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<!DOCTYPE html>
<html lang="en" class="RFC">
<head>
<meta charset="utf-8">
<meta content="Common,Latin" name="scripts">
<meta content="initial-scale=1.0" name="viewport">
<title>RFC 8816: Secure Telephone Identity Revisited (STIR) Out-of-Band Architecture and Use Cases</title>
<meta content="Eric Rescorla" name="author">
<meta content="Jon Peterson" name="author">
<meta content="
       The Personal Assertion Token (PASSporT) format defines
a token that can be carried by signaling protocols, including SIP,
to cryptographically attest the identity of callers.
However, not all telephone calls use Internet signaling protocols,
and some calls use them for only part of their signaling
path, while some cannot reliably deliver SIP header fields end-to-end.
This document describes use cases that require the delivery of
PASSporT objects outside of the signaling path, and defines
architectures and semantics to provide
this functionality.
       
    " name="description">
<meta content="xml2rfc 3.5.0" name="generator">
<meta content="SIP" name="keyword">
<meta content="8816" name="rfc.number">
<!-- Generator version information:
  xml2rfc 3.5.0
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<link href="rfc8816.xml" rel="alternate" type="application/rfc+xml">
<link href="#copyright" rel="license">
<style type="text/css">/*

  NOTE: Changes at the bottom of this file overrides some earlier settings.

  Once the style has stabilized and has been adopted as an official RFC style,
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*/

