MUST</bcp14> be specified for the AEAD used with that cipher suite.  This document
defines algorithms for AEAD_AES_128_GCM, AEAD_AES_128_CCM, AEAD_AES_256_GCM (all
these AES AEADs are defined in <xref target="AEAD" format="default" sectionFormat="of" derivedContent="AEAD"/>), and AEAD_CHACHA20_POLY1305
(defined in <xref target="CHACHA" format="default" sectionFormat="of" derivedContent="CHACHA"/>).  Prior to TLS selecting a cipher suite, AES
header protection is used (<xref target="hp-aes" format="default" sectionFormat="of" derivedContent="Section 5.4.3"/>), matching the AEAD_AES_128_GCM packet
protection.</t>
        </section>
        <section anchor="hp-sample" numbered="true" toc="include" removeInRFC="false" pn="section-5.4.2">
          <name slugifiedName="name-header-protection-sample">Header Protection Sample</name>
          <t indent="0" pn="section-5.4.2-1">The header protection algorithm uses both the header protection key and a sample
of the ciphertext from the packet Payload field.</t>
          <t indent="0" pn="section-5.4.2-2">The same number of bytes are always sampled, but an allowance needs to be made
for the removal of protection by a receiving endpoint, which will not know the
length of the Packet Number field.  The sample of ciphertext is taken starting
from an offset of 4 bytes after the start of the Packet Number field.  That is,
in sampling packet ciphertext for header protection, the Packet Number field is
assumed to be 4 bytes long (its maximum possible encoded length).</t>
          <t indent="0" pn="section-5.4.2-3">An endpoint <bcp14>MUST</bcp14> discard packets that are not long enough to contain a complete
sample.</t>
          <t indent="0" pn="section-5.4.2-4">To ensure that sufficient data is available for sampling, packets are padded so
that the combined lengths of the encoded packet number and protected payload is
at least 4 bytes longer than the sample required for header protection.  The
cipher suites defined in <xref target="TLS13" format="default" sectionFormat="of" derivedContent="TLS13"/> -- other than TLS_AES_128_CCM_8_SHA256, for
which a header protection scheme is not defined in this document -- have 16-byte
expansions and 16-byte header protection samples.  This results in needing at
least 3 bytes of frames in the unprotected payload if the packet number is
encoded on a single byte, or 2 bytes of frames for a 2-byte packet number
encoding.</t>
          <t indent="0" pn="section-5.4.2-5">The sampled ciphertext can be determined by the following pseudocode:</t>
          <sourcecode type="pseudocode" markers="false" pn="section-5.4.2-6">
# pn_offset is the start of the Packet Number field.
sample_offset = pn_offset + 4

sample = packet[sample_offset..sample_offset+sample_length]
</sourcecode>
          <t indent="0" pn="section-5.4.2-7">Where the packet number offset of a short header packet can be calculated as:</t>
          <sourcecode type="pseudocode" markers="false" pn="section-5.4.2-8">
pn_offset = 1 + len(connection_id)
</sourcecode>
          <t indent="0" pn="section-5.4.2-9">And the packet number offset of a long header packet can be calculated as:</t>
          <sourcecode type="pseudocode" markers="false" pn="section-5.4.2-10">
pn_offset = 7 + len(destination_connection_id) +
                len(source_connection_id) +
                len(payload_length)
if packet_type == Initial:
    pn_offset += len(token_length) +
                 len(token)
</sourcecode>
          <t indent="0" pn="section-5.4.2-11">For example, for a packet with a short header, an 8-byte connection ID, and
protected with AEAD_AES_128_GCM, the sample takes bytes 13 to 28 inclusive
(using zero-based indexing).</t>
          <t indent="0" pn="section-5.4.2-12">Multiple QUIC packets might be included in the same UDP datagram. Each packet
is handled separately.</t>
        </section>
        <section anchor="hp-aes" numbered="true" toc="include" removeInRFC="false" pn="section-5.4.3">
          <name slugifiedName="name-aes-based-header-protection">AES-Based Header Protection</name>
          <t indent="0" pn="section-5.4.3-1">This section defines the packet protection algorithm for AEAD_AES_128_GCM,
AEAD_AES_128_CCM, and AEAD_AES_256_GCM. AEAD_AES_128_GCM and AEAD_AES_128_CCM
use 128-bit AES in Electronic Codebook (ECB) mode. AEAD_AES_256_GCM uses
256-bit AES in ECB mode.  AES is defined in <xref target="AES" format="default" sectionFormat="of" derivedContent="AES"/>.</t>
          <t indent="0" pn="section-5.4.3-2">This algorithm samples 16 bytes from the packet ciphertext. This value is used
as the input to AES-ECB.  In pseudocode, the header protection function is
defined as:</t>
          <sourcecode type="pseudocode" markers="false" pn="section-5.4.3-3">
header_protection(hp_key, sample):
  mask = AES-ECB(hp_key, sample)
</sourcecode>
        </section>
        <section anchor="hp-chacha" numbered="true" toc="include" removeInRFC="false" pn="section-5.4.4">
          <name slugifiedName="name-chacha20-based-header-prote">ChaCha20-Based Header Protection</name>
          <t indent="0" pn="section-5.4.4-1">When AEAD_CHACHA20_POLY1305 is in use, header protection uses the raw ChaCha20
function as defined in <xref section="2.4" sectionFormat="of" target="CHACHA" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8439#section-2.4" derivedContent="CHACHA"/>.  This uses a 256-bit key and
16 bytes sampled from the packet protection output.</t>
          <t indent="0" pn="section-5.4.4-2">The first 4 bytes of the sampled ciphertext are the block counter.  A ChaCha20
implementation could take a 32-bit integer in place of a byte sequence, in
which case, the byte sequence is interpreted as a little-endian value.</t>
          <t indent="0" pn="section-5.4.4-3">The remaining 12 bytes are used as the nonce. A ChaCha20 implementation might
take an array of three 32-bit integers in place of a byte sequence, in which
case, the nonce bytes are interpreted as a sequence of 32-bit little-endian
integers.</t>
          <t indent="0" pn="section-5.4.4-4">The encryption mask is produced by invoking ChaCha20 to protect 5 zero bytes. In
pseudocode, the header protection function is defined as:</t>
          <sourcecode type="pseudocode" markers="false" pn="section-5.4.4-5">
header_protection(hp_key, sample):
  counter = sample[0..3]
  nonce = sample[4..15]
  mask = ChaCha20(hp_key, counter, nonce, {0,0,0,0,0})
</sourcecode>
        </section>
      </section>
      <section anchor="receiving-protected-packets" numbered="true" toc="include" removeInRFC="false" pn="section-5.5">
        <name slugifiedName="name-receiving-protected-packets">Receiving Protected Packets</name>
        <t indent="0" pn="section-5.5-1">Once an endpoint successfully receives a packet with a given packet number, it
<bcp14>MUST</bcp14> discard all packets in the same packet number space with higher packet
numbers if they cannot be successfully unprotected with either the same key, or
-- if there is a key update -- a subsequent packet protection key; see
<xref target="key-update" format="default" sectionFormat="of" derivedContent="Section 6"/>.  Similarly, a packet that appears to trigger a key update but
cannot be unprotected successfully <bcp14>MUST</bcp14> be discarded.</t>
        <t indent="0" pn="section-5.5-2">Failure to unprotect a packet does not necessarily indicate the existence of a
protocol error in a peer or an attack.  The truncated packet number encoding
used in QUIC can cause packet numbers to be decoded incorrectly if they are
delayed significantly.</t>
      </section>
      <section anchor="using-early-data" numbered="true" toc="include" removeInRFC="false" pn="section-5.6">
        <name slugifiedName="name-use-of-0-rtt-keys">Use of 0-RTT Keys</name>
        <t indent="0" pn="section-5.6-1">If 0-RTT keys are available (see <xref target="enable-0rtt" format="default" sectionFormat="of" derivedContent="Section 4.6.1"/>), the lack of replay protection
means that restrictions on their use are necessary to avoid replay attacks on
the protocol.</t>
        <t indent="0" pn="section-5.6-2">Of the frames defined in <xref target="QUIC-TRANSPORT" format="default" sectionFormat="of" derivedContent="QUIC-TRANSPORT"/>, the STREAM, RESET_STREAM,
STOP_SENDING, and CONNECTION_CLOSE frames are potentially unsafe for use with
0-RTT as they carry application data. Application data that is received in
0-RTT could cause an application at the server to process the data multiple
times rather than just once. Additional actions taken by a server as a result
of processing replayed application data could have unwanted consequences. A
client therefore <bcp14>MUST NOT</bcp14> use 0-RTT for application data unless specifically
requested by the application that is in use.</t>
        <t indent="0" pn="section-5.6-3">An application protocol that uses QUIC <bcp14>MUST</bcp14> include a profile that defines
acceptable use of 0-RTT; otherwise, 0-RTT can only be used to carry QUIC frames
that do not carry application data. For example, a profile for HTTP is
described in <xref target="HTTP-REPLAY" format="default" sectionFormat="of" derivedContent="HTTP-REPLAY"/> and used for HTTP/3; see
<xref section="10.9" sectionFormat="of" target="QUIC-HTTP" format="default" derivedLink="https://datatracker.ietf.org/doc/html/draft-ietf-quic-http-34#section-10.9" derivedContent="QUIC-HTTP"/>.</t>
        <t indent="0" pn="section-5.6-4">Though replaying packets might result in additional connection attempts, the
effect of processing replayed frames that do not carry application data is
limited to changing the state of the affected connection. A TLS handshake
cannot be successfully completed using replayed packets.</t>
        <t indent="0" pn="section-5.6-5">A client <bcp14>MAY</bcp14> wish to apply additional restrictions on what data it sends prior
to the completion of the TLS handshake.</t>
        <t indent="0" pn="section-5.6-6">A client otherwise treats 0-RTT keys as equivalent to 1-RTT keys, except that
it cannot send certain frames with 0-RTT keys; see
<xref section="12.5" sectionFormat="of" target="QUIC-TRANSPORT" format="default" derivedLink="https://rfc-editor.org/rfc/rfc9000#section-12.5" derivedContent="QUIC-TRANSPORT"/>.</t>
        <t indent="0" pn="section-5.6-7">A client that receives an indication that its 0-RTT data has been accepted by a
server can send 0-RTT data until it receives all of the server's handshake
messages.  A client <bcp14>SHOULD</bcp14> stop sending 0-RTT data if it receives an indication