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 h4 {
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/* Make pilcrows on dd visible */
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/* Make the placement of figcaption match that of a table's caption
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<table class="ears">
<thead><tr>
<td class="left">RFC 8816</td>
<td class="center">STIR Out-of-Band</td>
<td class="right">February 2021</td>
</tr></thead>
<tfoot><tr>
<td class="left">Rescorla &amp; Peterson</td>
<td class="center">Informational</td>
<td class="right">[Page]</td>
</tr></tfoot>
</table>
<div id="external-metadata" class="document-information"></div>
<div id="internal-metadata" class="document-information">
<dl id="identifiers">
<dt class="label-stream">Stream:</dt>
<dd class="stream">Internet Engineering Task Force (IETF)</dd>
<dt class="label-rfc">RFC:</dt>
<dd class="rfc"><a href="https://www.rfc-editor.org/rfc/rfc8816" class="eref">8816</a></dd>
<dt class="label-category">Category:</dt>
<dd class="category">Informational</dd>
<dt class="label-published">Published:</dt>
<dd class="published">
<time datetime="2021-02" class="published">February 2021</time>
    </dd>
<dt class="label-issn">ISSN:</dt>
<dd class="issn">2070-1721</dd>
<dt class="label-authors">Authors:</dt>
<dd class="authors">
<div class="author">
      <div class="author-name">E. Rescorla</div>
<div class="org">Mozilla</div>
</div>
<div class="author">
      <div class="author-name">J. Peterson</div>
<div class="org">Neustar</div>
</div>
</dd>
</dl>
</div>
<h1 id="rfcnum">RFC 8816</h1>
<h1 id="title">Secure Telephone Identity Revisited (STIR) Out-of-Band Architecture and Use Cases</h1>
<section id="section-abstract">
      <h2 id="abstract"><a href="#abstract" class="selfRef">Abstract</a></h2>
<p id="section-abstract-1">The Personal Assertion Token (PASSporT) format defines
a token that can be carried by signaling protocols, including SIP,
to cryptographically attest the identity of callers.
However, not all telephone calls use Internet signaling protocols,
and some calls use them for only part of their signaling
path, while some cannot reliably deliver SIP header fields end-to-end.
This document describes use cases that require the delivery of
PASSporT objects outside of the signaling path, and defines
architectures and semantics to provide
this functionality.<a href="#section-abstract-1" class="pilcrow">¶</a></p>
</section>
<div id="status-of-memo">
<section id="section-boilerplate.1">
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<a href="#name-status-of-this-memo" class="section-name selfRef">Status of This Memo</a>
        </h2>
<p id="section-boilerplate.1-1">
            This document is not an Internet Standards Track specification; it is
            published for informational purposes.<a href="#section-boilerplate.1-1" class="pilcrow">¶</a></p>
<p id="section-boilerplate.1-2">
            This document is a product of the Internet Engineering Task Force
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            Internet Engineering Steering Group (IESG).  Not all documents
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            Information about the current status of this document, any
            errata, and how to provide feedback on it may be obtained at
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</section>
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            This document is subject to BCP 78 and the IETF Trust's Legal
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            warranty as described in the Simplified BSD License.<a href="#section-boilerplate.2-2" class="pilcrow">¶</a></p>
</section>
</div>
<div id="toc">
<section id="section-toc.1">
        <a href="#" onclick="scroll(0,0)" class="toplink">▲</a><h2 id="name-table-of-contents">
<a href="#name-table-of-contents" class="section-name selfRef">Table of Contents</a>
        </h2>
<nav class="toc"><ul class="ulEmpty compact toc">
<li class="ulEmpty compact toc" id="section-toc.1-1.1">
            <p id="section-toc.1-1.1.1" class="keepWithNext"><a href="#section-1" class="xref">1</a>.  <a href="#name-introduction" class="xref">Introduction</a><a href="#section-toc.1-1.1.1" class="pilcrow">¶</a></p>
</li>
          <li class="ulEmpty compact toc" id="section-toc.1-1.2">
            <p id="section-toc.1-1.2.1" class="keepWithNext"><a href="#section-2" class="xref">2</a>.  <a href="#name-terminology" class="xref">Terminology</a><a href="#section-toc.1-1.2.1" class="pilcrow">¶</a></p>
</li>
          <li class="ulEmpty compact toc" id="section-toc.1-1.3">
            <p id="section-toc.1-1.3.1" class="keepWithNext"><a href="#section-3" class="xref">3</a>.  <a href="#name-operating-environments" class="xref">Operating Environments</a><a href="#section-toc.1-1.3.1" class="pilcrow">¶</a></p>
</li>
          <li class="ulEmpty compact toc" id="section-toc.1-1.4">
            <p id="section-toc.1-1.4.1"><a href="#section-4" class="xref">4</a>.  <a href="#name-dataflows" class="xref">Dataflows</a><a href="#section-toc.1-1.4.1" class="pilcrow">¶</a></p>
</li>
          <li class="ulEmpty compact toc" id="section-toc.1-1.5">
            <p id="section-toc.1-1.5.1"><a href="#section-5" class="xref">5</a>.  <a href="#name-use-cases" class="xref">Use Cases</a><a href="#section-toc.1-1.5.1" class="pilcrow">¶</a></p>
<ul class="ulEmpty compact toc">
<li class="ulEmpty compact toc" id="section-toc.1-1.5.2.1">
                <p id="section-toc.1-1.5.2.1.1"><a href="#section-5.1" class="xref">5.1</a>.  <a href="#name-case-1-voip-to-pstn-call" class="xref">Case 1: VoIP to PSTN Call</a><a href="#section-toc.1-1.5.2.1.1" class="pilcrow">¶</a></p>
</li>
              <li class="ulEmpty compact toc" id="section-toc.1-1.5.2.2">
                <p id="section-toc.1-1.5.2.2.1"><a href="#section-5.2" class="xref">5.2</a>.  <a href="#name-case-2-two-smart-pstn-endpo" class="xref">Case 2: Two Smart PSTN Endpoints</a><a href="#section-toc.1-1.5.2.2.1" class="pilcrow">¶</a></p>
</li>
              <li class="ulEmpty compact toc" id="section-toc.1-1.5.2.3">
                <p id="section-toc.1-1.5.2.3.1"><a href="#section-5.3" class="xref">5.3</a>.  <a href="#name-case-3-pstn-to-voip-call" class="xref">Case 3: PSTN to VoIP Call</a><a href="#section-toc.1-1.5.2.3.1" class="pilcrow">¶</a></p>
</li>
              <li class="ulEmpty compact toc" id="section-toc.1-1.5.2.4">
                <p id="section-toc.1-1.5.2.4.1"><a href="#section-5.4" class="xref">5.4</a>.  <a href="#name-case-4-gateway-out-of-band" class="xref">Case 4: Gateway Out-of-Band</a><a href="#section-toc.1-1.5.2.4.1" class="pilcrow">¶</a></p>
</li>
              <li class="ulEmpty compact toc" id="section-toc.1-1.5.2.5">
                <p id="section-toc.1-1.5.2.5.1"><a href="#section-5.5" class="xref">5.5</a>.  <a href="#name-case-5-enterprise-call-cent" class="xref">Case 5: Enterprise Call Center</a><a href="#section-toc.1-1.5.2.5.1" class="pilcrow">¶</a></p>
</li>
            </ul>
</li>
          <li class="ulEmpty compact toc" id="section-toc.1-1.6">
            <p id="section-toc.1-1.6.1"><a href="#section-6" class="xref">6</a>.  <a href="#name-storing-and-retrieving-pass" class="xref">Storing and Retrieving PASSporTs</a><a href="#section-toc.1-1.6.1" class="pilcrow">¶</a></p>
<ul class="ulEmpty compact toc">
<li class="ulEmpty compact toc" id="section-toc.1-1.6.2.1">
                <p id="section-toc.1-1.6.2.1.1"><a href="#section-6.1" class="xref">6.1</a>.  <a href="#name-storage" class="xref">Storage</a><a href="#section-toc.1-1.6.2.1.1" class="pilcrow">¶</a></p>
</li>
              <li class="ulEmpty compact toc" id="section-toc.1-1.6.2.2">
                <p id="section-toc.1-1.6.2.2.1"><a href="#section-6.2" class="xref">6.2</a>.  <a href="#name-retrieval" class="xref">Retrieval</a><a href="#section-toc.1-1.6.2.2.1" class="pilcrow">¶</a></p>
</li>
            </ul>
</li>
          <li class="ulEmpty compact toc" id="section-toc.1-1.7">
            <p id="section-toc.1-1.7.1"><a href="#section-7" class="xref">7</a>.  <a href="#name-solution-architecture" class="xref">Solution Architecture</a><a href="#section-toc.1-1.7.1" class="pilcrow">¶</a></p>
<ul class="ulEmpty compact toc">
<li class="ulEmpty compact toc" id="section-toc.1-1.7.2.1">
                <p id="section-toc.1-1.7.2.1.1"><a href="#section-7.1" class="xref">7.1</a>.  <a href="#name-credentials-and-phone-numbe" class="xref">Credentials and Phone Numbers</a><a href="#section-toc.1-1.7.2.1.1" class="pilcrow">¶</a></p>
</li>
              <li class="ulEmpty compact toc" id="section-toc.1-1.7.2.2">
                <p id="section-toc.1-1.7.2.2.1"><a href="#section-7.2" class="xref">7.2</a>.  <a href="#name-call-flow" class="xref">Call Flow</a><a href="#section-toc.1-1.7.2.2.1" class="pilcrow">¶</a></p>
</li>
              <li class="ulEmpty compact toc" id="section-toc.1-1.7.2.3">
                <p id="section-toc.1-1.7.2.3.1"><a href="#section-7.3" class="xref">7.3</a>.  <a href="#name-security-analysis" class="xref">Security Analysis</a><a href="#section-toc.1-1.7.2.3.1" class="pilcrow">¶</a></p>
</li>
              <li class="ulEmpty compact toc" id="section-toc.1-1.7.2.4">
                <p id="section-toc.1-1.7.2.4.1"><a href="#section-7.4" class="xref">7.4</a>.  <a href="#name-substitution-attacks" class="xref">Substitution Attacks</a><a href="#section-toc.1-1.7.2.4.1" class="pilcrow">¶</a></p>
</li>
              <li class="ulEmpty compact toc" id="section-toc.1-1.7.2.5">
                <p id="section-toc.1-1.7.2.5.1"><a href="#section-7.5" class="xref">7.5</a>.  <a href="#name-rate-control-for-cps-storag" class="xref">Rate Control for CPS Storage</a><a href="#section-toc.1-1.7.2.5.1" class="pilcrow">¶</a></p>
</li>
            </ul>
</li>
          <li class="ulEmpty compact toc" id="section-toc.1-1.8">
            <p id="section-toc.1-1.8.1"><a href="#section-8" class="xref">8</a>.  <a href="#name-authentication-and-verifica" class="xref">Authentication and Verification Service Behavior for Out-of-Band</a><a href="#section-toc.1-1.8.1" class="pilcrow">¶</a></p>
<ul class="ulEmpty compact toc">
<li class="ulEmpty compact toc" id="section-toc.1-1.8.2.1">
                <p id="section-toc.1-1.8.2.1.1"><a href="#section-8.1" class="xref">8.1</a>.  <a href="#name-authentication-service-as" class="xref">Authentication Service (AS)</a><a href="#section-toc.1-1.8.2.1.1" class="pilcrow">¶</a></p>
</li>
              <li class="ulEmpty compact toc" id="section-toc.1-1.8.2.2">
                <p id="section-toc.1-1.8.2.2.1"><a href="#section-8.2" class="xref">8.2</a>.  <a href="#name-verification-service-vs" class="xref">Verification Service (VS)</a><a href="#section-toc.1-1.8.2.2.1" class="pilcrow">¶</a></p>
</li>
              <li class="ulEmpty compact toc" id="section-toc.1-1.8.2.3">
                <p id="section-toc.1-1.8.2.3.1"><a href="#section-8.3" class="xref">8.3</a>.  <a href="#name-gateway-placement-services" class="xref">Gateway Placement Services</a><a href="#section-toc.1-1.8.2.3.1" class="pilcrow">¶</a></p>
</li>
            </ul>
</li>
          <li class="ulEmpty compact toc" id="section-toc.1-1.9">
            <p id="section-toc.1-1.9.1"><a href="#section-9" class="xref">9</a>.  <a href="#name-example-https-interface-to-" class="xref">Example HTTPS Interface to the CPS</a><a href="#section-toc.1-1.9.1" class="pilcrow">¶</a></p>
</li>
          <li class="ulEmpty compact toc" id="section-toc.1-1.10">
            <p id="section-toc.1-1.10.1"><a href="#section-10" class="xref">10</a>. <a href="#name-cps-discovery" class="xref">CPS Discovery</a><a href="#section-toc.1-1.10.1" class="pilcrow">¶</a></p>
</li>
          <li class="ulEmpty compact toc" id="section-toc.1-1.11">
            <p id="section-toc.1-1.11.1"><a href="#section-11" class="xref">11</a>. <a href="#name-encryption-key-lookup" class="xref">Encryption Key Lookup</a><a href="#section-toc.1-1.11.1" class="pilcrow">¶</a></p>
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            <p id="section-toc.1-1.12.1"><a href="#section-12" class="xref">12</a>. <a href="#name-iana-considerations" class="xref">IANA Considerations</a><a href="#section-toc.1-1.12.1" class="pilcrow">¶</a></p>
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          <li class="ulEmpty compact toc" id="section-toc.1-1.13">
            <p id="section-toc.1-1.13.1"><a href="#section-13" class="xref">13</a>. <a href="#name-privacy-considerations" class="xref">Privacy Considerations</a><a href="#section-toc.1-1.13.1" class="pilcrow">¶</a></p>
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            <p id="section-toc.1-1.14.1"><a href="#section-14" class="xref">14</a>. <a href="#name-security-considerations" class="xref">Security Considerations</a><a href="#section-toc.1-1.14.1" class="pilcrow">¶</a></p>
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            <p id="section-toc.1-1.15.1"><a href="#section-15" class="xref">15</a>. <a href="#name-informative-references" class="xref">Informative References</a><a href="#section-toc.1-1.15.1" class="pilcrow">¶</a></p>
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            <p id="section-toc.1-1.16.1"><a href="#section-appendix.a" class="xref"></a><a href="#name-acknowledgments" class="xref">Acknowledgments</a><a href="#section-toc.1-1.16.1" class="pilcrow">¶</a></p>
</li>
          <li class="ulEmpty compact toc" id="section-toc.1-1.17">
            <p id="section-toc.1-1.17.1"><a href="#section-appendix.b" class="xref"></a><a href="#name-authors-addresses" class="xref">Authors' Addresses</a><a href="#section-toc.1-1.17.1" class="pilcrow">¶</a></p>
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<section id="section-1">
      <h2 id="name-introduction">
<a href="#section-1" class="section-number selfRef">1. </a><a href="#name-introduction" class="section-name selfRef">Introduction</a>
      </h2>
<p id="section-1-1">The STIR problem statement <span>[<a href="#RFC7340" class="xref">RFC7340</a>]</span>
describes widespread problems enabled by impersonation in the telephone network,
including illegal robocalling, voicemail hacking, and swatting.
As telephone services are increasingly migrating onto the Internet,
and using Voice over IP (VoIP) protocols such as <span><a href="#RFC3261" class="xref">SIP</a> [<a href="#RFC3261" class="xref">RFC3261</a>]</span>,
it is necessary for these protocols to support stronger identity mechanisms to prevent impersonation.
For example, <span>[<a href="#RFC8224" class="xref">RFC8224</a>]</span> defines a SIP Identity header field
capable of carrying <span><a href="#RFC8225" class="xref">PASSporT objects</a> [<a href="#RFC8225" class="xref">RFC8225</a>]</span>
in SIP as a means to cryptographically attest that the originator of a
telephone call is authorized to use the calling party number (or, for native SIP cases,
SIP URI) associated with the originator of the call.<a href="#section-1-1" class="pilcrow">¶</a></p>
<p id="section-1-2">Not all telephone calls use SIP today, however, and even those that do use SIP do not always carry SIP signaling end-to-end.
   Calls from telephone numbers still routinely traverse the Public Switched Telephone Network (PSTN) at some
   point.  Broadly, calls fall into one of three categories:<a href="#section-1-2" class="pilcrow">¶</a></p>
<ol start="1" type="1" class="normal type-1" id="section-1-3">
        <li id="section-1-3.1">
        One or both of the endpoints is actually a PSTN endpoint.<a href="#section-1-3.1" class="pilcrow">¶</a>
</li>
        <li id="section-1-3.2">Both of the endpoints are non-PSTN (SIP, Jingle, etc.) but the call transits the PSTN at some point.<a href="#section-1-3.2" class="pilcrow">¶</a>
</li>
        <li id="section-1-3.3">Non-PSTN calls that do not transit the PSTN at all (such as native SIP end-to-end calls).<a href="#section-1-3.3" class="pilcrow">¶</a>
</li>
      </ol>
<p id="section-1-4">The first two categories represent the  majority of telephone calls associated with problems like illegal robocalling: many robocalls today originate on the Internet but terminate at PSTN endpoints.
   However, the core network elements that operate the PSTN are legacy devices that
   are unlikely to be upgradable at this point to support an in-band authentication system.
   As such, those devices largely cannot be modified to pass signatures originating on the Internet -- or indeed any in-band signaling
   data -- intact.  Even if fields for tunneling arbitrary data can be found in traditional PSTN signaling, in some cases legacy elements would strip the signatures from those fields; in
   others, they might damage them to the point where they cannot be
   verified.  For those first two categories above, any in-band authentication scheme does not
   seem practical in the current environment.<a href="#section-1-4" class="pilcrow">¶</a></p>
<p id="section-1-5">While the core network of the PSTN remains fixed, the endpoints of
   the telephone network are becoming increasingly programmable and
   sophisticated.  Landline "plain old telephone service" deployments,
   especially in the developed world, are shrinking, and increasingly
   being replaced by three classes of intelligent devices:  smart
   phones, IP Private Branch Exchanges (PBXs), and terminal adapters.  All three are general
   purpose computers, and typically all three have Internet access as
   well as access to the PSTN; they may be used for residential, mobile, or enterprise telephone services.
   Additionally, various kinds of gateways increasingly front for
   deployments of legacy PBX and PSTN switches. All of this provides a potential avenue for
   building an authentication system that implements stronger identity while leaving PSTN systems intact.<a href="#section-1-5" class="pilcrow">¶</a></p>
<p id="section-1-6"> This capability also provides an ideal transitional technology while in-band STIR adoption is ramping up. It permits early adopters to use the technology even when intervening network
elements are not yet STIR-aware, and through various kinds of gateways, it may allow providers with a significant PSTN investment to still secure their calls with STIR.<a href="#section-1-6" class="pilcrow">¶</a></p>
<p id="section-1-7">The techniques described in this document therefore build on the
<span><a href="#RFC8225" class="xref">PASSporT</a> [<a href="#RFC8225" class="xref">RFC8225</a>]</span> mechanism and the work of <span>[<a href="#RFC8224" class="xref">RFC8224</a>]</span> to describe a way that
a PASSporT object created in the originating network of a call can reach the terminating network even when it cannot be carried end-to-end in-band in the call signaling. This relies on
a new service defined in this document called a Call Placement Service (CPS) that permits the PASSporT object to be stored during call processing and retrieved for verification purposes.<a href="#section-1-7" class="pilcrow">¶</a></p>
<p id="section-1-8">Potential implementors should note that this document merely defines the operating environments in which this
       out-of-band STIR mechanism is intended to operate.  It provides use cases, gives a broad description of the components, and a potential solution architecture.
Various environments may have their own security requirements: a public deployment of out-of-band STIR faces far greater challenges than a constrained intra-network deployment.
        To flesh out the storage and retrieval of PASSporTs in the CPS within this
        context, this document includes a strawman protocol suitable for that purpose. Deploying this framework in any given environment
        would require additional specification outside the scope of this document.<a href="#section-1-8" class="pilcrow">¶</a></p>
</section>
<section id="section-2">
      <h2 id="name-terminology">
<a href="#section-2" class="section-number selfRef">2. </a><a href="#name-terminology" class="section-name selfRef">Terminology</a>
      </h2>
<p id="section-2-1">
    The key words "<span class="bcp14">MUST</span>", "<span class="bcp14">MUST NOT</span>", "<span class="bcp14">REQUIRED</span>", "<span class="bcp14">SHALL</span>", "<span class="bcp14">SHALL NOT</span>", "<span class="bcp14">SHOULD</span>", "<span class="bcp14">SHOULD NOT</span>", "<span class="bcp14">RECOMMENDED</span>", "<span class="bcp14">NOT RECOMMENDED</span>",
    "<span class="bcp14">MAY</span>", and "<span class="bcp14">OPTIONAL</span>" in this document are to be interpreted as
    described in BCP 14 <span>[<a href="#RFC2119" class="xref">RFC2119</a>]</span> <span>[<a href="#RFC8174" class="xref">RFC8174</a>]</span>
    when, and only when, they appear in all capitals, as shown here.<a href="#section-2-1" class="pilcrow">¶</a></p>
</section>
<section id="section-3">
      <h2 id="name-operating-environments">
<a href="#section-3" class="section-number selfRef">3. </a><a href="#name-operating-environments" class="section-name selfRef">Operating Environments</a>
      </h2>
<p id="section-3-1">This section describes the environments in which the proposed out-of-band STIR mechanism is intended to operate.  In the simplest setting, Alice
   calls Bob, and her call is routed through some set of gateways and/or the PSTN
   that do not support end-to-end delivery of STIR.  Both Alice
   and Bob have smart devices that can access the Internet (perhaps enterprise devices, or even end-user ones), but they do not have
   a clear telephone signaling connection between them:  Alice cannot inject any data into
   signaling that Bob can read, with the exception of the asserted destination and origination
   E.164 numbers. The calling party number might originate from her own device or from the network.  These numbers are effectively the only data that can be used for coordination between the endpoints.<a href="#section-3-1" class="pilcrow">¶</a></p>
<div class="artwork art-text alignLeft" id="section-3-2">
<pre>
                            +---------+
                           /           \
                       +---             +---+
  +----------+        /                      \        +----------+
  |          |       |        Gateways        |       |          |
  |   Alice  |&lt;-----&gt;|         and/or         |&lt;-----&gt;|    Bob   |
  | (caller) |       |          PSTN          |       | (callee) |
  +----------+        \                      /        +----------+
                       +---             +---+
                           \           /
                            +---------+
</pre><a href="#section-3-2" class="pilcrow">¶</a>
</div>
<p id="section-3-3">In a more complicated setting, Alice and/or Bob may not have a smart or
   programmable device, but instead just a traditional telephone. However, one or both of them are behind a STIR-aware gateway that can participate in out-of-band coordination, as shown below:<a href="#section-3-3" class="pilcrow">¶</a></p>
<div class="artwork art-text alignLeft" id="section-3-4">
<pre>
                           +---------+
                          /           \
                      +---             +---+
+----------+  +--+   /                      \   +--+  +----------+
|          |  |  |  |        Gateways        |  |  |  |          |
|   Alice  |&lt;-|GW|-&gt;|         and/or         |&lt;-|GW|-&gt;|    Bob   |
| (caller) |  |  |  |          PSTN          |  |  |  | (callee) |
+----------+  +--+   \                      /   +--+  +----------+
                      +---             +---+
                          \           /
                           +---------+
</pre><a href="#section-3-4" class="pilcrow">¶</a>
</div>
<p id="section-3-5">In such a case, Alice might have an analog (e.g., PSTN) connection to her
   gateway or switch that is responsible for her identity.  Similarly, the gateway
   would verify Alice's identity, generate the right calling party number
   information, and provide that number to Bob using ordinary
   Plain Old Telephone Service (POTS) mechanisms.<a href="#section-3-5" class="pilcrow">¶</a></p>
</section>
<div id="data">
<section id="section-4">
      <h2 id="name-dataflows">
<a href="#section-4" class="section-number selfRef">4. </a><a href="#name-dataflows" class="section-name selfRef">Dataflows</a>
      </h2>
<p id="section-4-1">Because in these operating environments, endpoints cannot pass cryptographic information to one another directly
  through signaling, any solution must
   involve some rendezvous mechanism to allow endpoints to communicate.
   We call this rendezvous service a Call Placement Service (CPS), a service where a record of call placement,