that 0-RTT data has been rejected.</t>
        <t indent="0" pn="section-5.6-8">A server <bcp14>MUST NOT</bcp14> use 0-RTT keys to protect packets; it uses 1-RTT keys to
protect acknowledgments of 0-RTT packets.  A client <bcp14>MUST NOT</bcp14> attempt to
decrypt 0-RTT packets it receives and instead <bcp14>MUST</bcp14> discard them.</t>
        <t indent="0" pn="section-5.6-9">Once a client has installed 1-RTT keys, it <bcp14>MUST NOT</bcp14> send any more 0-RTT
packets.</t>
        <aside pn="section-5.6-10">
          <t indent="0" pn="section-5.6-10.1">Note: 0-RTT data can be acknowledged by the server as it receives it, but any
  packets containing acknowledgments of 0-RTT data cannot have packet protection
  removed by the client until the TLS handshake is complete.  The 1-RTT keys
  necessary to remove packet protection cannot be derived until the client
  receives all server handshake messages.</t>
        </aside>
      </section>
      <section anchor="pre-hs-protected" numbered="true" toc="include" removeInRFC="false" pn="section-5.7">
        <name slugifiedName="name-receiving-out-of-order-prot">Receiving Out-of-Order Protected Packets</name>
        <t indent="0" pn="section-5.7-1">Due to reordering and loss, protected packets might be received by an endpoint
before the final TLS handshake messages are received.  A client will be unable
to decrypt 1-RTT packets from the server, whereas a server will be able to
decrypt 1-RTT packets from the client.  Endpoints in either role <bcp14>MUST NOT</bcp14>
decrypt 1-RTT packets from their peer prior to completing the handshake.</t>
        <t indent="0" pn="section-5.7-2">Even though 1-RTT keys are available to a server after receiving the first
handshake messages from a client, it is missing assurances on the client state:</t>
        <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.7-3">
          <li pn="section-5.7-3.1">The client is not authenticated, unless the server has chosen to use a
pre-shared key and validated the client's pre-shared key binder; see <xref section="4.2.11" sectionFormat="of" target="TLS13" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8446#section-4.2.11" derivedContent="TLS13"/>.</li>
          <li pn="section-5.7-3.2">The client has not demonstrated liveness, unless the server has validated the
client's address with a Retry packet or other means; see
<xref section="8.1" sectionFormat="of" target="QUIC-TRANSPORT" format="default" derivedLink="https://rfc-editor.org/rfc/rfc9000#section-8.1" derivedContent="QUIC-TRANSPORT"/>.</li>
          <li pn="section-5.7-3.3">Any received 0-RTT data that the server responds to might be due to a replay
attack.</li>
        </ul>
        <t indent="0" pn="section-5.7-4">Therefore, the server's use of 1-RTT keys before the handshake is complete is
limited to sending data.  A server <bcp14>MUST NOT</bcp14> process incoming 1-RTT protected
packets before the TLS handshake is complete.  Because sending acknowledgments
indicates that all frames in a packet have been processed, a server cannot send
acknowledgments for 1-RTT packets until the TLS handshake is complete.  Received
packets protected with 1-RTT keys <bcp14>MAY</bcp14> be stored and later decrypted and used
once the handshake is complete.</t>
        <aside pn="section-5.7-5">
          <t indent="0" pn="section-5.7-5.1">Note: TLS implementations might provide all 1-RTT secrets prior to handshake
  completion.  Even where QUIC implementations have 1-RTT read keys, those keys
  are not to be used prior to completing the handshake.</t>
        </aside>
        <t indent="0" pn="section-5.7-6">The requirement for the server to wait for the client Finished message creates
a dependency on that message being delivered.  A client can avoid the
potential for head-of-line blocking that this implies by sending its 1-RTT
packets coalesced with a Handshake packet containing a copy of the CRYPTO frame
that carries the Finished message, until one of the Handshake packets is
acknowledged.  This enables immediate server processing for those packets.</t>
        <t indent="0" pn="section-5.7-7">A server could receive packets protected with 0-RTT keys prior to receiving a
TLS ClientHello.  The server <bcp14>MAY</bcp14> retain these packets for later decryption in
anticipation of receiving a ClientHello.</t>
        <t indent="0" pn="section-5.7-8">A client generally receives 1-RTT keys at the same time as the handshake
completes.  Even if it has 1-RTT secrets, a client <bcp14>MUST NOT</bcp14> process
incoming 1-RTT protected packets before the TLS handshake is complete.</t>
      </section>
      <section anchor="retry-integrity" numbered="true" toc="include" removeInRFC="false" pn="section-5.8">
        <name slugifiedName="name-retry-packet-integrity">Retry Packet Integrity</name>
        <t indent="0" pn="section-5.8-1">Retry packets (see <xref section="17.2.5" sectionFormat="of" target="QUIC-TRANSPORT" format="default" derivedLink="https://rfc-editor.org/rfc/rfc9000#section-17.2.5" derivedContent="QUIC-TRANSPORT"/>) carry a Retry Integrity
Tag that provides two properties: it allows the discarding of packets that have
accidentally been corrupted by the network, and only an entity that observes an
Initial packet can send a valid Retry packet.</t>
        <t indent="0" pn="section-5.8-2">The Retry Integrity Tag is a 128-bit field that is computed as the output of
AEAD_AES_128_GCM <xref target="AEAD" format="default" sectionFormat="of" derivedContent="AEAD"/> used with the following inputs:</t>
        <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-5.8-3">
          <li pn="section-5.8-3.1">The secret key, K, is 128 bits equal to 0xbe0c690b9f66575a1d766b54e368c84e.</li>
          <li pn="section-5.8-3.2">The nonce, N, is 96 bits equal to 0x461599d35d632bf2239825bb.</li>
          <li pn="section-5.8-3.3">The plaintext, P, is empty.</li>
          <li pn="section-5.8-3.4">The associated data, A, is the contents of the Retry Pseudo-Packet, as
illustrated in <xref target="retry-pseudo" format="default" sectionFormat="of" derivedContent="Figure 8"/>:</li>
        </ul>
        <t indent="0" pn="section-5.8-4">The secret key and the nonce are values derived by calling HKDF-Expand-Label
using 0xd9c9943e6101fd200021506bcc02814c73030f25c79d71ce876eca876e6fca8e as the
secret, with labels being "quic key" and "quic iv" (<xref target="protection-keys" format="default" sectionFormat="of" derivedContent="Section 5.1"/>).</t>
        <figure anchor="retry-pseudo" align="left" suppress-title="false" pn="figure-8">
          <name slugifiedName="name-retry-pseudo-packet">Retry Pseudo-Packet</name>
          <artwork name="" type="" align="left" alt="" pn="section-5.8-5.1">
Retry Pseudo-Packet {
  ODCID Length (8),
  Original Destination Connection ID (0..160),
  Header Form (1) = 1,
  Fixed Bit (1) = 1,
  Long Packet Type (2) = 3,
  Unused (4),
  Version (32),
  DCID Len (8),
  Destination Connection ID (0..160),
  SCID Len (8),
  Source Connection ID (0..160),
  Retry Token (..),
}
</artwork>
        </figure>
        <t indent="0" pn="section-5.8-6">The Retry Pseudo-Packet is not sent over the wire. It is computed by taking
the transmitted Retry packet, removing the Retry Integrity Tag, and prepending
the two following fields:</t>
        <dl indent="3" newline="false" spacing="normal" pn="section-5.8-7">
          <dt pn="section-5.8-7.1">ODCID Length:</dt>
          <dd pn="section-5.8-7.2">
            <t indent="0" pn="section-5.8-7.2.1">The ODCID Length field contains the length in bytes of the Original
Destination Connection ID field that follows it, encoded as an 8-bit unsigned
integer.</t>
          </dd>
          <dt pn="section-5.8-7.3">Original Destination Connection ID:</dt>
          <dd pn="section-5.8-7.4">
            <t indent="0" pn="section-5.8-7.4.1">The Original Destination Connection ID contains the value of the Destination
Connection ID from the Initial packet that this Retry is in response to. The
length of this field is given in ODCID Length. The presence of this field
ensures that a valid Retry packet can only be sent by an entity that
observes the Initial packet.</t>
          </dd>
        </dl>
      </section>
    </section>
    <section anchor="key-update" numbered="true" toc="include" removeInRFC="false" pn="section-6">
      <name slugifiedName="name-key-update">Key Update</name>
      <t indent="0" pn="section-6-1">Once the handshake is confirmed (see <xref target="handshake-confirmed" format="default" sectionFormat="of" derivedContent="Section 4.1.2"/>), an endpoint <bcp14>MAY</bcp14>
initiate a key update.</t>
      <t indent="0" pn="section-6-2">The Key Phase bit indicates which packet protection keys are used to protect the
packet.  The Key Phase bit is initially set to 0 for the first set of 1-RTT
packets and toggled to signal each subsequent key update.</t>
      <t indent="0" pn="section-6-3">The Key Phase bit allows a recipient to detect a change in keying material
without needing to receive the first packet that triggered the change.  An
endpoint that notices a changed Key Phase bit updates keys and decrypts the
packet that contains the changed value.</t>
      <t indent="0" pn="section-6-4">Initiating a key update results in both endpoints updating keys.  This differs
from TLS where endpoints can update keys independently.</t>
      <t indent="0" pn="section-6-5">This mechanism replaces the key update mechanism of TLS, which relies on
KeyUpdate messages sent using 1-RTT encryption keys.  Endpoints <bcp14>MUST NOT</bcp14> send a
TLS KeyUpdate message.  Endpoints <bcp14>MUST</bcp14> treat the receipt of a TLS KeyUpdate
message as a connection error of type 0x010a, equivalent to a
fatal TLS alert of unexpected_message; see <xref target="tls-errors" format="default" sectionFormat="of" derivedContent="Section 4.8"/>.</t>
      <t indent="0" pn="section-6-6"><xref target="ex-key-update" format="default" sectionFormat="of" derivedContent="Figure 9"/> shows a key update process, where the initial set of keys used
(identified with @M) are replaced by updated keys (identified with @N).  The
value of the Key Phase bit is indicated in brackets [].</t>
      <figure anchor="ex-key-update" align="left" suppress-title="false" pn="figure-9">
        <name slugifiedName="name-key-update-2">Key Update</name>
        <artwork name="" type="" align="left" alt="" pn="section-6-7.1">
   Initiating Peer                    Responding Peer