   in this case a PASSporT, can be stored for future retrieval.  In
   principle, this service could communicate any information, but minimally we
   expect it to include a full-form PASSporT that attests
   the caller, callee, and the time of the call.  The callee can use the
   existence of a PASSporT for a given incoming call as rough validation of
   the asserted origin of that call.  (See <a href="#lookup" class="xref">Section 11</a> for limitations of
   this design.)<a href="#section-4-1" class="pilcrow">¶</a></p>
<p id="section-4-2">This architecture does not mandate that any particular sort
of entity operate a CPS or mandate any means to discover a CPS. A CPS
could be run internally within a network or made publicly available.
One or more CPSes could be run by a carrier, as repositories for PASSporTs
for calls sent to its customers, or a CPS could be built into an enterprise
PBX or even a smartphone. To the degree possible, it is
specified here generically as an idea that may have applicability to a variety of STIR deployments.<a href="#section-4-2" class="pilcrow">¶</a></p>
<p id="section-4-3">There are roughly two plausible dataflow architectures for the CPS:<a href="#section-4-3" class="pilcrow">¶</a></p>
<ol start="1" type="1" class="normal type-1" id="section-4-4">
        <li id="section-4-4.1">The callee registers with the CPS.  When the caller wishes to
      place a call to the callee, it sends the PASSporT to the CPS, which immediately
      forwards it to the callee.<a href="#section-4-4.1" class="pilcrow">¶</a>
</li>
        <li id="section-4-4.2">The caller stores the PASSporT with the CPS at the time of call
      placement.  When the callee receives the call, it contacts the CPS
      and retrieves the PASSporT.<a href="#section-4-4.2" class="pilcrow">¶</a>
</li>
      </ol>
<p id="section-4-5">While the first architecture is roughly isomorphic to current VoIP
   protocols, it shares their drawbacks.  Specifically, the callee must
   maintain a full-time connection to the CPS to serve as a notification
   channel.  This comes with the usual networking costs to the callee
   and is especially problematic for mobile endpoints. Indeed, if the endpoints had the capabilities
   to implement such an architecture, they could surely just use SIP or some other
   protocol to set up a secure session; even if the media were going through the traditional PSTN, a
   "shadow" SIP session could convey the PASSporT. Thus, we focus
   on the second architecture in which the PSTN incoming call serves as
   the notification channel, and the callee can then contact the CPS to
   retrieve the PASSporT. In specialized environments, for example, a call center that receives a large volume of
   incoming calls that originated in the PSTN, the notification channel approach might be viable.<a href="#section-4-5" class="pilcrow">¶</a></p>
</section>
</div>
<div id="uses">
<section id="section-5">
      <h2 id="name-use-cases">
<a href="#section-5" class="section-number selfRef">5. </a><a href="#name-use-cases" class="section-name selfRef">Use Cases</a>
      </h2>
<p id="section-5-1">The following are the motivating use cases for this mechanism. Bear in mind that,
just as in <span>[<a href="#RFC8224" class="xref">RFC8224</a>]</span>, there may be multiple Identity header fields in a single SIP
INVITE, so there may be multiple PASSporTs in this out-of-band mechanism associated with a single call. For example, a SIP user agent might create a PASSporT for a call with an end-user
credential, and as the call exits the originating administrative domain,
the network authentication service might create its own PASSporT for the same call. As such, these use cases may overlap
in the processing of a single call.<a href="#section-5-1" class="pilcrow">¶</a></p>
<div id="case-ip-pstn">
<section id="section-5.1">
        <h3 id="name-case-1-voip-to-pstn-call">
<a href="#section-5.1" class="section-number selfRef">5.1. </a><a href="#name-case-1-voip-to-pstn-call" class="section-name selfRef">Case 1: VoIP to PSTN Call</a>
        </h3>
<p id="section-5.1-1">A call originates in a SIP environment in a STIR-aware administrative domain. The local authentication service for that administrative domain creates a PASSporT that is carried
in band in the call per <span>[<a href="#RFC8224" class="xref">RFC8224</a>]</span>. The call is routed out of the originating administrative domain and reaches a gateway to the PSTN.
Eventually, the call will terminate on a mobile smartphone that supports this out-of-band mechanism.<a href="#section-5.1-1" class="pilcrow">¶</a></p>
<p id="section-5.1-2">In this use case, the originating authentication service
can store the PASSporT with the appropriate CPS (per the practices of
<a href="#cps" class="xref">Section 10</a>) for the target telephone number
as a fallback in case SIP signaling will not reach end-to-end. When
the destination mobile smartphone receives the call over the PSTN, it
consults the CPS and discovers a PASSporT from the originating telephone number waiting for it.
It uses this PASSporT to verify the calling party number.<a href="#section-5.1-2" class="pilcrow">¶</a></p>
</section>
</div>
<div id="case-pstn-pstn-gw">
<section id="section-5.2">
        <h3 id="name-case-2-two-smart-pstn-endpo">
<a href="#section-5.2" class="section-number selfRef">5.2. </a><a href="#name-case-2-two-smart-pstn-endpo" class="section-name selfRef">Case 2: Two Smart PSTN Endpoints</a>
        </h3>
<p id="section-5.2-1">A call originates with an enterprise PBX that has both
Internet access and a built-in gateway to the PSTN, which communicates
through traditional telephone signaling protocols.
The PBX immediately routes the call to the PSTN, but before it does,
it provisions a PASSporT on the CPS associated with the target telephone number.<a href="#section-5.2-1" class="pilcrow">¶</a></p>
<p id="section-5.2-2">After normal PSTN routing, the call lands on a smart mobile handset that supports the STIR out-of-band mechanism. It queries the appropriate CPS over the Internet to determine if a call has been placed to it
by a STIR-aware device. It finds the PASSporT provisioned by the
enterprise PBX and uses it to verify the calling party number.<a href="#section-5.2-2" class="pilcrow">¶</a></p>
</section>
</div>
<div id="case-pstn-ip">
<section id="section-5.3">
        <h3 id="name-case-3-pstn-to-voip-call">
<a href="#section-5.3" class="section-number selfRef">5.3. </a><a href="#name-case-3-pstn-to-voip-call" class="section-name selfRef">Case 3: PSTN to VoIP Call</a>
        </h3>
<p id="section-5.3-1">A call originates with an enterprise PBX that has both
Internet access and a built-in gateway to the PSTN. It will immediately
route the call to the PSTN, but before it does, it provisions
a PASSporT with the CPS associated with the target telephone number.
However, it turns out that the call will eventually route through
the PSTN to an Internet gateway, which will translate this into a SIP
call and deliver it to an administrative domain with a STIR verification service.<a href="#section-5.3-1" class="pilcrow">¶</a></p>
<p id="section-5.3-2">In this case, there are two subcases for how the PASSporT
might be retrieved. In subcase 1, the Internet gateway that receives
the call from the PSTN could query the appropriate CPS to determine
if the original caller created and provisioned a PASSporT for this call. If so,
it can retrieve the PASSporT and, when it creates a SIP INVITE for
this call, add a corresponding Identity header field per
<span>[<a href="#RFC8224" class="xref">RFC8224</a>]</span>. When the SIP INVITE reaches
the destination administrative domain, it will be able to verify the
PASSporT normally. Note that to avoid discrepancies with the Date
header field value, only a full-form PASSporT should be used for this purpose. In
subcase 2, the gateway does not retrieve the PASSporT itself, but
instead the verification service at the destination administrative
domain does so. Subcase 1 would perhaps be valuable for deployments where
the destination administrative domain supports in-band STIR but not out-of-band STIR.<a href="#section-5.3-2" class="pilcrow">¶</a></p>
</section>
</div>
<div id="case-gateways">
<section id="section-5.4">
        <h3 id="name-case-4-gateway-out-of-band">
<a href="#section-5.4" class="section-number selfRef">5.4. </a><a href="#name-case-4-gateway-out-of-band" class="section-name selfRef">Case 4: Gateway Out-of-Band</a>
        </h3>
<p id="section-5.4-1">A call originates in the SIP world in a STIR-aware administrative domain.
The local authentication service for that administrative domain creates a PASSporT that is carried
in band in the call per <span>[<a href="#RFC8224" class="xref">RFC8224</a>]</span>. The call is routed
out of the originating administrative domain and eventually reaches a gateway to the PSTN.<a href="#section-5.4-1" class="pilcrow">¶</a></p>
<p id="section-5.4-2">In this case, the originating authentication service does not support the out-of-band mechanism, so instead the gateway to the PSTN extracts the PASSporT from the SIP request and provisions it to the CPS. (When the call reaches the gateway to the PSTN, the gateway might first check the CPS to see if a PASSporT object had already been provisioned for this call, and only provision a PASSporT if none is present).<a href="#section-5.4-2" class="pilcrow">¶</a></p>
<p id="section-5.4-3">Ultimately, the call may terminate on the PSTN or be routed back to a SIP environment. In the former case, perhaps the destination endpoint queries the CPS to retrieve the PASSporT provisioned by the first gateway. If the call ultimately returns to a SIP environment, it might be the gateway from the PSTN back to the Internet that retrieves the PASSporT from the CPS and attaches it to the new SIP INVITE it creates, or it might be the terminating administrative domain's verification service that checks the CPS when an INVITE arrives with no Identity header field. Either way, the PASSporT can survive the gap in SIP coverage caused by the PSTN leg of the call.<a href="#section-5.4-3" class="pilcrow">¶</a></p>
</section>
</div>
<div id="case-enterprise">
<section id="section-5.5">
        <h3 id="name-case-5-enterprise-call-cent">
<a href="#section-5.5" class="section-number selfRef">5.5. </a><a href="#name-case-5-enterprise-call-cent" class="section-name selfRef">Case 5: Enterprise Call Center</a>
        </h3>
<p id="section-5.5-1">A call originates from a mobile user, and a STIR authentication service operated by their carrier creates a PASSporT for the call. As the carrier forwards the call via SIP, it attaches the PASSporT to the SIP call with an Identity header field. As a fallback in case the call will not go end-to-end over SIP, the carrier also stores the PASSporT in a CPS.<a href="#section-5.5-1" class="pilcrow">¶</a></p>
<p id="section-5.5-2">The call is then routed over SIP for a time, before it
transitions to the PSTN and ultimately is handled by a legacy PBX at a
high-volume call center. The call center supports the out-of-band service,
and has a high-volume interface to a CPS to retrieve PASSporTs for incoming
calls; agents at the call center use a general purpose computer to manage
inbound calls and can receive STIR notifications through it. When the PASSporT
arrives at the CPS, it is sent through a subscription/notification interface
to a system that can correlate incoming calls with valid PASSporTs. The call
center agent sees that a valid call from the originating number has arrived.<a href="#section-5.5-2" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</div>
<div id="authz">
<section id="section-6">
      <h2 id="name-storing-and-retrieving-pass">
<a href="#section-6" class="section-number selfRef">6. </a><a href="#name-storing-and-retrieving-pass" class="section-name selfRef">Storing and Retrieving PASSporTs</a>
      </h2>
<p id="section-6-1">The use cases show a variety of entities accessing the CPS to
store and retrieve PASSporTs. The question of how the CPS authorizes the
storage and retrieval of PASSporTs is thus a key design decision in the architecture.
The STIR architecture assumes that service providers and, in some cases,
end-user devices will have credentials suitable for attesting authority