@M [0] QUIC Packets

... Update to @N
@N [1] QUIC Packets
                      --------&gt;
                                         Update to @N ...
                                      QUIC Packets [1] @N
                      &lt;--------
                                      QUIC Packets [1] @N
                                    containing ACK
                      &lt;--------
... Key Update Permitted

@N [1] QUIC Packets
         containing ACK for @N packets
                      --------&gt;
                                 Key Update Permitted ...
</artwork>
      </figure>
      <section anchor="key-update-initiate" numbered="true" toc="include" removeInRFC="false" pn="section-6.1">
        <name slugifiedName="name-initiating-a-key-update">Initiating a Key Update</name>
        <t indent="0" pn="section-6.1-1">Endpoints maintain separate read and write secrets for packet protection.  An
endpoint initiates a key update by updating its packet protection write secret
and using that to protect new packets.  The endpoint creates a new write secret
from the existing write secret as performed in <xref section="7.2" sectionFormat="of" target="TLS13" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8446#section-7.2" derivedContent="TLS13"/>.  This
uses the KDF function provided by TLS with a label of "quic ku".  The
corresponding key and IV are created from that secret as defined in
<xref target="protection-keys" format="default" sectionFormat="of" derivedContent="Section 5.1"/>.  The header protection key is not updated.</t>
        <t indent="0" pn="section-6.1-2">For example, to update write keys with TLS 1.3, HKDF-Expand-Label is used as:</t>
        <sourcecode type="pseudocode" markers="false" pn="section-6.1-3">
secret_&lt;n+1&gt; = HKDF-Expand-Label(secret_&lt;n&gt;, "quic ku",
                                 "", Hash.length)
</sourcecode>
        <t indent="0" pn="section-6.1-4">The endpoint toggles the value of the Key Phase bit and uses the updated key and
IV to protect all subsequent packets.</t>
        <t indent="0" pn="section-6.1-5">An endpoint <bcp14>MUST NOT</bcp14> initiate a key update prior to having confirmed the
handshake (<xref target="handshake-confirmed" format="default" sectionFormat="of" derivedContent="Section 4.1.2"/>).  An endpoint <bcp14>MUST NOT</bcp14> initiate a subsequent
key update unless it has received an acknowledgment for a packet that was sent
protected with keys from the current key phase.  This ensures that keys are
available to both peers before another key update can be initiated.  This can be
implemented by tracking the lowest packet number sent with each key phase and
the highest acknowledged packet number in the 1-RTT space: once the latter is
higher than or equal to the former, another key update can be initiated.</t>
        <aside pn="section-6.1-6">
          <t indent="0" pn="section-6.1-6.1">Note: Keys of packets other than the 1-RTT packets are never updated; their keys
  are derived solely from the TLS handshake state.</t>
        </aside>
        <t indent="0" pn="section-6.1-7">The endpoint that initiates a key update also updates the keys that it uses for
receiving packets.  These keys will be needed to process packets the peer sends
after updating.</t>
        <t indent="0" pn="section-6.1-8">An endpoint <bcp14>MUST</bcp14> retain old keys until it has successfully unprotected a packet
sent using the new keys.  An endpoint <bcp14>SHOULD</bcp14> retain old keys for some time
after unprotecting a packet sent using the new keys.  Discarding old keys too
early can cause delayed packets to be discarded.  Discarding packets will be
interpreted as packet loss by the peer and could adversely affect performance.</t>
      </section>
      <section anchor="responding-to-a-key-update" numbered="true" toc="include" removeInRFC="false" pn="section-6.2">
        <name slugifiedName="name-responding-to-a-key-update">Responding to a Key Update</name>
        <t indent="0" pn="section-6.2-1">A peer is permitted to initiate a key update after receiving an acknowledgment
of a packet in the current key phase.  An endpoint detects a key update when
processing a packet with a key phase that differs from the value used to protect
the last packet it sent.  To process this packet, the endpoint uses the next
packet protection key and IV.  See <xref target="receive-key-generation" format="default" sectionFormat="of" derivedContent="Section 6.3"/> for considerations
about generating these keys.</t>
        <t indent="0" pn="section-6.2-2">If a packet is successfully processed using the next key and IV, then the peer
has initiated a key update.  The endpoint <bcp14>MUST</bcp14> update its send keys to the
corresponding key phase in response, as described in <xref target="key-update-initiate" format="default" sectionFormat="of" derivedContent="Section 6.1"/>.
Sending keys <bcp14>MUST</bcp14> be updated before sending an acknowledgment for the packet
that was received with updated keys.  By acknowledging the packet that triggered
the key update in a packet protected with the updated keys, the endpoint signals
that the key update is complete.</t>
        <t indent="0" pn="section-6.2-3">An endpoint can defer sending the packet or acknowledgment according to its
normal packet sending behavior; it is not necessary to immediately generate a
packet in response to a key update.  The next packet sent by the endpoint will
use the updated keys.  The next packet that contains an acknowledgment will
cause the key update to be completed.  If an endpoint detects a second update
before it has sent any packets with updated keys containing an
acknowledgment for the packet that initiated the key update, it indicates that
its peer has updated keys twice without awaiting confirmation.  An endpoint <bcp14>MAY</bcp14>
treat such consecutive key updates as a connection error of type
KEY_UPDATE_ERROR.</t>
        <t indent="0" pn="section-6.2-4">An endpoint that receives an acknowledgment that is carried in a packet
protected with old keys where any acknowledged packet was protected with newer
keys <bcp14>MAY</bcp14> treat that as a connection error of type KEY_UPDATE_ERROR.  This
indicates that a peer has received and acknowledged a packet that initiates a
key update, but has not updated keys in response.</t>
      </section>
      <section anchor="receive-key-generation" numbered="true" toc="include" removeInRFC="false" pn="section-6.3">
        <name slugifiedName="name-timing-of-receive-key-gener">Timing of Receive Key Generation</name>
        <t indent="0" pn="section-6.3-1">Endpoints responding to an apparent key update <bcp14>MUST NOT</bcp14> generate a timing
side-channel signal that might indicate that the Key Phase bit was invalid (see
<xref target="hp-side-channel" format="default" sectionFormat="of" derivedContent="Section 9.5"/>).  Endpoints can use randomized packet protection keys in
place of discarded keys when key updates are not yet permitted.  Using
randomized keys ensures that attempting to remove packet protection does not
result in timing variations, and results in packets with an invalid Key Phase
bit being rejected.</t>
        <t indent="0" pn="section-6.3-2">The process of creating new packet protection keys for receiving packets could
reveal that a key update has occurred. An endpoint <bcp14>MAY</bcp14> generate new keys as
part of packet processing, but this creates a timing signal that could be used
by an attacker to learn when key updates happen and thus leak the value of the
Key Phase bit.</t>
        <t indent="0" pn="section-6.3-3">Endpoints are generally expected to have current and next receive packet
protection keys available. For a short period after a key update completes, up
to the PTO, endpoints <bcp14>MAY</bcp14> defer generation of the next set of
receive packet protection keys. This allows endpoints
to retain only two sets of receive keys; see <xref target="old-keys-recv" format="default" sectionFormat="of" derivedContent="Section 6.5"/>.</t>
        <t indent="0" pn="section-6.3-4">Once generated, the next set of packet protection keys <bcp14>SHOULD</bcp14> be retained, even
if the packet that was received was subsequently discarded.  Packets containing
apparent key updates are easy to forge, and while the process of key update does
not require significant effort, triggering this process could be used by an
attacker for DoS.</t>
        <t indent="0" pn="section-6.3-5">For this reason, endpoints <bcp14>MUST</bcp14> be able to retain two sets of packet protection
keys for receiving packets: the current and the next.  Retaining the previous
keys in addition to these might improve performance, but this is not essential.</t>
      </section>
      <section anchor="old-keys-send" numbered="true" toc="include" removeInRFC="false" pn="section-6.4">
        <name slugifiedName="name-sending-with-updated-keys">Sending with Updated Keys</name>
        <t indent="0" pn="section-6.4-1">An endpoint never sends packets that are protected with old keys.  Only the
current keys are used.  Keys used for protecting packets can be discarded
immediately after switching to newer keys.</t>
        <t indent="0" pn="section-6.4-2">Packets with higher packet numbers <bcp14>MUST</bcp14> be protected with either the same or
newer packet protection keys than packets with lower packet numbers.  An
endpoint that successfully removes protection with old keys when newer keys were
used for packets with lower packet numbers <bcp14>MUST</bcp14> treat this as a connection error
of type KEY_UPDATE_ERROR.</t>
      </section>
      <section anchor="old-keys-recv" numbered="true" toc="include" removeInRFC="false" pn="section-6.5">
        <name slugifiedName="name-receiving-with-different-ke">Receiving with Different Keys</name>
        <t indent="0" pn="section-6.5-1">For receiving packets during a key update, packets protected with older keys
might arrive if they were delayed by the network.  Retaining old packet
protection keys allows these packets to be successfully processed.</t>
        <t indent="0" pn="section-6.5-2">As packets protected with keys from the next key phase use the same Key Phase
value as those protected with keys from the previous key phase, it is necessary
to distinguish between the two if packets protected with old keys are to be
processed.  This can be done using packet numbers.  A recovered packet number
that is lower than any packet number from the current key phase uses the
previous packet protection keys; a recovered packet number that is higher than
any packet number from the current key phase requires the use of the next packet
protection keys.</t>
        <t indent="0" pn="section-6.5-3">Some care is necessary to ensure that any process for selecting between
previous, current, and next packet protection keys does not expose a timing side
channel that might reveal which keys were used to remove packet protection.  See
<xref target="hp-side-channel" format="default" sectionFormat="of" derivedContent="Section 9.5"/> for more information.</t>
        <t indent="0" pn="section-6.5-4">Alternatively, endpoints can retain only two sets of packet protection keys,
swapping previous for next after enough time has passed to allow for reordering
in the network.  In this case, the Key Phase bit alone can be used to select
keys.</t>
        <t indent="0" pn="section-6.5-5">An endpoint <bcp14>MAY</bcp14> allow a period of approximately the Probe Timeout (PTO; see
<xref target="QUIC-RECOVERY" format="default" sectionFormat="of" derivedContent="QUIC-RECOVERY"/>) after promoting the next set of receive keys to be current
before it creates the subsequent set of packet protection keys. These updated
keys <bcp14>MAY</bcp14> replace the previous keys at that time. With the caveat that PTO is a
subjective measure -- that is, a peer could have a different view of the RTT --
this time is expected to be long enough that any reordered packets would be
declared lost by a peer even if they were acknowledged and short enough to allow
a peer to initiate further key updates.</t>
        <t indent="0" pn="section-6.5-6">Endpoints need to allow for the possibility that a peer might not be able to
decrypt packets that initiate a key update during the period when the peer
retains old keys.  Endpoints <bcp14>SHOULD</bcp14> wait three times the PTO before initiating a
key update after receiving an acknowledgment that confirms that the previous key
update was received.  Failing to allow sufficient time could lead to packets
being discarded.</t>
        <t indent="0" pn="section-6.5-7">An endpoint <bcp14>SHOULD</bcp14> retain old read keys for no more than three times the PTO
after having received a packet protected using the new keys. After this period,
old read keys and their corresponding secrets <bcp14>SHOULD</bcp14> be discarded.</t>
      </section>
      <section anchor="aead-limits" numbered="true" toc="include" removeInRFC="false" pn="section-6.6">
        <name slugifiedName="name-limits-on-aead-usage">Limits on AEAD Usage</name>
        <t indent="0" pn="section-6.6-1">This document sets usage limits for AEAD algorithms to ensure that overuse does
not give an adversary a disproportionate advantage in attacking the
confidentiality and integrity of communications when using QUIC.</t>
        <t indent="0" pn="section-6.6-2">The usage limits defined in TLS 1.3 exist for protection against attacks
on confidentiality and apply to successful applications of AEAD protection. The
integrity protections in authenticated encryption also depend on limiting the
number of attempts to forge packets. TLS achieves this by closing connections
after any record fails an authentication check. In comparison, QUIC ignores any
packet that cannot be authenticated, allowing multiple forgery attempts.</t>
        <t indent="0" pn="section-6.6-3">QUIC accounts for AEAD confidentiality and integrity limits separately. The
confidentiality limit applies to the number of packets encrypted with a given
key. The integrity limit applies to the number of packets decrypted within a
given connection. Details on enforcing these limits for each AEAD algorithm
follow below.</t>
        <t indent="0" pn="section-6.6-4">Endpoints <bcp14>MUST</bcp14> count the number of encrypted packets for each set of keys. If
the total number of encrypted packets with the same key exceeds the
confidentiality limit for the selected AEAD, the endpoint <bcp14>MUST</bcp14> stop using those
keys. Endpoints <bcp14>MUST</bcp14> initiate a key update before sending more protected packets
than the confidentiality limit for the selected AEAD permits. If a key update
is not possible or integrity limits are reached, the endpoint <bcp14>MUST</bcp14> stop using
the connection and only send stateless resets in response to receiving packets.
It is <bcp14>RECOMMENDED</bcp14> that endpoints immediately close the connection with a
connection error of type AEAD_LIMIT_REACHED before reaching a state where key
updates are not possible.</t>
        <t indent="0" pn="section-6.6-5">For AEAD_AES_128_GCM and AEAD_AES_256_GCM, the confidentiality limit is
2<sup>23</sup> encrypted packets; see <xref target="gcm-bounds" format="default" sectionFormat="of" derivedContent="Appendix B.1"/>. For
AEAD_CHACHA20_POLY1305, the confidentiality limit is greater than the number of
possible packets (2<sup>62</sup>) and so can be disregarded. For
AEAD_AES_128_CCM, the confidentiality limit is 2<sup>21.5</sup> encrypted
packets; see <xref target="ccm-bounds" format="default" sectionFormat="of" derivedContent="Appendix B.2"/>. Applying a limit reduces the probability that an
attacker can distinguish the AEAD in use from a random permutation; see
<xref target="AEBounds" format="default" sectionFormat="of" derivedContent="AEBounds"/>, <xref target="ROBUST" format="default" sectionFormat="of" derivedContent="ROBUST"/>, and <xref target="GCM-MU" format="default" sectionFormat="of" derivedContent="GCM-MU"/>.</t>
        <t indent="0" pn="section-6.6-6">In addition to counting packets sent, endpoints <bcp14>MUST</bcp14> count the number of
received packets that fail authentication during the lifetime of a connection.
If the total number of received packets that fail authentication within the
connection, across all keys, exceeds the integrity limit for the selected AEAD,
the endpoint <bcp14>MUST</bcp14> immediately close the connection with a connection error of
type AEAD_LIMIT_REACHED and not process any more packets.</t>
        <t indent="0" pn="section-6.6-7">For AEAD_AES_128_GCM and AEAD_AES_256_GCM, the integrity limit is 2<sup>52</sup>
invalid packets; see <xref target="gcm-bounds" format="default" sectionFormat="of" derivedContent="Appendix B.1"/>. For AEAD_CHACHA20_POLY1305, the integrity
limit is 2<sup>36</sup> invalid packets; see <xref target="AEBounds" format="default" sectionFormat="of" derivedContent="AEBounds"/>. For AEAD_AES_128_CCM,
the integrity limit is 2<sup>21.5</sup> invalid packets; see
<xref target="ccm-bounds" format="default" sectionFormat="of" derivedContent="Appendix B.2"/>. Applying this limit reduces the probability that an attacker can
successfully forge a packet; see <xref target="AEBounds" format="default" sectionFormat="of" derivedContent="AEBounds"/>, <xref target="ROBUST" format="default" sectionFormat="of" derivedContent="ROBUST"/>, and <xref target="GCM-MU" format="default" sectionFormat="of" derivedContent="GCM-MU"/>.</t>
        <t indent="0" pn="section-6.6-8">Endpoints that limit the size of packets <bcp14>MAY</bcp14> use higher confidentiality and
integrity limits; see <xref target="aead-analysis" format="default" sectionFormat="of" derivedContent="Appendix B"/> for details.</t>
        <t indent="0" pn="section-6.6-9">Future analyses and specifications <bcp14>MAY</bcp14> relax confidentiality or integrity limits
for an AEAD.</t>
        <t indent="0" pn="section-6.6-10">Any TLS cipher suite that is specified for use with QUIC <bcp14>MUST</bcp14> define limits on
the use of the associated AEAD function that preserves margins for
confidentiality and integrity. That is, limits <bcp14>MUST</bcp14> be specified for the number
of packets that can be authenticated and for the number of packets that can fail
authentication.  Providing a reference to any analysis upon which values are
based -- and any assumptions used in that analysis -- allows limits to be
adapted to varying usage conditions.</t>
      </section>
      <section anchor="key-update-error" numbered="true" toc="include" removeInRFC="false" pn="section-6.7">
        <name slugifiedName="name-key-update-error-code">Key Update Error Code</name>
        <t indent="0" pn="section-6.7-1">The KEY_UPDATE_ERROR error code (0x0e) is used to signal errors related to key
updates.</t>
      </section>
    </section>
    <section anchor="security-of-initial-messages" numbered="true" toc="include" removeInRFC="false" pn="section-7">
      <name slugifiedName="name-security-of-initial-message">Security of Initial Messages</name>
      <t indent="0" pn="section-7-1">Initial packets are not protected with a secret key, so they are subject to
potential tampering by an attacker.  QUIC provides protection against attackers
that cannot read packets but does not attempt to provide additional protection
against attacks where the attacker can observe and inject packets.  Some forms
of tampering -- such as modifying the TLS messages themselves -- are detectable,
but some -- such as modifying ACKs -- are not.</t>
      <t indent="0" pn="section-7-2">For example, an attacker could inject a packet containing an ACK frame to
make it appear that a packet had not been received or to create a false
impression of the state of the connection (e.g., by modifying the ACK Delay).
Note that such a packet could cause a legitimate packet to be dropped as a
duplicate.  Implementations <bcp14>SHOULD</bcp14> use caution in relying on any data that is
contained in Initial packets that is not otherwise authenticated.</t>
      <t indent="0" pn="section-7-3">It is also possible for the attacker to tamper with data that is carried in
Handshake packets, but because that sort of tampering requires modifying TLS
handshake messages, any such tampering will cause the TLS handshake to fail.</t>
    </section>
    <section anchor="quic-specific-adjustments-to-the-tls-handshake" numbered="true" toc="include" removeInRFC="false" pn="section-8">
      <name slugifiedName="name-quic-specific-adjustments-t">QUIC-Specific Adjustments to the TLS Handshake</name>
      <t indent="0" pn="section-8-1">Certain aspects of the TLS handshake are different when used with QUIC.</t>
      <t indent="0" pn="section-8-2">QUIC also requires additional features from TLS.  In addition to negotiation of
cryptographic parameters, the TLS handshake carries and authenticates values for
QUIC transport parameters.</t>
      <section anchor="protocol-negotiation" numbered="true" toc="include" removeInRFC="false" pn="section-8.1">
        <name slugifiedName="name-protocol-negotiation">Protocol Negotiation</name>
        <t indent="0" pn="section-8.1-1">QUIC requires that the cryptographic handshake provide authenticated protocol
negotiation.  TLS uses Application-Layer Protocol Negotiation
<xref target="ALPN" format="default" sectionFormat="of" derivedContent="ALPN"/> to select an application protocol.  Unless another mechanism
is used for agreeing on an application protocol, endpoints <bcp14>MUST</bcp14> use ALPN for
this purpose.</t>
        <t indent="0" pn="section-8.1-2">When using ALPN, endpoints <bcp14>MUST</bcp14> immediately close a connection (see <xref section="10.2" sectionFormat="of" target="QUIC-TRANSPORT" format="default" derivedLink="https://rfc-editor.org/rfc/rfc9000#section-10.2" derivedContent="QUIC-TRANSPORT"/>) with a no_application_protocol TLS alert (QUIC error
code 0x0178; see <xref target="tls-errors" format="default" sectionFormat="of" derivedContent="Section 4.8"/>) if an application protocol is not negotiated.
While <xref target="ALPN" format="default" sectionFormat="of" derivedContent="ALPN"/> only specifies that servers use this alert, QUIC clients <bcp14>MUST</bcp14>
use error 0x0178 to terminate a connection when ALPN negotiation fails.</t>
        <t indent="0" pn="section-8.1-3">An application protocol <bcp14>MAY</bcp14> restrict the QUIC versions that it can operate over.
Servers <bcp14>MUST</bcp14> select an application protocol compatible with the QUIC version
that the client has selected.  The server <bcp14>MUST</bcp14> treat the inability to select a
compatible application protocol as a connection error of type 0x0178
(no_application_protocol).  Similarly, a client <bcp14>MUST</bcp14> treat the selection of an
incompatible application protocol by a server as a connection error of type
0x0178.</t>
      </section>
      <section anchor="quic_parameters" numbered="true" toc="include" removeInRFC="false" pn="section-8.2">
        <name slugifiedName="name-quic-transport-parameters-e">QUIC Transport Parameters Extension</name>
        <t indent="0" pn="section-8.2-1">QUIC transport parameters are carried in a TLS extension. Different versions of
QUIC might define a different method for negotiating transport configuration.</t>
        <t indent="0" pn="section-8.2-2">Including transport parameters in the TLS handshake provides integrity
protection for these values.</t>
        <sourcecode type="tls-presentation" markers="false" pn="section-8.2-3">
   enum {
      quic_transport_parameters(0x39), (65535)
   } ExtensionType;
</sourcecode>
        <t indent="0" pn="section-8.2-4">The extension_data field of the quic_transport_parameters extension contains a
value that is defined by the version of QUIC that is in use.</t>
        <t indent="0" pn="section-8.2-5">The quic_transport_parameters extension is carried in the ClientHello and the
EncryptedExtensions messages during the handshake. Endpoints <bcp14>MUST</bcp14> send the
quic_transport_parameters extension; endpoints that receive ClientHello or
EncryptedExtensions messages without the quic_transport_parameters extension
<bcp14>MUST</bcp14> close the connection with an error of type 0x016d (equivalent to a fatal
TLS missing_extension alert, see <xref target="tls-errors" format="default" sectionFormat="of" derivedContent="Section 4.8"/>).</t>
        <t indent="0" pn="section-8.2-6">Transport parameters become available prior to the completion of the handshake.
A server might use these values earlier than handshake completion. However, the
value of transport parameters is not authenticated until the handshake
completes, so any use of these parameters cannot depend on their authenticity.
Any tampering with transport parameters will cause the handshake to fail.</t>
        <t indent="0" pn="section-8.2-7">Endpoints <bcp14>MUST NOT</bcp14> send this extension in a TLS connection that does not use
QUIC (such as the use of TLS with TCP defined in <xref target="TLS13" format="default" sectionFormat="of" derivedContent="TLS13"/>).  A fatal
unsupported_extension alert <bcp14>MUST</bcp14> be sent by an implementation that supports this
extension if the extension is received when the transport is not QUIC.</t>
        <t indent="0" pn="section-8.2-8">Negotiating the quic_transport_parameters extension causes the EndOfEarlyData to
be removed; see <xref target="remove-eoed" format="default" sectionFormat="of" derivedContent="Section 8.3"/>.</t>
      </section>
      <section anchor="remove-eoed" numbered="true" toc="include" removeInRFC="false" pn="section-8.3">
        <name slugifiedName="name-removing-the-endofearlydata">Removing the EndOfEarlyData Message</name>
        <t indent="0" pn="section-8.3-1">The TLS EndOfEarlyData message is not used with QUIC.  QUIC does not rely on
this message to mark the end of 0-RTT data or to signal the change to Handshake
keys.</t>
        <t indent="0" pn="section-8.3-2">Clients <bcp14>MUST NOT</bcp14> send the EndOfEarlyData message.  A server <bcp14>MUST</bcp14> treat receipt
of a CRYPTO frame in a 0-RTT packet as a connection error of type
PROTOCOL_VIOLATION.</t>
        <t indent="0" pn="section-8.3-3">As a result, EndOfEarlyData does not appear in the TLS handshake transcript.</t>
      </section>
      <section anchor="compat-mode" numbered="true" toc="include" removeInRFC="false" pn="section-8.4">
        <name slugifiedName="name-prohibit-tls-middlebox-comp">Prohibit TLS Middlebox Compatibility Mode</name>
        <t indent="0" pn="section-8.4-1">Appendix D.4 of <xref target="TLS13" format="default" sectionFormat="of" derivedContent="TLS13"/> describes an alteration to the TLS 1.3 handshake as
a workaround for bugs in some middleboxes. The TLS 1.3 middlebox compatibility
mode involves setting the legacy_session_id field to a 32-byte value in the
ClientHello and ServerHello, then sending a change_cipher_spec record. Both
field and record carry no semantic content and are ignored.</t>
        <t indent="0" pn="section-8.4-2">This mode has no use in QUIC as it only applies to middleboxes that interfere
with TLS over TCP. QUIC also provides no means to carry a change_cipher_spec
record. A client <bcp14>MUST NOT</bcp14> request the use of the TLS 1.3 compatibility mode. A
server <bcp14>SHOULD</bcp14> treat the receipt of a TLS ClientHello with a non-empty
legacy_session_id field as a connection error of type PROTOCOL_VIOLATION.</t>
      </section>
    </section>
    <section anchor="security-considerations" numbered="true" toc="include" removeInRFC="false" pn="section-9">
      <name slugifiedName="name-security-considerations">Security Considerations</name>
      <t indent="0" pn="section-9-1">All of the security considerations that apply to TLS also apply to the use of
TLS in QUIC. Reading all of <xref target="TLS13" format="default" sectionFormat="of" derivedContent="TLS13"/> and its appendices is the best way to
gain an understanding of the security properties of QUIC.</t>
      <t indent="0" pn="section-9-2">This section summarizes some of the more important security aspects specific to
the TLS integration, though there are many security-relevant details in the
remainder of the document.</t>
      <section anchor="session-linkability" numbered="true" toc="include" removeInRFC="false" pn="section-9.1">
        <name slugifiedName="name-session-linkability">Session Linkability</name>
        <t indent="0" pn="section-9.1-1">Use of TLS session tickets allows servers and possibly other entities to
correlate connections made by the same client; see <xref target="resumption" format="default" sectionFormat="of" derivedContent="Section 4.5"/> for details.</t>
      </section>
      <section anchor="replay" numbered="true" toc="include" removeInRFC="false" pn="section-9.2">
        <name slugifiedName="name-replay-attacks-with-0-rtt">Replay Attacks with 0-RTT</name>
        <t indent="0" pn="section-9.2-1">As described in <xref section="8" sectionFormat="of" target="TLS13" format="default" derivedLink="https://rfc-editor.org/rfc/rfc8446#section-8" derivedContent="TLS13"/>, use of TLS early data comes with an
exposure to replay attack.  The use of 0-RTT in QUIC is similarly vulnerable to
replay attack.</t>
        <t indent="0" pn="section-9.2-2">Endpoints <bcp14>MUST</bcp14> implement and use the replay protections described in <xref target="TLS13" format="default" sectionFormat="of" derivedContent="TLS13"/>,
however it is recognized that these protections are imperfect.  Therefore,
additional consideration of the risk of replay is needed.</t>
        <t indent="0" pn="section-9.2-3">QUIC is not vulnerable to replay attack, except via the application protocol
information it might carry.  The management of QUIC protocol state based on the
frame types defined in <xref target="QUIC-TRANSPORT" format="default" sectionFormat="of" derivedContent="QUIC-TRANSPORT"/> is not vulnerable to replay.
Processing of QUIC frames is idempotent and cannot result in invalid connection
states if frames are replayed, reordered, or lost.  QUIC connections do not
produce effects that last beyond the lifetime of the connection, except for
those produced by the application protocol that QUIC serves.</t>
        <t indent="0" pn="section-9.2-4">TLS session tickets and address validation tokens are used to carry QUIC
configuration information between connections, specifically, to enable a server
to efficiently recover state that is used in connection establishment and
address validation.  These <bcp14>MUST NOT</bcp14> be used to communicate application semantics
between endpoints; clients <bcp14>MUST</bcp14> treat them as opaque values.  The potential for
reuse of these tokens means that they require stronger protections against
replay.</t>
        <t indent="0" pn="section-9.2-5">A server that accepts 0-RTT on a connection incurs a higher cost than accepting
a connection without 0-RTT.  This includes higher processing and computation
costs.  Servers need to consider the probability of replay and all associated
costs when accepting 0-RTT.</t>
        <t indent="0" pn="section-9.2-6">Ultimately, the responsibility for managing the risks of replay attacks with
0-RTT lies with an application protocol.  An application protocol that uses QUIC
<bcp14>MUST</bcp14> describe how the protocol uses 0-RTT and the measures that are employed to
protect against replay attack.  An analysis of replay risk needs to consider
all QUIC protocol features that carry application semantics.</t>
        <t indent="0" pn="section-9.2-7">Disabling 0-RTT entirely is the most effective defense against replay attack.</t>
        <t indent="0" pn="section-9.2-8">QUIC extensions <bcp14>MUST</bcp14> either describe how replay attacks affect their operation
or prohibit the use of the extension in 0-RTT.  Application protocols <bcp14>MUST</bcp14>
either prohibit the use of extensions that carry application semantics in 0-RTT
or provide replay mitigation strategies.</t>
      </section>
      <section anchor="reflection" numbered="true" toc="include" removeInRFC="false" pn="section-9.3">
        <name slugifiedName="name-packet-reflection-attack-mi">Packet Reflection Attack Mitigation</name>
        <t indent="0" pn="section-9.3-1">A small ClientHello that results in a large block of handshake messages from a
server can be used in packet reflection attacks to amplify the traffic generated
by an attacker.</t>
        <t indent="0" pn="section-9.3-2">QUIC includes three defenses against this attack. First, the packet containing
a ClientHello <bcp14>MUST</bcp14> be padded to a minimum size. Second, if responding to an
unverified source address, the server is forbidden to send more than three
times as many bytes as the number of bytes it has received (see <xref section="8.1" sectionFormat="of" target="QUIC-TRANSPORT" format="default" derivedLink="https://rfc-editor.org/rfc/rfc9000#section-8.1" derivedContent="QUIC-TRANSPORT"/>). Finally, because acknowledgments of Handshake packets are
authenticated, a blind attacker cannot forge them. Put together, these defenses
limit the level of amplification.</t>
      </section>
      <section anchor="header-protect-analysis" numbered="true" toc="include" removeInRFC="false" pn="section-9.4">
        <name slugifiedName="name-header-protection-analysis">Header Protection Analysis</name>
        <t indent="0" pn="section-9.4-1"><xref target="NAN" format="default" sectionFormat="of" derivedContent="NAN"/> analyzes authenticated encryption
algorithms that provide nonce privacy, referred to as "Hide Nonce" (HN)
transforms. The general header protection construction in this document is
one of those algorithms (HN1). Header protection is applied after the packet
protection AEAD, sampling a set of bytes (<tt>sample</tt>) from the AEAD output and
encrypting the header field using a pseudorandom function (PRF) as follows:</t>
        <sourcecode type="pseudocode" markers="false" pn="section-9.4-2">
protected_field = field XOR PRF(hp_key, sample)
</sourcecode>
        <t indent="0" pn="section-9.4-3">The header protection variants in this document use a pseudorandom permutation
(PRP) in place of a generic PRF. However, since all PRPs are also PRFs <xref target="IMC" format="default" sectionFormat="of" derivedContent="IMC"/>,
these variants do not deviate from the HN1 construction.</t>
        <t indent="0" pn="section-9.4-4">As <tt>hp_key</tt> is distinct from the packet protection key, it follows that header