over telephone numbers per <span>[<a href="#RFC8226" class="xref">RFC8226</a>]</span>.
These credentials provide the most obvious way that a CPS can authorize
the storage and retrieval of PASSporTs. However, as use cases 3, 4, and 5
in <a href="#uses" class="xref">Section 5</a> show, it may sometimes make sense
for the entity storing or retrieving PASSporTs to be an intermediary rather
than a device associated with either the originating or terminating side of
a call; those intermediaries often would not have access to STIR
credentials covering the telephone numbers in question. Requiring authorization
based on a credential to store PASSporTs is therefore undesirable, though
potentially acceptable if sufficient steps are taken to mitigate any privacy
risk of leaking data.<a href="#section-6-1" class="pilcrow">¶</a></p>
<p id="section-6-2">It is an explicit design goal of this mechanism to minimize
the potential privacy exposure of using a CPS. Ideally, the out-of-band
 mechanism should not result in a worse privacy situation than in-band
STIR <span>[<a href="#RFC8224" class="xref">RFC8224</a>]</span>: for in-band, we might say
that a SIP entity is authorized to receive a PASSporT if it is an intermediate
or final target of the routing of a SIP request. As the originator of a
call cannot necessarily predict the routing path a call will follow, an
out-of-band mechanism could conceivably even improve on the privacy story.<a href="#section-6-2" class="pilcrow">¶</a></p>
<p id="section-6-3">Broadly, the architecture recommended here thus is one focused
on permitting any entity to store encrypted PASSporTs at the CPS, indexed
under the called number. PASSporTs will be encrypted with a public key
associated with the called number, so these PASSporTs may safely be retrieved
by any entity because only holders of the corresponding private key will be
able to decrypt the PASSporT.  This also prevents the CPS itself from
learning the contents of PASSporTs, and thus metadata about calls in
progress, which makes the CPS a less attractive target for pervasive
monitoring (see <span>[<a href="#RFC7258" class="xref">RFC7258</a>]</span>). As a first
step, transport-level security can provide confidentiality from eavesdroppers
for both the storing and retrieval of PASSporTs. To bolster the privacy story,
to prevent denial-of-service flooding of the CPS, and to complicate traffic
analysis, a few additional mechanisms are also recommended below.<a href="#section-6-3" class="pilcrow">¶</a></p>
<div id="stor">
<section id="section-6.1">
        <h3 id="name-storage">
<a href="#section-6.1" class="section-number selfRef">6.1. </a><a href="#name-storage" class="section-name selfRef">Storage</a>
        </h3>
<p id="section-6.1-1">There are a few dimensions to authorizing the storage of PASSporTs.
Encrypting PASSporTs prior to storage entails that a CPS has no way to tell
if a PASSporT is valid; it simply conveys encrypted blocks that it cannot
access itself and can make no authorization decision based on the PASSporT
contents. There is certainly no prospect for the CPS to verify the PASSporTs itself.<a href="#section-6.1-1" class="pilcrow">¶</a></p>
<p id="section-6.1-2">Note that this architecture requires clients that store PASSporTs
to have access to an encryption key associated with the intended called party
to be used to encrypt the PASSporT. Discovering this key requires the existence
of a key lookup service (see <a href="#lookup" class="xref">Section 11</a>),
depending on how the CPS is architected; however, some kind of key store or
repository could be implemented adjacent to it and perhaps even incorporated
into its operation. Key discovery is made more complicated by the fact that
there can potentially be multiple entities that have
 authority over a telephone number: a carrier, a reseller, an enterprise,
and an end user might all have credentials permitting them to attest that they
are allowed to originate calls from a number, say. PASSporTs for out-of-band use
therefore might need to be encrypted with multiple keys in the hopes that one
will be decipherable by the relying party.<a href="#section-6.1-2" class="pilcrow">¶</a></p>
<p id="section-6.1-3">Again, the most obvious way to authorize storage is to require
the originator to authenticate themselves to the CPS with their STIR credential.
However, since the call is indexed at the CPS under the called number,
this can weaken the privacy story of the architecture, as it reveals to
the CPS both the identity of the caller and the callee. Moreover, it does not work
for the gateway use cases described above; to support those use cases, we must
effectively allow any entity to store PASSporTs at a CPS. This does not degrade
the anti-impersonation security of STIR, because entities who do not possess
the necessary credentials to sign the PASSporT will not be able to create
PASSporTs that will be treated as valid by verifiers. In this architecture,
it does not matter whether the CPS received a PASSporT from the authentication
service that created it or from an intermediary gateway downstream in the
routing path as in case 4 above. However, if literally anyone can store
PASSporTs in the CPS, an attacker could easily flood the CPS with millions
of bogus PASSporTs indexed under a calling number, and thereby prevent the called
party from finding a valid PASSporT for an incoming call buried in a haystack of fake entries.<a href="#section-6.1-3" class="pilcrow">¶</a></p>
<p id="section-6.1-4">The solution architecture must therefore include some sort of traffic
control system to prevent flooding. Preferably, this should not require
authenticating the source, as this will reveal to the CPS both the source and
destination of traffic. A potential solution is discussed below in <a href="#rate-control" class="xref">Section 7.5</a>.<a href="#section-6.1-4" class="pilcrow">¶</a></p>
</section>
</div>
<div id="retr">
<section id="section-6.2">
        <h3 id="name-retrieval">
<a href="#section-6.2" class="section-number selfRef">6.2. </a><a href="#name-retrieval" class="section-name selfRef">Retrieval</a>
        </h3>
<p id="section-6.2-1">For retrieval of PASSporTs, this architecture assumes that clients will
contact the CPS through some sort of polling or notification interface to receive all
 current PASSporTs for calls destined to a particular telephone number, or block of numbers.<a href="#section-6.2-1" class="pilcrow">¶</a></p>
<p id="section-6.2-2">As PASSporTs stored at the CPS are encrypted with a key belonging
to the intended destination, the CPS can safely allow anyone to download PASSporTs
for a called number without much fear of compromising private information
about calls in progress -- provided that the CPS always returns at least one
encrypted blob in response to a request, even if there was no call in progress.
Otherwise, entities could poll the CPS constantly, or eavesdrop on traffic,
to learn whether or not calls were in progress. The CPS <span class="bcp14">MUST</span> generate
at least one unique and plausible encrypted response to all retrieval requests,
and these dummy encrypted PASSporTs <span class="bcp14">MUST NOT</span> be repeated for
later calls. An encryption scheme needs to be carefully chosen to make messages
look indistinguishable from random when encrypted, so that information about the
called party is not discoverable from legitimate encrypted PASSporTs.<a href="#section-6.2-2" class="pilcrow">¶</a></p>
<p id="section-6.2-3">Because the entity placing a call may discover multiple keys
associated with the called party number, multiple valid PASSporTs may be
stored in the CPS. A particular called party who retrieves PASSporTs from
the CPS may have access to only one of those keys. Thus, the presence of
one or more PASSporTs that the called party cannot decrypt -- which would
be indistinguishable from the "dummy" PASSporTs created by the CPS when
no calls are in progress - does not entail that there is no call in progress.
A retriever likely will need to decrypt all PASSporTs retrieved from the CPS,
and may find only one that is valid.<a href="#section-6.2-3" class="pilcrow">¶</a></p>
<p id="section-6.2-4">In order to prevent the CPS from learning the numbers that a callee
controls, callees might also request PASSporTs for numbers that they do not own,
that they have no hope of decrypting. Implementations could even allow a callee
to request PASSporTs for a range or prefix of numbers: a trade-off where that
callee is willing to sift through bulk quantities of undecryptable PASSporTs
for the sake of hiding from the CPS which numbers it controls.<a href="#section-6.2-4" class="pilcrow">¶</a></p>
<p id="section-6.2-5">Note that in out-of-band call forwarding cases, special behavior is
required to manage the relationship between PASSporTs using the diversion
extension <span>[<a href="#I-D.ietf-stir-passport-divert" class="xref">PASSPORT-DIVERT</a>]</span>.
The originating authentication service encrypts the initial PASSporT with the
public encryption key of the intended destination, but once a call is forwarded,
it may go to a destination that does not possess the corresponding private key
and thus could not decrypt the original PASSporT. This requires the retargeting
entity to generate encrypted PASSporTs that show a secure chain of diversion:
a retargeting storer <span class="bcp14">SHOULD</span> use the "div-o" PASSporT type,
with its "opt" extension, as specified in
<span>[<a href="#I-D.ietf-stir-passport-divert" class="xref">PASSPORT-DIVERT</a>]</span>, in order to nest
the original PASSporT within the encrypted diversion PASSporT.<a href="#section-6.2-5" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</div>
<div id="arch">
<section id="section-7">
      <h2 id="name-solution-architecture">
<a href="#section-7" class="section-number selfRef">7. </a><a href="#name-solution-architecture" class="section-name selfRef">Solution Architecture</a>
      </h2>
<p id="section-7-1">In this section, we discuss a high-level architecture for providing the service
   described in the previous sections.  This discussion is deliberately
   sketchy, focusing on broad concepts and skipping over details.  The
   intent here is merely to provide an overall architecture, not an implementable
   specification. A more concrete example of how this might be specified is given in <a href="#web" class="xref">Section 9</a>.<a href="#section-7-1" class="pilcrow">¶</a></p>
<div id="phone">
<section id="section-7.1">
        <h3 id="name-credentials-and-phone-numbe">
<a href="#section-7.1" class="section-number selfRef">7.1. </a><a href="#name-credentials-and-phone-numbe" class="section-name selfRef">Credentials and Phone Numbers</a>
        </h3>
<p id="section-7.1-1">We start from the premise of the
   <span><a href="#RFC7340" class="xref">STIR problem statement</a> [<a href="#RFC7340" class="xref">RFC7340</a>]</span> that phone numbers can be
   associated with credentials that can be used to attest
   ownership of numbers.  For purposes of exposition, we will assume
   that ownership is associated with the endpoint (e.g., a smartphone),
   but it might well be associated with a provider or gateway acting for the
   endpoint instead.  It might be the case that multiple entities are
   able to act for a given number, provided that they have the
   appropriate authority. <span>[<a href="#RFC8226" class="xref">RFC8226</a>]</span> describes
   a credential system suitable for this purpose; the question of how an entity is determined
   to have control of a given number is out of scope for this document.<a href="#section-7.1-1" class="pilcrow">¶</a></p>
</section>
</div>
<div id="solve">
<section id="section-7.2">
        <h3 id="name-call-flow">
<a href="#section-7.2" class="section-number selfRef">7.2. </a><a href="#name-call-flow" class="section-name selfRef">Call Flow</a>
        </h3>
<p id="section-7.2-1">An overview of the basic calling and verification process is shown
   below.  In this diagram, we assume that Alice has the number
   +1.111.555.1111 and Bob has the number +2.222.555.2222.<a href="#section-7.2-1" class="pilcrow">¶</a></p>
<div class="artwork art-text alignLeft" id="section-7.2-2">
<pre>
Alice                    Call Placement Service                  Bob
--------------------------------------------------------------------