protection achieves AE2 security as defined in <xref target="NAN" format="default" sectionFormat="of" derivedContent="NAN"/> and therefore guarantees
privacy of <tt>field</tt>, the protected packet header. Future header protection
variants based on this construction <bcp14>MUST</bcp14> use a PRF to ensure equivalent
security guarantees.</t>
        <t indent="0" pn="section-9.4-5">Use of the same key and ciphertext sample more than once risks compromising
header protection. Protecting two different headers with the same key and
ciphertext sample reveals the exclusive OR of the protected fields.  Assuming
that the AEAD acts as a PRF, if L bits are sampled, the odds of two ciphertext
samples being identical approach 2<sup>-L/2</sup>, that is, the birthday bound.
For the algorithms described in this document, that probability is one in
2<sup>64</sup>.</t>
        <t indent="0" pn="section-9.4-6">To prevent an attacker from modifying packet headers, the header is transitively
authenticated using packet protection; the entire packet header is part of the
authenticated additional data.  Protected fields that are falsified or modified
can only be detected once the packet protection is removed.</t>
      </section>
      <section anchor="hp-side-channel" numbered="true" toc="include" removeInRFC="false" pn="section-9.5">
        <name slugifiedName="name-header-protection-timing-si">Header Protection Timing Side Channels</name>
        <t indent="0" pn="section-9.5-1">An attacker could guess values for packet numbers or Key Phase and have an
endpoint confirm guesses through timing side channels.  Similarly, guesses for
the packet number length can be tried and exposed.  If the recipient of a packet
discards packets with duplicate packet numbers without attempting to remove
packet protection, they could reveal through timing side channels that the
packet number matches a received packet.  For authentication to be free from
side channels, the entire process of header protection removal, packet number
recovery, and packet protection removal <bcp14>MUST</bcp14> be applied together without timing
and other side channels.</t>
        <t indent="0" pn="section-9.5-2">For the sending of packets, construction and protection of packet payloads and
packet numbers <bcp14>MUST</bcp14> be free from side channels that would reveal the packet
number or its encoded size.</t>
        <t indent="0" pn="section-9.5-3">During a key update, the time taken to generate new keys could reveal through
timing side channels that a key update has occurred.  Alternatively, where an
attacker injects packets, this side channel could reveal the value of the Key
Phase on injected packets.  After receiving a key update, an endpoint <bcp14>SHOULD</bcp14>
generate and save the next set of receive packet protection keys, as described
in <xref target="receive-key-generation" format="default" sectionFormat="of" derivedContent="Section 6.3"/>.  By generating new keys before a key update is
received, receipt of packets will not create timing signals that leak the value
of the Key Phase.</t>
        <t indent="0" pn="section-9.5-4">This depends on not doing this key generation during packet processing, and it
can require that endpoints maintain three sets of packet protection keys for
receiving: for the previous key phase, for the current key phase, and for the
next key phase.  Endpoints can instead choose to defer generation of the next
receive packet protection keys until they discard old keys so that only two sets
of receive keys need to be retained at any point in time.</t>
      </section>
      <section anchor="key-diversity" numbered="true" toc="include" removeInRFC="false" pn="section-9.6">
        <name slugifiedName="name-key-diversity">Key Diversity</name>
        <t indent="0" pn="section-9.6-1">In using TLS, the central key schedule of TLS is used.  As a result of the TLS
handshake messages being integrated into the calculation of secrets, the
inclusion of the QUIC transport parameters extension ensures that the handshake
and 1-RTT keys are not the same as those that might be produced by a server
running TLS over TCP.  To avoid the possibility of cross-protocol key
synchronization, additional measures are provided to improve key separation.</t>
        <t indent="0" pn="section-9.6-2">The QUIC packet protection keys and IVs are derived using a different label than
the equivalent keys in TLS.</t>
        <t indent="0" pn="section-9.6-3">To preserve this separation, a new version of QUIC <bcp14>SHOULD</bcp14> define new labels for
key derivation for packet protection key and IV, plus the header protection
keys.  This version of QUIC uses the string "quic".  Other versions can use a
version-specific label in place of that string.</t>
        <t indent="0" pn="section-9.6-4">The initial secrets use a key that is specific to the negotiated QUIC version.
New QUIC versions <bcp14>SHOULD</bcp14> define a new salt value used in calculating initial
secrets.</t>
      </section>
      <section anchor="randomness" numbered="true" toc="include" removeInRFC="false" pn="section-9.7">
        <name slugifiedName="name-randomness">Randomness</name>
        <t indent="0" pn="section-9.7-1">QUIC depends on endpoints being able to generate secure random numbers, both
directly for protocol values such as the connection ID, and transitively via
TLS. See <xref target="RFC4086" format="default" sectionFormat="of" derivedContent="RFC4086"/> for guidance on secure random number generation.</t>
      </section>
    </section>
    <section anchor="iana-considerations" numbered="true" toc="include" removeInRFC="false" pn="section-10">
      <name slugifiedName="name-iana-considerations">IANA Considerations</name>
      <t indent="0" pn="section-10-1">IANA has registered a codepoint of 57 (or 0x39) for the
quic_transport_parameters extension (defined in <xref target="quic_parameters" format="default" sectionFormat="of" derivedContent="Section 8.2"/>) in the "TLS
ExtensionType Values" registry <xref target="TLS-REGISTRIES" format="default" sectionFormat="of" derivedContent="TLS-REGISTRIES"/>.</t>
      <t indent="0" pn="section-10-2">The Recommended column for this extension is marked Yes. The TLS 1.3 Column
includes CH (ClientHello) and EE (EncryptedExtensions).</t>
      <table anchor="iana-tls-ext" align="center" pn="table-2">
        <name slugifiedName="name-tls-extensiontype-values-re">TLS ExtensionType Values Registry Entry</name>
        <thead>
          <tr>
            <th align="right" colspan="1" rowspan="1">Value</th>
            <th align="left" colspan="1" rowspan="1">Extension Name</th>
            <th align="left" colspan="1" rowspan="1">TLS 1.3</th>
            <th align="left" colspan="1" rowspan="1">Recommended</th>
            <th align="left" colspan="1" rowspan="1">Reference</th>
          </tr>
        </thead>
        <tbody>
          <tr>
            <td align="right" colspan="1" rowspan="1">57</td>
            <td align="left" colspan="1" rowspan="1">quic_transport_parameters</td>
            <td align="left" colspan="1" rowspan="1">CH, EE</td>
            <td align="left" colspan="1" rowspan="1">Y</td>
            <td align="left" colspan="1" rowspan="1">This document</td>
          </tr>
        </tbody>
      </table>
    </section>
  </middle>
  <back>
    <references pn="section-11">
      <name slugifiedName="name-references">References</name>
      <references pn="section-11.1">
        <name slugifiedName="name-normative-references">Normative References</name>
        <reference anchor="AEAD" target="https://www.rfc-editor.org/info/rfc5116" quoteTitle="true" derivedAnchor="AEAD">
          <front>
            <title>An Interface and Algorithms for Authenticated Encryption</title>
            <author fullname="D. McGrew" initials="D." surname="McGrew">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="January" year="2008"/>
            <abstract>
              <t indent="0">This document defines algorithms for Authenticated Encryption with Associated Data (AEAD), and defines a uniform interface and a registry for such algorithms.  The interface and registry can be used as an application-independent set of cryptoalgorithm suites.  This approach provides advantages in efficiency and security, and promotes the reuse of crypto implementations.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="5116"/>
          <seriesInfo name="DOI" value="10.17487/RFC5116"/>
        </reference>
        <reference anchor="AES" quoteTitle="true" target="https://doi.org/10.6028/nist.fips.197" derivedAnchor="AES">
          <front>
            <title>Advanced encryption standard (AES)</title>
            <author>
              <organization showOnFrontPage="true"/>
            </author>
            <date month="November" year="2001"/>
          </front>
          <seriesInfo name="National Institute of Standards and Technology" value="report"/>
          <seriesInfo name="DOI" value="10.6028/nist.fips.197"/>
        </reference>
        <reference anchor="ALPN" target="https://www.rfc-editor.org/info/rfc7301" quoteTitle="true" derivedAnchor="ALPN">
          <front>
            <title>Transport Layer Security (TLS) Application-Layer Protocol Negotiation Extension</title>
            <author fullname="S. Friedl" initials="S." surname="Friedl">
              <organization showOnFrontPage="true"/>
            </author>
            <author fullname="A. Popov" initials="A." surname="Popov">
              <organization showOnFrontPage="true"/>
            </author>
            <author fullname="A. Langley" initials="A." surname="Langley">
              <organization showOnFrontPage="true"/>
            </author>
            <author fullname="E. Stephan" initials="E." surname="Stephan">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="July" year="2014"/>
            <abstract>
              <t indent="0">This document describes a Transport Layer Security (TLS) extension for application-layer protocol negotiation within the TLS handshake. For instances in which multiple application protocols are supported on the same TCP or UDP port, this extension allows the application layer to negotiate which protocol will be used within the TLS connection.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="7301"/>
          <seriesInfo name="DOI" value="10.17487/RFC7301"/>
        </reference>
        <reference anchor="CHACHA" target="https://www.rfc-editor.org/info/rfc8439" quoteTitle="true" derivedAnchor="CHACHA">
          <front>
            <title>ChaCha20 and Poly1305 for IETF Protocols</title>
            <author fullname="Y. Nir" initials="Y." surname="Nir">
              <organization showOnFrontPage="true"/>
            </author>
            <author fullname="A. Langley" initials="A." surname="Langley">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="June" year="2018"/>
            <abstract>
              <t indent="0">This document defines the ChaCha20 stream cipher as well as the use of the Poly1305 authenticator, both as stand-alone algorithms and as a "combined mode", or Authenticated Encryption with Associated Data (AEAD) algorithm.</t>
              <t indent="0">RFC 7539, the predecessor of this document, was meant to serve as a stable reference and an implementation guide.  It was a product of the Crypto Forum Research Group (CFRG).  This document merges the errata filed against RFC 7539 and adds a little text to the Security Considerations section.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="8439"/>
          <seriesInfo name="DOI" value="10.17487/RFC8439"/>
        </reference>
        <reference anchor="HKDF" target="https://www.rfc-editor.org/info/rfc5869" quoteTitle="true" derivedAnchor="HKDF">
          <front>
            <title>HMAC-based Extract-and-Expand Key Derivation Function (HKDF)</title>
            <author fullname="H. Krawczyk" initials="H." surname="Krawczyk">
              <organization showOnFrontPage="true"/>
            </author>
            <author fullname="P. Eronen" initials="P." surname="Eronen">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="May" year="2010"/>
            <abstract>
              <t indent="0">This document specifies a simple Hashed Message Authentication Code (HMAC)-based key derivation function (HKDF), which can be used as a building block in various protocols and applications.  The key derivation function (KDF) is intended to support a wide range of applications and requirements, and is conservative in its use of cryptographic hash functions.  This document is not an Internet  Standards Track specification; it is published for informational  purposes.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="5869"/>
          <seriesInfo name="DOI" value="10.17487/RFC5869"/>
        </reference>
        <reference anchor="QUIC-RECOVERY" target="https://www.rfc-editor.org/info/rfc9002" quoteTitle="true" derivedAnchor="QUIC-RECOVERY">
          <front>
            <title>QUIC Loss Detection and Congestion Control</title>
            <author initials="J." surname="Iyengar" fullname="Jana Iyengar" role="editor">
              <organization showOnFrontPage="true">Fastly</organization>
            </author>
            <author initials="I." surname="Swett" fullname="Ian Swett" role="editor">
              <organization showOnFrontPage="true">Google</organization>
            </author>
            <date year="2021" month="May"/>
          </front>
          <seriesInfo name="RFC" value="9002"/>
          <seriesInfo name="DOI" value="10.17487/RFC9002"/>
        </reference>
        <reference anchor="QUIC-TRANSPORT" target="https://www.rfc-editor.org/info/rfc9000" quoteTitle="true" derivedAnchor="QUIC-TRANSPORT">
          <front>
            <title>QUIC: A UDP-Based Multiplexed and Secure Transport</title>
            <author initials="J." surname="Iyengar" fullname="Jana Iyengar" role="editor">
              <organization showOnFrontPage="true">Fastly</organization>
            </author>
            <author initials="M." surname="Thomson" fullname="Martin Thomson" role="editor">
              <organization showOnFrontPage="true">Mozilla</organization>
            </author>
            <date year="2021" month="May"/>
          </front>
          <seriesInfo name="RFC" value="9000"/>
          <seriesInfo name="DOI" value="10.17487/RFC9000"/>
        </reference>
        <reference anchor="RFC2119" target="https://www.rfc-editor.org/info/rfc2119" quoteTitle="true" derivedAnchor="RFC2119">
          <front>
            <title>Key words for use in RFCs to Indicate Requirement Levels</title>
            <author fullname="S. Bradner" initials="S." surname="Bradner">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="March" year="1997"/>
            <abstract>
              <t indent="0">In many standards track documents several words are used to signify the requirements in the specification.  These words are often capitalized. This document defines these words as they should be interpreted in IETF documents.  This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.</t>
            </abstract>
          </front>
          <seriesInfo name="BCP" value="14"/>
          <seriesInfo name="RFC" value="2119"/>
          <seriesInfo name="DOI" value="10.17487/RFC2119"/>
        </reference>
        <reference anchor="RFC4086" target="https://www.rfc-editor.org/info/rfc4086" quoteTitle="true" derivedAnchor="RFC4086">
          <front>
            <title>Randomness Requirements for Security</title>
            <author fullname="D. Eastlake 3rd" initials="D." surname="Eastlake 3rd">
              <organization showOnFrontPage="true"/>
            </author>
            <author fullname="J. Schiller" initials="J." surname="Schiller">
              <organization showOnFrontPage="true"/>
            </author>
            <author fullname="S. Crocker" initials="S." surname="Crocker">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="June" year="2005"/>
            <abstract>
              <t indent="0">Security systems are built on strong cryptographic algorithms that foil pattern analysis attempts.  However, the security of these systems is dependent on generating secret quantities for passwords, cryptographic keys, and similar quantities.  The use of pseudo-random processes to generate secret quantities can result in pseudo-security. A sophisticated attacker may find it easier to reproduce the environment that produced the secret quantities and to search the resulting small set of possibilities than to locate the quantities in the whole of the potential number space.</t>
              <t indent="0">Choosing random quantities to foil a resourceful and motivated adversary is surprisingly difficult.  This document points out many pitfalls in using poor entropy sources or traditional pseudo-random number generation techniques for generating such quantities.  It recommends the use of truly random hardware techniques and shows that the existing hardware on many systems can be used for this purpose. It provides suggestions to ameliorate the problem when a hardware solution is not available, and it gives examples of how large such quantities need to be for some applications.  This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.</t>
            </abstract>
          </front>
          <seriesInfo name="BCP" value="106"/>
          <seriesInfo name="RFC" value="4086"/>
          <seriesInfo name="DOI" value="10.17487/RFC4086"/>
        </reference>
        <reference anchor="RFC8174" target="https://www.rfc-editor.org/info/rfc8174" quoteTitle="true" derivedAnchor="RFC8174">
          <front>
            <title>Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words</title>
            <author fullname="B. Leiba" initials="B." surname="Leiba">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="May" year="2017"/>
            <abstract>
              <t indent="0">RFC 2119 specifies common key words that may be used in protocol  specifications.  This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the  defined special meanings.</t>
            </abstract>
          </front>
          <seriesInfo name="BCP" value="14"/>
          <seriesInfo name="RFC" value="8174"/>
          <seriesInfo name="DOI" value="10.17487/RFC8174"/>
        </reference>
        <reference anchor="SHA" quoteTitle="true" target="https://doi.org/10.6028/nist.fips.180-4" derivedAnchor="SHA">
          <front>
            <title>Secure Hash Standard</title>
            <author fullname="Quynh H. Dang" initials="Q." surname="Dang">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="July" year="2015"/>
          </front>
          <seriesInfo name="National Institute of Standards and Technology" value="report"/>
          <seriesInfo name="DOI" value="10.6028/nist.fips.180-4"/>
        </reference>
        <reference anchor="TLS-REGISTRIES" target="https://www.rfc-editor.org/info/rfc8447" quoteTitle="true" derivedAnchor="TLS-REGISTRIES">
          <front>
            <title>IANA Registry Updates for TLS and DTLS</title>
            <author fullname="J. Salowey" initials="J." surname="Salowey">
              <organization showOnFrontPage="true"/>
            </author>
            <author fullname="S. Turner" initials="S." surname="Turner">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="August" year="2018"/>
            <abstract>
              <t indent="0">This document describes a number of changes to TLS and DTLS IANA registries that range from adding notes to the registry all the way to changing the registration policy.  These changes were mostly motivated by WG review of the TLS- and DTLS-related registries undertaken as part of the TLS 1.3 development process.</t>
              <t indent="0">This document updates the following RFCs: 3749, 5077, 4680, 5246, 5705, 5878, 6520, and 7301.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="8447"/>
          <seriesInfo name="DOI" value="10.17487/RFC8447"/>
        </reference>
        <reference anchor="TLS13" target="https://www.rfc-editor.org/info/rfc8446" quoteTitle="true" derivedAnchor="TLS13">
          <front>
            <title>The Transport Layer Security (TLS) Protocol Version 1.3</title>
            <author fullname="E. Rescorla" initials="E." surname="Rescorla">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="August" year="2018"/>
            <abstract>
              <t indent="0">This document specifies version 1.3 of the Transport Layer Security (TLS) protocol.  TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery.</t>
              <t indent="0">This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961.  This document also specifies new requirements for TLS 1.2 implementations.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="8446"/>
          <seriesInfo name="DOI" value="10.17487/RFC8446"/>
        </reference>
      </references>
      <references pn="section-11.2">
        <name slugifiedName="name-informative-references">Informative References</name>
        <reference anchor="AEBounds" target="https://www.isg.rhul.ac.uk/~kp/TLS-AEbounds.pdf" quoteTitle="true" derivedAnchor="AEBounds">
          <front>
            <title>Limits on Authenticated Encryption Use in TLS</title>
            <author initials="A." surname="Luykx">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="K." surname="Paterson">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2017" month="August" day="28"/>
          </front>
        </reference>
        <reference anchor="ASCII" target="https://www.rfc-editor.org/info/rfc20" quoteTitle="true" derivedAnchor="ASCII">
          <front>
            <title>ASCII format for network interchange</title>
            <author fullname="V.G. Cerf" initials="V.G." surname="Cerf">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="October" year="1969"/>
          </front>
          <seriesInfo name="STD" value="80"/>
          <seriesInfo name="RFC" value="20"/>
          <seriesInfo name="DOI" value="10.17487/RFC0020"/>
        </reference>
        <reference anchor="CCM-ANALYSIS" quoteTitle="true" target="https://doi.org/10.1007/3-540-36492-7_7" derivedAnchor="CCM-ANALYSIS">
          <front>
            <title>On the Security of CTR + CBC-MAC</title>
            <author initials="J." surname="Jonsson" fullname="Jakob Jonsson">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2003"/>
          </front>
          <seriesInfo name="DOI" value="10.1007/3-540-36492-7_7"/>
          <refcontent>Selected Areas in Cryptography</refcontent>
          <refcontent>SAC 2002</refcontent>
          <refcontent>Lecture Notes in Computer Science, vol 2595</refcontent>
          <refcontent>pp. 76-93</refcontent>
        </reference>
        <reference anchor="COMPRESS" target="https://www.rfc-editor.org/info/rfc8879" quoteTitle="true" derivedAnchor="COMPRESS">
          <front>
            <title>TLS Certificate Compression</title>
            <author fullname="A. Ghedini" initials="A." surname="Ghedini">
              <organization showOnFrontPage="true"/>
            </author>
            <author fullname="V. Vasiliev" initials="V." surname="Vasiliev">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="December" year="2020"/>
            <abstract>
              <t indent="0">In TLS handshakes, certificate chains often take up the majority of the bytes transmitted.</t>
              <t indent="0">This document describes how certificate chains can be compressed to reduce the amount of data transmitted and avoid some round trips.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="8879"/>
          <seriesInfo name="DOI" value="10.17487/RFC8879"/>
        </reference>
        <reference anchor="GCM-MU" quoteTitle="true" target="https://doi.org/10.1145/3243734.3243816" derivedAnchor="GCM-MU">
          <front>
            <title>The Multi-user Security of GCM, Revisited: Tight Bounds for Nonce Randomization</title>
            <author initials="V." surname="Hoang" fullname="Viet Tung Hoang">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="S." surname="Tessaro" fullname="Stefano Tessaro">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="A." surname="Thiruvengadam" fullname="Aishwarya Thiruvengadam">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2018"/>
          </front>
          <seriesInfo name="DOI" value="10.1145/3243734.3243816"/>
          <refcontent>CCS '18: Proceedings of the 2018 ACM SIGSAC Conference on Computer and Communications Security</refcontent>
          <refcontent>pp. 1429-1440</refcontent>
        </reference>
        <reference anchor="HTTP-REPLAY" target="https://www.rfc-editor.org/info/rfc8470" quoteTitle="true" derivedAnchor="HTTP-REPLAY">
          <front>
            <title>Using Early Data in HTTP</title>
            <author fullname="M. Thomson" initials="M." surname="Thomson">
              <organization showOnFrontPage="true"/>
            </author>
            <author fullname="M. Nottingham" initials="M." surname="Nottingham">
              <organization showOnFrontPage="true"/>
            </author>
            <author fullname="W. Tarreau" initials="W." surname="Tarreau">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="September" year="2018"/>
            <abstract>
              <t indent="0">Using TLS early data creates an exposure to the possibility of a replay attack.  This document defines mechanisms that allow clients to communicate with servers about HTTP requests that are sent in early data.  Techniques are described that use these mechanisms to mitigate the risk of replay.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="8470"/>
          <seriesInfo name="DOI" value="10.17487/RFC8470"/>
        </reference>
        <reference anchor="HTTP2-TLS13" target="https://www.rfc-editor.org/info/rfc8740" quoteTitle="true" derivedAnchor="HTTP2-TLS13">
          <front>
            <title>Using TLS 1.3 with HTTP/2</title>
            <author fullname="D. Benjamin" initials="D." surname="Benjamin">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="February" year="2020"/>
            <abstract>
              <t indent="0">This document updates RFC 7540 by forbidding TLS 1.3 post-handshake authentication, as an analog to the existing TLS 1.2 renegotiation restriction.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="8740"/>
          <seriesInfo name="DOI" value="10.17487/RFC8740"/>
        </reference>
        <reference anchor="IMC" quoteTitle="true" derivedAnchor="IMC">
          <front>
            <title>Introduction to Modern Cryptography, Second Edition</title>
            <author initials="J." surname="Katz">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="Y." surname="Lindell">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2014" month="November" day="06"/>
          </front>
          <seriesInfo name="ISBN" value="978-1466570269"/>
        </reference>
        <reference anchor="NAN" quoteTitle="true" target="https://doi.org/10.1007/978-3-030-26948-7_9" derivedAnchor="NAN">
          <front>
            <title>Nonces Are Noticed: AEAD Revisited</title>
            <author initials="M." surname="Bellare" fullname="Mihir Bellare">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="R." surname="Ng" fullname="Ruth Ng">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="B." surname="Tackmann" fullname="Björn Tackmann">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2019"/>
          </front>
          <seriesInfo name="DOI" value="10.1007/978-3-030-26948-7_9"/>
          <refcontent>Advances in Cryptology - CRYPTO 2019</refcontent>
          <refcontent>Lecture Notes in Computer Science, vol 11692</refcontent>
          <refcontent>pp. 235-265</refcontent>
        </reference>
        <reference anchor="QUIC-HTTP" quoteTitle="true" target="https://datatracker.ietf.org/doc/html/draft-ietf-quic-http-34" derivedAnchor="QUIC-HTTP">
          <front>
            <title>Hypertext Transfer Protocol Version 3 (HTTP/3)</title>
            <author initials="M." surname="Bishop" fullname="Mike Bishop" role="editor">
              <organization showOnFrontPage="true">Akamai Technologies</organization>
            </author>
            <date year="2021" month="February" day="2"/>
          </front>
          <seriesInfo name="Internet-Draft" value="draft-ietf-quic-http-34"/>
          <refcontent>Work in Progress</refcontent>
        </reference>
        <reference anchor="RFC2818" target="https://www.rfc-editor.org/info/rfc2818" quoteTitle="true" derivedAnchor="RFC2818">
          <front>
            <title>HTTP Over TLS</title>
            <author fullname="E. Rescorla" initials="E." surname="Rescorla">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="May" year="2000"/>
            <abstract>
              <t indent="0">This memo describes how to use Transport Layer Security (TLS) to secure Hypertext Transfer Protocol (HTTP) connections over the Internet.  This memo provides information for the Internet community.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="2818"/>
          <seriesInfo name="DOI" value="10.17487/RFC2818"/>
        </reference>
        <reference anchor="RFC5280" target="https://www.rfc-editor.org/info/rfc5280" quoteTitle="true" derivedAnchor="RFC5280">
          <front>
            <title>Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile</title>
            <author fullname="D. Cooper" initials="D." surname="Cooper">
              <organization showOnFrontPage="true"/>
            </author>
            <author fullname="S. Santesson" initials="S." surname="Santesson">
              <organization showOnFrontPage="true"/>
            </author>
            <author fullname="S. Farrell" initials="S." surname="Farrell">
              <organization showOnFrontPage="true"/>
            </author>
            <author fullname="S. Boeyen" initials="S." surname="Boeyen">
              <organization showOnFrontPage="true"/>
            </author>
            <author fullname="R. Housley" initials="R." surname="Housley">
              <organization showOnFrontPage="true"/>
            </author>
            <author fullname="W. Polk" initials="W." surname="Polk">
              <organization showOnFrontPage="true"/>
            </author>
            <date month="May" year="2008"/>
            <abstract>
              <t indent="0">This memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet.  An overview of this approach and model is provided as an introduction.  The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms.  Standard certificate extensions are described and two Internet-specific extensions are defined.  A set of required certificate extensions is specified.  The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions.  An algorithm for X.509 certification path validation is described.  An ASN.1 module and examples are provided in the appendices.  [STANDARDS-TRACK]</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="5280"/>
          <seriesInfo name="DOI" value="10.17487/RFC5280"/>
        </reference>
        <reference anchor="ROBUST" target="https://eprint.iacr.org/2020/718" quoteTitle="true" derivedAnchor="ROBUST">
          <front>
            <title>Robust Channels: Handling Unreliable Networks in the Record Layers of QUIC and DTLS 1.3</title>
            <author initials="M." surname="Fischlin">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="F." surname="Günther">
              <organization showOnFrontPage="true"/>
            </author>
            <author initials="C." surname="Janson">
              <organization showOnFrontPage="true"/>
            </author>
            <date year="2020" month="May" day="16"/>
          </front>
        </reference>
      </references>
    </references>
    <section anchor="test-vectors" numbered="true" toc="include" removeInRFC="false" pn="section-appendix.a">
      <name slugifiedName="name-sample-packet-protection">Sample Packet Protection</name>
      <t indent="0" pn="section-appendix.a-1">This section shows examples of packet protection so that implementations can be
verified incrementally. Samples of Initial packets from both client and server
plus a Retry packet are defined. These packets use an 8-byte client-chosen
Destination Connection ID of 0x8394c8f03e515708. Some intermediate values are
included. All values are shown in hexadecimal.</t>
      <section anchor="keys" numbered="true" toc="include" removeInRFC="false" pn="section-a.1">
        <name slugifiedName="name-keys">Keys</name>
        <t indent="0" pn="section-a.1-1">The labels generated during the execution of the HKDF-Expand-Label function
(that is, HkdfLabel.label) and part of the value given to the HKDF-Expand
function in order to produce its output are:</t>
        <dl indent="3" newline="false" spacing="normal" pn="section-a.1-2">
          <dt pn="section-a.1-2.1">client in:</dt>
          <dd pn="section-a.1-2.2">
            <t indent="0" pn="section-a.1-2.2.1">00200f746c73313320636c69656e7420696e00</t>
          </dd>
          <dt pn="section-a.1-2.3">server in:</dt>
          <dd pn="section-a.1-2.4">
            <t indent="0" pn="section-a.1-2.4.1">00200f746c7331332073657276657220696e00</t>
          </dd>
          <dt pn="section-a.1-2.5">quic key:</dt>
          <dd pn="section-a.1-2.6">
            <t indent="0" pn="section-a.1-2.6.1">00100e746c7331332071756963206b657900</t>
          </dd>
          <dt pn="section-a.1-2.7">quic iv:</dt>
          <dd pn="section-a.1-2.8">
            <t indent="0" pn="section-a.1-2.8.1">000c0d746c733133207175696320697600</t>
          </dd>
          <dt pn="section-a.1-2.9">quic hp:</dt>
          <dd pn="section-a.1-2.10">
            <t indent="0" pn="section-a.1-2.10.1">00100d746c733133207175696320687000</t>
          </dd>
        </dl>
        <t indent="0" pn="section-a.1-3">The initial secret is common:</t>
        <artwork name="" type="" align="left" alt="" pn="section-a.1-4">
initial_secret = HKDF-Extract(initial_salt, cid)
    = 7db5df06e7a69e432496adedb0085192
      3595221596ae2ae9fb8115c1e9ed0a44
</artwork>
        <t indent="0" pn="section-a.1-5">The secrets for protecting client packets are:</t>
        <artwork name="" type="" align="left" alt="" pn="section-a.1-6">
client_initial_secret
    = HKDF-Expand-Label(initial_secret, "client in", "", 32)
    = c00cf151ca5be075ed0ebfb5c80323c4
      2d6b7db67881289af4008f1f6c357aea