Store Encrypted PASSporT for 2.222.555.2222 -&gt;

Call from 1.111.555.1111 ------------------------------------------&gt;


                                 &lt;-------------- Request PASSporT(s)
                                  for 2.222.555.2222

                                 Obtain Encrypted PASSporT --------&gt;
                                    (2.222.555.2222, 1.111.555.1111)

                                  [Ring phone with verified callerid
                                                   = 1.111.555.1111]
</pre><a href="#section-7.2-2" class="pilcrow">¶</a>
</div>
<p id="section-7.2-3">When Alice wishes to make a call to Bob, she contacts the CPS and
   stores an encrypted PASSporT on
   the CPS indexed under Bob's number. The CPS then awaits retrievals for
   that number.<a href="#section-7.2-3" class="pilcrow">¶</a></p>
<p id="section-7.2-4">When Alice places the call, Bob's phone would usually ring and display
   Alice's number (+1.111.555.1111), which is informed by the existing
   PSTN mechanisms for relaying a calling party number (e.g., the
   Calling Party's Number (CIN) field of
   the Initial Address Message (IAM)).  Instead,
   Bob's phone transparently contacts the CPS and requests any current
   PASSporTs for calls to his number.  The CPS responds with any such PASSporTs
   (or dummy PASSporTs if no relevant ones are currently stored).
If such a PASSporT exists, and the verification service in Bob's phone decrypts it using
   his private key, validates it, then
   Bob's phone can present the calling party number
   information as valid.  Otherwise, the call is unverifiable.  Note
   that this does not necessarily mean that the call is bogus; because
   we expect incremental deployment, many legitimate calls will be
   unverifiable.<a href="#section-7.2-4" class="pilcrow">¶</a></p>
</section>
</div>
<div id="sec">
<section id="section-7.3">
        <h3 id="name-security-analysis">
<a href="#section-7.3" class="section-number selfRef">7.3. </a><a href="#name-security-analysis" class="section-name selfRef">Security Analysis</a>
        </h3>
<p id="section-7.3-1">The primary attack we seek to prevent is an attacker convincing the
   callee that a given call is from some other caller C. There are two
   scenarios to be concerned with:<a href="#section-7.3-1" class="pilcrow">¶</a></p>
<ol start="1" type="1" class="normal type-1" id="section-7.3-2">
          <li id="section-7.3-2.1">The attacker wishes to impersonate a target when no call from that
         target is in progress.<a href="#section-7.3-2.1" class="pilcrow">¶</a>
</li>
          <li id="section-7.3-2.2">The attacker wishes to substitute himself for an existing call setup.<a href="#section-7.3-2.2" class="pilcrow">¶</a>
</li>
        </ol>
<p id="section-7.3-3">If an attacker can inject fake PASSporTs into the CPS or in the
   communication from the CPS to the callee, he can mount either attack.
   As PASSporTs should be
   digitally signed by an appropriate authority for the number and verified by the callee
   (see <a href="#phone" class="xref">Section 7.1</a>), this should not arise in ordinary operations.
   Any attacker who is aware of calls in progress can attempt to mount a race to substitute themselves
   as described in <a href="#sub" class="xref">Section 7.4</a>. For privacy and robustness reasons,
   using <span><a href="#RFC8446" class="xref">TLS</a> [<a href="#RFC8446" class="xref">RFC8446</a>]</span> on the originating
   side when storing the PASSporT at the CPS is <span class="bcp14">RECOMMENDED</span>.<a href="#section-7.3-3" class="pilcrow">¶</a></p>
<p id="section-7.3-4">The entire system depends on the security of the credential
   infrastructure.  If the authentication credentials for a given number
   are compromised, then an attacker can impersonate calls from that
   number. However, that is no different from in-band STIR <span>[<a href="#RFC8224" class="xref">RFC8224</a>]</span>.<a href="#section-7.3-4" class="pilcrow">¶</a></p>
<p id="section-7.3-5">A secondary attack we must also prevent is denial-of-service against the CPS, which requires some form of rate control solution that will not degrade the privacy properties of the architecture.<a href="#section-7.3-5" class="pilcrow">¶</a></p>
</section>
</div>
<div id="sub">
<section id="section-7.4">
        <h3 id="name-substitution-attacks">
<a href="#section-7.4" class="section-number selfRef">7.4. </a><a href="#name-substitution-attacks" class="section-name selfRef">Substitution Attacks</a>
        </h3>
<p id="section-7.4-1">All that the receipt of the PASSporT from the CPS proves to the called party
   is that Alice is trying to call
   Bob (or at least was as of very recently) -- it does not prove that
   any particular incoming call is from Alice.  Consider the scenario
   in which we have a service that provides an automatic callback to a
   user-provided number.  In that case, the attacker can try to arrange for a
   false caller-id value, as shown below:<a href="#section-7.4-1" class="pilcrow">¶</a></p>
<div class="artwork art-text alignLeft" id="section-7.4-2">
<pre>
 Attacker            Callback Service           CPS               Bob
 --------------------------------------------------------------------
 Place call to Bob ----------&gt;
  (from 111.555.1111)
                             Store PASSporT for
                             CS:Bob -------------&gt;

 Call from Attacker (forged CS caller-id info)  --------------------&gt;

                             Call from CS ------------------------&gt; X


                                                &lt;-- Retrieve PASSporT
                                                           for CS:Bob

                        PASSporT for CS:Bob ------------------------&gt;

                                         [Ring phone with callerid =
                                            111.555.1111]
</pre><a href="#section-7.4-2" class="pilcrow">¶</a>
</div>
<p id="section-7.4-3">In order to mount this attack, the attacker contacts the Callback
   Service (CS) and provides it with Bob's number.  This causes the CS
   to initiate a call to Bob. As before, the CS contacts the CPS to
   insert an appropriate PASSporT and then initiates a call to Bob. Because
   it is a valid CS injecting the PASSporT, none of the security checks
   mentioned above help.  However, the attacker simultaneously initiates
   a call to Bob using forged caller-id information corresponding to the
   CS.  If he wins the race with the CS, then Bob's phone will attempt
   to verify the attacker's call (and succeed since they are
   indistinguishable), and the CS's call will go to busy/voice mail/call
   waiting.<a href="#section-7.4-3" class="pilcrow">¶</a></p>
<p id="section-7.4-4">
   In order to prevent a passive attacker from using traffic analysis or
   similar means to learn precisely when a call is placed, it is essential
   that the connection between the caller and the CPS be encrypted as recommended above.
   Authentication services could store dummy PASSporTs at the CPS at random intervals in order
   to make it more difficult for an eavesdropper to use traffic analysis to determine
   that a call was about to be placed.<a href="#section-7.4-4" class="pilcrow">¶</a></p>
<p id="section-7.4-5">
   Note that in a SIP environment, the callee might notice that
   there were multiple INVITEs and thus detect this attack, but in some PSTN
   interworking scenarios, or highly intermediated networks, only one call setup
   attempt will reach the target. Also note that the success of this substitution
   attack depends on the attacker landing their call within the narrow window
   that the PASSporT is retained in the CPS, so
   shortening that window will reduce the
   opportunity for the attack. Finally, smart endpoints could implement some sort of
   state coordination to ensure that both sides believe the call is in progress, though
   methods of supporting that are outside the scope of this document.<a href="#section-7.4-5" class="pilcrow">¶</a></p>
</section>
</div>
<div id="rate-control">
<section id="section-7.5">
        <h3 id="name-rate-control-for-cps-storag">
<a href="#section-7.5" class="section-number selfRef">7.5. </a><a href="#name-rate-control-for-cps-storag" class="section-name selfRef">Rate Control for CPS Storage</a>
        </h3>
<p id="section-7.5-1">In order to prevent the flooding of a CPS with bogus PASSporTs,
we propose the use of "blind signatures" (see <span>[<a href="#RFC5636" class="xref">RFC5636</a>]</span>).
A sender will initially authenticate to the CPS using its STIR credentials
and acquire a signed token from the CPS that will be presented later
when storing a PASSporT. The flow looks as follows:<a href="#section-7.5-1" class="pilcrow">¶</a></p>
<div class="artwork art-text alignLeft" id="section-7.5-2">
<pre>
    Sender                                 CPS

    Authenticate to CPS ---------------------&gt;
    Blinded(K_temp) -------------------------&gt;
    &lt;------------- Sign(K_cps, Blinded(K_temp))
    [Disconnect]