key = HKDF-Expand-Label(client_initial_secret, "quic key", "", 16)
    = 1f369613dd76d5467730efcbe3b1a22d

iv  = HKDF-Expand-Label(client_initial_secret, "quic iv", "", 12)
    = fa044b2f42a3fd3b46fb255c

hp  = HKDF-Expand-Label(client_initial_secret, "quic hp", "", 16)
    = 9f50449e04a0e810283a1e9933adedd2
</artwork>
        <t indent="0" pn="section-a.1-7">The secrets for protecting server packets are:</t>
        <artwork name="" type="" align="left" alt="" pn="section-a.1-8">
server_initial_secret
    = HKDF-Expand-Label(initial_secret, "server in", "", 32)
    = 3c199828fd139efd216c155ad844cc81
      fb82fa8d7446fa7d78be803acdda951b

key = HKDF-Expand-Label(server_initial_secret, "quic key", "", 16)
    = cf3a5331653c364c88f0f379b6067e37

iv  = HKDF-Expand-Label(server_initial_secret, "quic iv", "", 12)
    = 0ac1493ca1905853b0bba03e

hp  = HKDF-Expand-Label(server_initial_secret, "quic hp", "", 16)
    = c206b8d9b9f0f37644430b490eeaa314
</artwork>
      </section>
      <section anchor="sample-client-initial" numbered="true" toc="include" removeInRFC="false" pn="section-a.2">
        <name slugifiedName="name-client-initial">Client Initial</name>
        <t indent="0" pn="section-a.2-1">The client sends an Initial packet.  The unprotected payload of this packet
contains the following CRYPTO frame, plus enough PADDING frames to make a
1162-byte payload:</t>
        <artwork name="" type="" align="left" alt="" pn="section-a.2-2">
060040f1010000ed0303ebf8fa56f129 39b9584a3896472ec40bb863cfd3e868
04fe3a47f06a2b69484c000004130113 02010000c000000010000e00000b6578
616d706c652e636f6dff01000100000a 00080006001d00170018001000070005
04616c706e0005000501000000000033 00260024001d00209370b2c9caa47fba
baf4559fedba753de171fa71f50f1ce1 5d43e994ec74d748002b000302030400
0d0010000e0403050306030203080408 050806002d00020101001c0002400100
3900320408ffffffffffffffff050480 00ffff07048000ffff08011001048000
75300901100f088394c8f03e51570806 048000ffff
</artwork>
        <t indent="0" pn="section-a.2-3">The unprotected header indicates a length of 1182 bytes: the 4-byte packet
number, 1162 bytes of frames, and the 16-byte authentication tag.  The header
includes the connection ID and a packet number of 2:</t>
        <artwork name="" type="" align="left" alt="" pn="section-a.2-4">
c300000001088394c8f03e5157080000449e00000002
</artwork>
        <t indent="0" pn="section-a.2-5">Protecting the payload produces output that is sampled for header protection.
Because the header uses a 4-byte packet number encoding, the first 16 bytes of
the protected payload is sampled and then applied to the header as follows:</t>
        <artwork name="" type="" align="left" alt="" pn="section-a.2-6">
sample = d1b1c98dd7689fb8ec11d242b123dc9b

mask = AES-ECB(hp, sample)[0..4]
     = 437b9aec36

header[0] ^= mask[0] &amp; 0x0f
     = c0
header[18..21] ^= mask[1..4]
     = 7b9aec34
header = c000000001088394c8f03e5157080000449e7b9aec34
</artwork>
        <t indent="0" pn="section-a.2-7">The resulting protected packet is:</t>
        <artwork name="" type="" align="left" alt="" pn="section-a.2-8">
c000000001088394c8f03e5157080000 449e7b9aec34d1b1c98dd7689fb8ec11
d242b123dc9bd8bab936b47d92ec356c 0bab7df5976d27cd449f63300099f399
1c260ec4c60d17b31f8429157bb35a12 82a643a8d2262cad67500cadb8e7378c
8eb7539ec4d4905fed1bee1fc8aafba1 7c750e2c7ace01e6005f80fcb7df6212
30c83711b39343fa028cea7f7fb5ff89 eac2308249a02252155e2347b63d58c5
457afd84d05dfffdb20392844ae81215 4682e9cf012f9021a6f0be17ddd0c208
4dce25ff9b06cde535d0f920a2db1bf3 62c23e596d11a4f5a6cf3948838a3aec
4e15daf8500a6ef69ec4e3feb6b1d98e 610ac8b7ec3faf6ad760b7bad1db4ba3
485e8a94dc250ae3fdb41ed15fb6a8e5 eba0fc3dd60bc8e30c5c4287e53805db
059ae0648db2f64264ed5e39be2e20d8 2df566da8dd5998ccabdae053060ae6c
7b4378e846d29f37ed7b4ea9ec5d82e7 961b7f25a9323851f681d582363aa5f8
9937f5a67258bf63ad6f1a0b1d96dbd4 faddfcefc5266ba6611722395c906556
be52afe3f565636ad1b17d508b73d874 3eeb524be22b3dcbc2c7468d54119c74
68449a13d8e3b95811a198f3491de3e7 fe942b330407abf82a4ed7c1b311663a
c69890f4157015853d91e923037c227a 33cdd5ec281ca3f79c44546b9d90ca00
f064c99e3dd97911d39fe9c5d0b23a22 9a234cb36186c4819e8b9c5927726632
291d6a418211cc2962e20fe47feb3edf 330f2c603a9d48c0fcb5699dbfe58964
25c5bac4aee82e57a85aaf4e2513e4f0 5796b07ba2ee47d80506f8d2c25e50fd
14de71e6c418559302f939b0e1abd576 f279c4b2e0feb85c1f28ff18f58891ff
ef132eef2fa09346aee33c28eb130ff2 8f5b766953334113211996d20011a198
e3fc433f9f2541010ae17c1bf202580f 6047472fb36857fe843b19f5984009dd
c324044e847a4f4a0ab34f719595de37 252d6235365e9b84392b061085349d73
203a4a13e96f5432ec0fd4a1ee65accd d5e3904df54c1da510b0ff20dcc0c77f
cb2c0e0eb605cb0504db87632cf3d8b4 dae6e705769d1de354270123cb11450e
fc60ac47683d7b8d0f811365565fd98c 4c8eb936bcab8d069fc33bd801b03ade
a2e1fbc5aa463d08ca19896d2bf59a07 1b851e6c239052172f296bfb5e724047
90a2181014f3b94a4e97d117b4381303 68cc39dbb2d198065ae3986547926cd2
162f40a29f0c3c8745c0f50fba3852e5 66d44575c29d39a03f0cda721984b6f4
40591f355e12d439ff150aab7613499d bd49adabc8676eef023b15b65bfc5ca0
6948109f23f350db82123535eb8a7433 bdabcb909271a6ecbcb58b936a88cd4e
8f2e6ff5800175f113253d8fa9ca8885 c2f552e657dc603f252e1a8e308f76f0
be79e2fb8f5d5fbbe2e30ecadd220723 c8c0aea8078cdfcb3868263ff8f09400
54da48781893a7e49ad5aff4af300cd8 04a6b6279ab3ff3afb64491c85194aab
760d58a606654f9f4400e8b38591356f bf6425aca26dc85244259ff2b19c41b9
f96f3ca9ec1dde434da7d2d392b905dd f3d1f9af93d1af5950bd493f5aa731b4
056df31bd267b6b90a079831aaf579be 0a39013137aac6d404f518cfd4684064
7e78bfe706ca4cf5e9c5453e9f7cfd2b 8b4c8d169a44e55c88d4a9a7f9474241
e221af44860018ab0856972e194cd934
</artwork>
      </section>
      <section anchor="server-initial" numbered="true" toc="include" removeInRFC="false" pn="section-a.3">
        <name slugifiedName="name-server-initial">Server Initial</name>
        <t indent="0" pn="section-a.3-1">The server sends the following payload in response, including an ACK frame, a
CRYPTO frame, and no PADDING frames:</t>
        <artwork name="" type="" align="left" alt="" pn="section-a.3-2">
02000000000600405a020000560303ee fce7f7b37ba1d1632e96677825ddf739
88cfc79825df566dc5430b9a045a1200 130100002e00330024001d00209d3c94
0d89690b84d08a60993c144eca684d10 81287c834d5311bcf32bb9da1a002b00
020304
</artwork>
        <t indent="0" pn="section-a.3-3">The header from the server includes a new connection ID and a 2-byte packet
number encoding for a packet number of 1:</t>
        <artwork name="" type="" align="left" alt="" pn="section-a.3-4">
c1000000010008f067a5502a4262b50040750001
</artwork>
        <t indent="0" pn="section-a.3-5">As a result, after protection, the header protection sample is taken starting
from the third protected byte:</t>
        <artwork name="" type="" align="left" alt="" pn="section-a.3-6">
sample = 2cd0991cd25b0aac406a5816b6394100
mask   = 2ec0d8356a
header = cf000000010008f067a5502a4262b5004075c0d9
</artwork>
        <t indent="0" pn="section-a.3-7">The final protected packet is then:</t>
        <artwork name="" type="" align="left" alt="" pn="section-a.3-8">
cf000000010008f067a5502a4262b500 4075c0d95a482cd0991cd25b0aac406a
5816b6394100f37a1c69797554780bb3 8cc5a99f5ede4cf73c3ec2493a1839b3
dbcba3f6ea46c5b7684df3548e7ddeb9 c3bf9c73cc3f3bded74b562bfb19fb84
022f8ef4cdd93795d77d06edbb7aaf2f 58891850abbdca3d20398c276456cbc4
2158407dd074ee
</artwork>
      </section>
      <section anchor="retry" numbered="true" toc="include" removeInRFC="false" pn="section-a.4">
        <name slugifiedName="name-retry">Retry</name>
        <t indent="0" pn="section-a.4-1">This shows a Retry packet that might be sent in response to the Initial packet
in <xref target="sample-client-initial" format="default" sectionFormat="of" derivedContent="Appendix A.2"/>. The integrity check includes the client-chosen
connection ID value of 0x8394c8f03e515708, but that value is not
included in the final Retry packet:</t>
        <artwork name="" type="" align="left" alt="" pn="section-a.4-2">
ff000000010008f067a5502a4262b574 6f6b656e04a265ba2eff4d829058fb3f
0f2496ba
</artwork>
      </section>
      <section anchor="chacha20-poly1305-short-header-packet" numbered="true" toc="include" removeInRFC="false" pn="section-a.5">
        <name slugifiedName="name-chacha20-poly1305-short-hea">ChaCha20-Poly1305 Short Header Packet</name>
        <t indent="0" pn="section-a.5-1">This example shows some of the steps required to protect a packet with
a short header.  This example uses AEAD_CHACHA20_POLY1305.</t>
        <t indent="0" pn="section-a.5-2">In this example, TLS produces an application write secret from which a server
uses HKDF-Expand-Label to produce four values: a key, an IV, a header
protection key, and the secret that will be used after keys are updated (this
last value is not used further in this example).</t>
        <artwork name="" type="" align="left" alt="" pn="section-a.5-3">
secret
    = 9ac312a7f877468ebe69422748ad00a1
      5443f18203a07d6060f688f30f21632b

key = HKDF-Expand-Label(secret, "quic key", "", 32)
    = c6d98ff3441c3fe1b2182094f69caa2e
      d4b716b65488960a7a984979fb23e1c8

iv  = HKDF-Expand-Label(secret, "quic iv", "", 12)
    = e0459b3474bdd0e44a41c144

hp  = HKDF-Expand-Label(secret, "quic hp", "", 32)
    = 25a282b9e82f06f21f488917a4fc8f1b
      73573685608597d0efcb076b0ab7a7a4

ku  = HKDF-Expand-Label(secret, "quic ku", "", 32)
    = 1223504755036d556342ee9361d25342
      1a826c9ecdf3c7148684b36b714881f9
</artwork>
        <t indent="0" pn="section-a.5-4">The following shows the steps involved in protecting a minimal packet with an
empty Destination Connection ID. This packet contains a single PING frame (that
is, a payload of just 0x01) and has a packet number of 654360564. In this
example, using a packet number of length 3 (that is, 49140 is encoded) avoids
having to pad the payload of the packet; PADDING frames would be needed if the
packet number is encoded on fewer bytes.</t>
        <artwork name="" type="" align="left" alt="" pn="section-a.5-5">
pn                 = 654360564 (decimal)
nonce              = e0459b3474bdd0e46d417eb0
unprotected header = 4200bff4
payload plaintext  = 01
payload ciphertext = 655e5cd55c41f69080575d7999c25a5bfb
</artwork>
        <t indent="0" pn="section-a.5-6">The resulting ciphertext is the minimum size possible. One byte is skipped to
produce the sample for header protection.</t>
        <artwork name="" type="" align="left" alt="" pn="section-a.5-7">
sample = 5e5cd55c41f69080575d7999c25a5bfb
mask   = aefefe7d03
header = 4cfe4189
</artwork>
        <t indent="0" pn="section-a.5-8">The protected packet is the smallest possible packet size of 21 bytes.</t>
        <artwork name="" type="" align="left" alt="" pn="section-a.5-9">
packet = 4cfe4189655e5cd55c41f69080575d7999c25a5bfb
</artwork>
      </section>
    </section>
    <section anchor="aead-analysis" numbered="true" toc="include" removeInRFC="false" pn="section-appendix.b">
      <name slugifiedName="name-aead-algorithm-analysis">AEAD Algorithm Analysis</name>
      <t indent="0" pn="section-appendix.b-1">This section documents analyses used in deriving AEAD algorithm limits for
AEAD_AES_128_GCM, AEAD_AES_128_CCM, and AEAD_AES_256_GCM. The analyses that
follow use symbols for multiplication (*), division (/), and exponentiation (^),
plus parentheses for establishing precedence. The following symbols are also
used:</t>
      <dl indent="3" newline="false" spacing="normal" pn="section-appendix.b-2">
        <dt pn="section-appendix.b-2.1">t:</dt>
        <dd pn="section-appendix.b-2.2">
          <t indent="0" pn="section-appendix.b-2.2.1">The size of the authentication tag in bits. For these ciphers, t is 128.</t>
        </dd>
        <dt pn="section-appendix.b-2.3">n:</dt>
        <dd pn="section-appendix.b-2.4">
          <t indent="0" pn="section-appendix.b-2.4.1">The size of the block function in bits. For these ciphers, n is 128.</t>
        </dd>
        <dt pn="section-appendix.b-2.5">k:</dt>
        <dd pn="section-appendix.b-2.6">
          <t indent="0" pn="section-appendix.b-2.6.1">The size of the key in bits. This is 128 for AEAD_AES_128_GCM and
AEAD_AES_128_CCM; 256 for AEAD_AES_256_GCM.</t>
        </dd>
        <dt pn="section-appendix.b-2.7">l:</dt>
        <dd pn="section-appendix.b-2.8">
          <t indent="0" pn="section-appendix.b-2.8.1">The number of blocks in each packet (see below).</t>
        </dd>
        <dt pn="section-appendix.b-2.9">q:</dt>
        <dd pn="section-appendix.b-2.10">
          <t indent="0" pn="section-appendix.b-2.10.1">The number of genuine packets created and protected by endpoints. This value
is the bound on the number of packets that can be protected before updating
keys.</t>
        </dd>
        <dt pn="section-appendix.b-2.11">v:</dt>
        <dd pn="section-appendix.b-2.12">
          <t indent="0" pn="section-appendix.b-2.12.1">The number of forged packets that endpoints will accept. This value is the
bound on the number of forged packets that an endpoint can reject before
updating keys.</t>
        </dd>
        <dt pn="section-appendix.b-2.13">o:</dt>
        <dd pn="section-appendix.b-2.14">
          <t indent="0" pn="section-appendix.b-2.14.1">The amount of offline ideal cipher queries made by an adversary.</t>
        </dd>
      </dl>
      <t indent="0" pn="section-appendix.b-3">The analyses that follow rely on a count of the number of block operations
involved in producing each message. This analysis is performed for packets of
size up to 2<sup>11</sup> (l = 2<sup>7</sup>) and 2<sup>16</sup> (l =
2<sup>12</sup>). A size of 2<sup>11</sup> is expected to be a limit that matches
common deployment patterns, whereas the 2<sup>16</sup> is the maximum possible
size of a QUIC packet. Only endpoints that strictly limit packet size can use
the larger confidentiality and integrity limits that are derived using the
smaller packet size.</t>
      <t indent="0" pn="section-appendix.b-4">For AEAD_AES_128_GCM and AEAD_AES_256_GCM, the message length (l) is the length
of the associated data in blocks plus the length of the plaintext in blocks.</t>
      <t indent="0" pn="section-appendix.b-5">For AEAD_AES_128_CCM, the total number of block cipher operations is the sum of
the following: the length of the associated data in blocks, the length of the
ciphertext in blocks, the length of the plaintext in blocks, plus 1. In this
analysis, this is simplified to a value of twice the length of the packet in
blocks (that is, <tt>2l = 2<sup>8</sup></tt> for packets that are limited to
2<sup>11</sup> bytes, or <tt>2l = 2<sup>13</sup></tt> otherwise). This
simplification is based on the packet containing all of the associated data and
ciphertext. This results in a one to three block overestimation of the number of
operations per packet.</t>
      <section anchor="gcm-bounds" numbered="true" toc="include" removeInRFC="false" pn="section-b.1">
        <name slugifiedName="name-analysis-of-aead_aes_128_gc">Analysis of AEAD_AES_128_GCM and AEAD_AES_256_GCM Usage Limits</name>
        <t indent="0" pn="section-b.1-1"><xref target="GCM-MU" format="default" sectionFormat="of" derivedContent="GCM-MU"/> specifies concrete bounds for AEAD_AES_128_GCM and AEAD_AES_256_GCM
as used in TLS 1.3 and QUIC. This section documents this analysis using several
simplifying assumptions:</t>
        <ul spacing="normal" bare="false" empty="false" indent="3" pn="section-b.1-2">
          <li pn="section-b.1-2.1">The number of ciphertext blocks an attacker uses in forgery attempts is
bounded by v * l, which is the number of forgery attempts multiplied by the
size of each packet (in blocks).</li>
          <li pn="section-b.1-2.2">The amount of offline work done by an attacker does not dominate other factors
in the analysis.</li>
        </ul>
        <t indent="0" pn="section-b.1-3">The bounds in <xref target="GCM-MU" format="default" sectionFormat="of" derivedContent="GCM-MU"/> are tighter and more complete than those used in
<xref target="AEBounds" format="default" sectionFormat="of" derivedContent="AEBounds"/>, which allows for larger limits than those described in
<xref target="TLS13" format="default" sectionFormat="of" derivedContent="TLS13"/>.</t>
        <section anchor="confidentiality-limit" numbered="true" toc="include" removeInRFC="false" pn="section-b.1.1">
          <name slugifiedName="name-confidentiality-limit">Confidentiality Limit</name>
          <t indent="0" pn="section-b.1.1-1">For confidentiality, Theorem (4.3) in <xref target="GCM-MU" format="default" sectionFormat="of" derivedContent="GCM-MU"/> establishes that, for a single
user that does not repeat nonces, the dominant term in determining the
distinguishing advantage between a real and random AEAD algorithm gained by an
attacker is:</t>
          <artwork name="" type="" align="left" alt="" pn="section-b.1.1-2">
2 * (q * l)^2 / 2^n
</artwork>
          <t indent="0" pn="section-b.1.1-3">For a target advantage of 2<sup>-57</sup>, this results in the relation:</t>
          <artwork name="" type="" align="left" alt="" pn="section-b.1.1-4">
q &lt;= 2^35 / l
</artwork>
          <t indent="0" pn="section-b.1.1-5">Thus, endpoints that do not send packets larger than 2<sup>11</sup> bytes cannot
protect more than 2<sup>28</sup> packets in a single connection without causing
an attacker to gain a more significant advantage than the target of
2<sup>-57</sup>. The limit for endpoints that allow for the packet size to be as
large as 2<sup>16</sup> is instead 2<sup>23</sup>.</t>
        </section>
        <section anchor="integrity-limit" numbered="true" toc="include" removeInRFC="false" pn="section-b.1.2">
          <name slugifiedName="name-integrity-limit">Integrity Limit</name>
          <t indent="0" pn="section-b.1.2-1">For integrity, Theorem (4.3) in <xref target="GCM-MU" format="default" sectionFormat="of" derivedContent="GCM-MU"/> establishes that an attacker gains
an advantage in successfully forging a packet of no more than the following:</t>
          <artwork name="" type="" align="left" alt="" pn="section-b.1.2-2">
(1 / 2^(8 * n)) + ((2 * v) / 2^(2 * n))
        + ((2 * o * v) / 2^(k + n)) + (n * (v + (v * l)) / 2^k)
</artwork>
          <t indent="0" pn="section-b.1.2-3">The goal is to limit this advantage to 2<sup>-57</sup>.  For AEAD_AES_128_GCM,
the fourth term in this inequality dominates the rest, so the others can be
removed without significant effect on the result. This produces the following
approximation:</t>
          <artwork name="" type="" align="left" alt="" pn="section-b.1.2-4">
v &lt;= 2^64 / l
</artwork>
          <t indent="0" pn="section-b.1.2-5">Endpoints that do not attempt to remove protection from packets larger than
2<sup>11</sup> bytes can attempt to remove protection from at most
2<sup>57</sup> packets. Endpoints that do not restrict the size of processed
packets can attempt to remove protection from at most 2<sup>52</sup> packets.</t>
          <t indent="0" pn="section-b.1.2-6">For AEAD_AES_256_GCM, the same term dominates, but the larger value of k
produces the following approximation:</t>
          <artwork name="" type="" align="left" alt="" pn="section-b.1.2-7">
v &lt;= 2^192 / l
</artwork>
          <t indent="0" pn="section-b.1.2-8">This is substantially larger than the limit for AEAD_AES_128_GCM.  However, this
document recommends that the same limit be applied to both functions as either
limit is acceptably large.</t>
        </section>
      </section>
      <section anchor="ccm-bounds" numbered="true" toc="include" removeInRFC="false" pn="section-b.2">
        <name slugifiedName="name-analysis-of-aead_aes_128_cc">Analysis of AEAD_AES_128_CCM Usage Limits</name>
        <t indent="0" pn="section-b.2-1">TLS <xref target="TLS13" format="default" sectionFormat="of" derivedContent="TLS13"/> and <xref target="AEBounds" format="default" sectionFormat="of" derivedContent="AEBounds"/> do not specify limits on usage
for AEAD_AES_128_CCM. However, any AEAD that is used with QUIC requires limits
on use that ensure that both confidentiality and integrity are preserved. This
section documents that analysis.</t>
        <t indent="0" pn="section-b.2-2"><xref target="CCM-ANALYSIS" format="default" sectionFormat="of" derivedContent="CCM-ANALYSIS"/> is used as the basis of this
analysis. The results of that analysis are used to derive usage limits that are
based on those chosen in <xref target="TLS13" format="default" sectionFormat="of" derivedContent="TLS13"/>.</t>
        <t indent="0" pn="section-b.2-3">For confidentiality, Theorem 2 in <xref target="CCM-ANALYSIS" format="default" sectionFormat="of" derivedContent="CCM-ANALYSIS"/> establishes that an attacker
gains a distinguishing advantage over an ideal pseudorandom permutation (PRP) of
no more than the following:</t>
        <artwork name="" type="" align="left" alt="" pn="section-b.2-4">
(2l * q)^2 / 2^n
</artwork>
        <t indent="0" pn="section-b.2-5">The integrity limit in Theorem 1 in <xref target="CCM-ANALYSIS" format="default" sectionFormat="of" derivedContent="CCM-ANALYSIS"/> provides an attacker a
strictly higher advantage for the same number of messages. As the targets for
the confidentiality advantage and the integrity advantage are the same, only
Theorem 1 needs to be considered.</t>
        <t indent="0" pn="section-b.2-6">Theorem 1 establishes that an attacker gains an advantage over an
ideal PRP of no more than the following:</t>
        <artwork name="" type="" align="left" alt="" pn="section-b.2-7">
v / 2^t + (2l * (v + q))^2 / 2^n
</artwork>
        <t indent="0" pn="section-b.2-8">As <tt>t</tt> and <tt>n</tt> are both 128, the first term is negligible relative to the
second, so that term can be removed without a significant effect on the result.</t>
        <t indent="0" pn="section-b.2-9">This produces a relation that combines both encryption and decryption attempts
with the same limit as that produced by the theorem for confidentiality alone.
For a target advantage of 2<sup>-57</sup>, this results in the following:</t>
        <artwork name="" type="" align="left" alt="" pn="section-b.2-10">
v + q &lt;= 2^34.5 / l
</artwork>
        <t indent="0" pn="section-b.2-11">By setting <tt>q = v</tt>, values for both confidentiality and integrity limits can be
produced. Endpoints that limit packets to 2<sup>11</sup> bytes therefore have
both confidentiality and integrity limits of 2<sup>26.5</sup> packets. Endpoints
that do not restrict packet size have a limit of 2<sup>21.5</sup>.</t>
      </section>
    </section>
    <section numbered="false" anchor="contributors" toc="include" removeInRFC="false" pn="section-appendix.c">
      <name slugifiedName="name-contributors">Contributors</name>
      <t indent="0" pn="section-appendix.c-1">The IETF QUIC Working Group received an enormous amount of support from many
people. The following people provided substantive contributions to this
document:</t>
      <ul spacing="compact" bare="false" empty="false" indent="3" pn="section-appendix.c-2">
        <li pn="section-appendix.c-2.1">
          <t indent="0" pn="section-appendix.c-2.1.1"><contact fullname="Adam Langley"/></t>
        </li>
        <li pn="section-appendix.c-2.2">
          <t indent="0" pn="section-appendix.c-2.2.1"><contact fullname="Alessandro Ghedini"/></t>
        </li>
        <li pn="section-appendix.c-2.3">
          <t indent="0" pn="section-appendix.c-2.3.1"><contact fullname="Christian Huitema"/></t>
        </li>
        <li pn="section-appendix.c-2.4">
          <t indent="0" pn="section-appendix.c-2.4.1"><contact fullname="Christopher Wood"/></t>
        </li>
        <li pn="section-appendix.c-2.5">
          <t indent="0" pn="section-appendix.c-2.5.1"><contact fullname="David Schinazi"/></t>
        </li>
        <li pn="section-appendix.c-2.6">
          <t indent="0" pn="section-appendix.c-2.6.1"><contact fullname="Dragana Damjanovic"/></t>
        </li>
        <li pn="section-appendix.c-2.7">
          <t indent="0" pn="section-appendix.c-2.7.1"><contact fullname="Eric Rescorla"/></t>
        </li>
        <li pn="section-appendix.c-2.8">
          <t indent="0" pn="section-appendix.c-2.8.1"><contact fullname="Felix Günther"/></t>
        </li>
        <li pn="section-appendix.c-2.9">
          <t indent="0" pn="section-appendix.c-2.9.1"><contact fullname="Ian Swett"/></t>
        </li>
        <li pn="section-appendix.c-2.10">
          <t indent="0" pn="section-appendix.c-2.10.1"><contact fullname="Jana Iyengar"/></t>
        </li>
        <li pn="section-appendix.c-2.11">
          <t indent="0" pn="section-appendix.c-2.11.1"><contact asciiFullname="Kazuho Oku" fullname="奥 一穂"/></t>
        </li>
        <li pn="section-appendix.c-2.12">
          <t indent="0" pn="section-appendix.c-2.12.1"><contact fullname="Marten Seemann"/></t>
        </li>
        <li pn="section-appendix.c-2.13">
          <t indent="0" pn="section-appendix.c-2.13.1"><contact fullname="Martin Duke"/></t>
        </li>
        <li pn="section-appendix.c-2.14">
          <t indent="0" pn="section-appendix.c-2.14.1"><contact fullname="Mike Bishop"/></t>
        </li>
        <li pn="section-appendix.c-2.15">
          <t indent="0" pn="section-appendix.c-2.15.1"><contact fullname="Mikkel Fahnøe Jørgensen"/></t>
        </li>
        <li pn="section-appendix.c-2.16">
          <t indent="0" pn="section-appendix.c-2.16.1"><contact fullname="Nick Banks"/></t>
        </li>
        <li pn="section-appendix.c-2.17">
          <t indent="0" pn="section-appendix.c-2.17.1"><contact fullname="Nick Harper"/></t>
        </li>
        <li pn="section-appendix.c-2.18">
          <t indent="0" pn="section-appendix.c-2.18.1"><contact fullname="Roberto Peon"/></t>
        </li>
        <li pn="section-appendix.c-2.19">
          <t indent="0" pn="section-appendix.c-2.19.1"><contact fullname="Rui Paulo"/></t>
        </li>
        <li pn="section-appendix.c-2.20">
          <t indent="0" pn="section-appendix.c-2.20.1"><contact fullname="Ryan Hamilton"/></t>
        </li>
        <li pn="section-appendix.c-2.21">
          <t indent="0" pn="section-appendix.c-2.21.1"><contact fullname="Victor Vasiliev"/></t>
        </li>
      </ul>
    </section>
    <section anchor="authors-addresses" numbered="false" removeInRFC="false" toc="include" pn="section-appendix.d">
      <name slugifiedName="name-authors-addresses">Authors' Addresses</name>
      <author initials="M." surname="Thomson" fullname="Martin Thomson" role="editor">
        <organization showOnFrontPage="true">Mozilla</organization>
        <address>
          <email>mt@lowentropy.net</email>
        </address>
      </author>
      <author initials="S." surname="Turner" fullname="Sean Turner" role="editor">
        <organization showOnFrontPage="true">sn3rd</organization>
        <address>
          <email>sean@sn3rd.com</email>
        </address>
      </author>
    </section>
  </back>
</rfc>