    Sign(K_cps, K_temp)
    Sign(K_temp, E(K_receiver, PASSporT)) ---&gt;
</pre><a href="#section-7.5-2" class="pilcrow">¶</a>
</div>
<p id="section-7.5-3">At an initial time when no call is yet in progress, a potential client connects to the CPS, authenticates,
and sends a blinded version of a freshly generated public key. The
CPS returns a signed version of that blinded key. The sender can
then unblind the key and get a signature on K_temp from the CPS.<a href="#section-7.5-3" class="pilcrow">¶</a></p>
<p id="section-7.5-4">
Then later, when a client wants to store a PASSporT, it connects
to the CPS anonymously (preferably over a network connection that cannot be correlated with the token acquisition) and
sends both the signed K_temp and its own signature over the
encrypted PASSporT. The CPS verifies both signatures and, if they
verify, stores the encrypted passport (discarding the signatures).<a href="#section-7.5-4" class="pilcrow">¶</a></p>
<p id="section-7.5-5">
This design lets the CPS rate limit how many PASSporTs a given
sender can store just by counting how many times K_temp appears;
perhaps CPS policy might reject storage attempts and require acquisition
of a new K_temp after storing more than a certain number of PASSporTs
indexed under the same destination number in a short interval.
This does not, of course, allow the CPS to tell when bogus data
is being provisioned by an attacker,
simply the rate at which data is being provisioned. Potentially,
feedback mechanisms could be developed that would allow the called
parties to tell the CPS when they are receiving unusual or bogus
PASSporTs.<a href="#section-7.5-5" class="pilcrow">¶</a></p>
<p id="section-7.5-6">
This architecture also assumes that the CPS will age out PASSporTs.
A CPS <span class="bcp14">SHOULD NOT</span> keep any stored PASSporT for longer
than the recommended freshness
policy for the "iat" value as described in
<span>[<a href="#RFC8224" class="xref">RFC8224</a>]</span> (i.e., sixty seconds)
unless some local policy for a CPS deployment requires a longer or shorter interval.
Any reduction in this window makes substitution attacks
(see <a href="#sub" class="xref">Section 7.4</a>) harder to mount,
but making the window too small might conceivably age PASSporTs out
while a heavily redirected call is still alerting.<a href="#section-7.5-6" class="pilcrow">¶</a></p>
<p id="section-7.5-7">
An alternative potential approach to blind signatures would be
the use of verifiable oblivious pseudorandom functions (VOPRFs, per
<span>[<a href="#I-D.privacy-pass" class="xref">PRIVACY-PASS</a>]</span>), which may prove faster.<a href="#section-7.5-7" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</div>
<div id="u8224">
<section id="section-8">
      <h2 id="name-authentication-and-verifica">
<a href="#section-8" class="section-number selfRef">8. </a><a href="#name-authentication-and-verifica" class="section-name selfRef">Authentication and Verification Service Behavior for Out-of-Band</a>
      </h2>
<p id="section-8-1">
<span>[<a href="#RFC8224" class="xref">RFC8224</a>]</span> defines an authentication service and a verification service as functions that act in the context of SIP requests and responses. This specification thus provides a more generic description of authentication service and verification service behavior that might or might not involve any SIP transactions, but depends only on placing a request for communications from
an originating identity to one or more destination identities.<a href="#section-8-1" class="pilcrow">¶</a></p>
<div id="as">
<section id="section-8.1">
        <h3 id="name-authentication-service-as">
<a href="#section-8.1" class="section-number selfRef">8.1. </a><a href="#name-authentication-service-as" class="section-name selfRef">Authentication Service (AS)</a>
        </h3>
<p id="section-8.1-1">
Out-of-band authentication services perform steps similar to those defined in <span>[<a href="#RFC8224" class="xref">RFC8224</a>]</span> with some exceptions:<a href="#section-8.1-1" class="pilcrow">¶</a></p>
<p id="section-8.1-2">
Step 1: The authentication service <span class="bcp14">MUST</span> determine whether it is
    authoritative for the identity of the originator of the request, that is, the identity it will populate in the "orig" claim of the PASSporT.
It can do so only if it possesses the private key of one or more credentials that can be used
    to sign for that identity, be it a domain or a telephone number or some other identifier. For example, the authentication service could hold the private key associated with a <span><a href="#RFC8225" class="xref">STIR certificate</a> [<a href="#RFC8225" class="xref">RFC8225</a>]</span>.<a href="#section-8.1-2" class="pilcrow">¶</a></p>
<p id="section-8.1-3">
Step 2: The authentication service <span class="bcp14">MUST</span> determine that the
originator of communications can claim the originating identity. This is a policy
decision made by the authentication service that depends on its relationship to
the originator. For an out-of-band application built into the
calling device, for example, this is the same check performed in Step 1: does the
calling device hold a private key, one corresponding to a STIR certificate,
that can sign for the originating identity?<a href="#section-8.1-3" class="pilcrow">¶</a></p>
<div id="as-step3">
<p id="section-8.1-4">
Step 3: The authentication service <span class="bcp14">MUST</span> acquire the public encryption key
of the destination, which will be used to encrypt the PASSporT (see <a href="#lookup" class="xref">Section 11</a>).
It <span class="bcp14">MUST</span> also discover (see <a href="#cps" class="xref">Section 10</a>)
the CPS associated with the destination. The authentication service
may already have the encryption key and destination CPS cached, or may need
to query a service to acquire the key. Note that per <a href="#rate-control" class="xref">Section 7.5</a>,
the authentication service may also need to acquire a token for PASSporT
storage from the CPS upon CPS discovery. It is anticipated that the discovery mechanism
(see <a href="#cps" class="xref">Section 10</a>) used to find the appropriate
CPS will also find the proper key server for the public key of the destination.
In some cases, a destination may have multiple public encryption keys associated with it.
In that case, the authentication service <span class="bcp14">MUST</span> collect all of those keys.<a href="#section-8.1-4" class="pilcrow">¶</a></p>
</div>
<p id="section-8.1-5">
Step 4: The authentication service <span class="bcp14">MUST</span> create the PASSporT object. This includes acquiring the system time to populate the "iat" claim, and populating the "orig" and "dest" claims as
described in <span>[<a href="#RFC8225" class="xref">RFC8225</a>]</span>. The authentication service <span class="bcp14">MUST</span> then encrypt the PASSporT. If in Step 3 the authentication service discovered multiple public keys for the destination, it
        <span class="bcp14">MUST</span> create one encrypted copy for each public key it discovered.<a href="#section-8.1-5" class="pilcrow">¶</a></p>
<p id="section-8.1-6">
Finally, the authentication service stores the encrypted PASSporT(s) at the CPS
discovered in Step 3. Only after that is completed should any call be initiated.
Note that a call might be initiated over SIP, and the authentication
service would place the same PASSporT in the Identity header field value of the SIP request --
though SIP would carry a cleartext version rather than an encrypted version
sent to the CPS. In that case, out-of-band would serve as a fallback mechanism
if the request was not conveyed over SIP end-to-end. Also, note that the
authentication service <span class="bcp14">MAY</span> use a compact form of the PASSporT
for a SIP request, whereas the version stored at the CPS <span class="bcp14">MUST</span>
always be a full-form PASSporT.<a href="#section-8.1-6" class="pilcrow">¶</a></p>
</section>
</div>
<div id="vs">
<section id="section-8.2">
        <h3 id="name-verification-service-vs">
<a href="#section-8.2" class="section-number selfRef">8.2. </a><a href="#name-verification-service-vs" class="section-name selfRef">Verification Service (VS)</a>
        </h3>
<p id="section-8.2-1">
When a call arrives, an out-of-band verification service performs steps similar to those defined in <span>[<a href="#RFC8224" class="xref">RFC8224</a>]</span> with some exceptions:<a href="#section-8.2-1" class="pilcrow">¶</a></p>
<div id="vs-step1">
<p id="section-8.2-2">
Step 1: The verification service contacts the CPS and requests all current PASSporTs for its destination number; or alternatively it may receive PASSporTs through a push interface from the CPS in some deployments. The verification service <span class="bcp14">MUST</span> then decrypt all PASSporTs using its private key. Some PASSporTs may not be decryptable for any number of reasons: they may be intended for a different verification service, or they may be "dummy" values inserted by the CPS for privacy purposes. The next few steps will narrow down the set of PASSporTs that the verification service will examine from that initial decryptable set.<a href="#section-8.2-2" class="pilcrow">¶</a></p>
</div>
<p id="section-8.2-3">
Step 2: The verification service <span class="bcp14">MUST</span> determine if any "ppt" extensions in the PASSporTs are unsupported. It takes only the set of supported PASSporTs and applies the next step to them.<a href="#section-8.2-3" class="pilcrow">¶</a></p>
<p id="section-8.2-4">
Step 3: The verification service <span class="bcp14">MUST</span> determine if there is an overlap between the calling party number presented in call signaling and the "orig" field of any decrypted PASSporTs. It takes the set of matching PASSporTs and applies the next step to them.<a href="#section-8.2-4" class="pilcrow">¶</a></p>
<p id="section-8.2-5">
Step 4: The verification service <span class="bcp14">MUST</span> determine if the credentials that signed each PASSporT are valid, and if the verification service trusts the CA that issued the credentials. It takes the set
of trusted PASSporTs to the next step.<a href="#section-8.2-5" class="pilcrow">¶</a></p>
<p id="section-8.2-6">
Step 5: The verification service <span class="bcp14">MUST</span> check the freshness of the "iat" claim of each PASSporT. The exact interval of time that determines freshness is left to local policy. It takes the set of fresh PASSporTs to the next step.<a href="#section-8.2-6" class="pilcrow">¶</a></p>
<p id="section-8.2-7">
Step 6: The verification service <span class="bcp14">MUST</span> check the validity of the signature over each PASSporT, as described in <span>[<a href="#RFC8225" class="xref">RFC8225</a>]</span>.<a href="#section-8.2-7" class="pilcrow">¶</a></p>
<p id="section-8.2-8">
Finally, the verification service will end up with one or more valid PASSporTs corresponding to the call it has received. In keeping with baseline STIR, this document does not dictate any particular treatment of calls that have valid PASSporTs associated with them; the handling of the call
   after the verification process depends on how the verification
   service is implemented and on local policy. However, it is anticipated that local policies could involve
   making different forwarding decisions in intermediary
   implementations, or changing how the user is alerted or how identity
   is rendered in user agent implementations.<a href="#section-8.2-8" class="pilcrow">¶</a></p>
</section>
</div>
<div id="hybrid">
<section id="section-8.3">
        <h3 id="name-gateway-placement-services">
<a href="#section-8.3" class="section-number selfRef">8.3. </a><a href="#name-gateway-placement-services" class="section-name selfRef">Gateway Placement Services</a>
        </h3>
<p id="section-8.3-1">
The STIR out-of-band mechanism also supports the presence of gateway placement services, which do not create PASSporTs themselves, but instead take PASSporTs out of signaling protocols and store them at a CPS before gatewaying to a protocol that cannot carry PASSporTs itself. For example, a SIP gateway that sends calls to the PSTN could receive a call with an Identity header field, extract a PASSporT from the Identity header field, and store that PASSporT at a CPS.<a href="#section-8.3-1" class="pilcrow">¶</a></p>
<p id="section-8.3-2">
To place a PASSporT at a CPS, a gateway <span class="bcp14">MUST</span> perform
<a href="#as-step3" class="xref">Step 3</a> of <a href="#as" class="xref">Section 8.1</a> above:
that is, it must discover the CPS and public key associated with the
destination of the call, and may need to acquire a PASSporT storage token
(see <a href="#stor" class="xref">Section 6.1</a>). Per <a href="#as-step3" class="xref">Step 3</a>
of <a href="#as" class="xref">Section 8.1</a>, this may entail discovering several keys.
The gateway then collects the in-band PASSporT(s) from the in-band signaling,
encrypts the PASSporT(s), and stores them at the CPS.<a href="#section-8.3-2" class="pilcrow">¶</a></p>
<p id="section-8.3-3">
A similar service could be performed by a gateway that retrieves PASSporTs from a CPS and inserts them into signaling protocols that support carrying PASSporTs in-band. This behavior may be defined by future specifications.<a href="#section-8.3-3" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</div>
<div id="web">
<section id="section-9">
      <h2 id="name-example-https-interface-to-">
<a href="#section-9" class="section-number selfRef">9. </a><a href="#name-example-https-interface-to-" class="section-name selfRef">Example HTTPS Interface to the CPS</a>
      </h2>
<p id="section-9-1">
As a rough example, we show a CPS implementation here that uses a
Representational State Transfer (REST) API <span>[<a href="#REST" class="xref">REST</a>]</span> to store and retrieve objects at the CPS.
The calling party stores the PASSporT at the CPS prior to initiating the call; the
PASSporT is stored at a location at the CPS that corresponds to the called number.
Note that it is possible for multiple parties to be calling a number at the same time, and that for called
numbers such as large call centers, many PASSporTs could legitimately be stored
simultaneously, and it might prove difficult to correlate these with incoming calls.<a href="#section-9-1" class="pilcrow">¶</a></p>
<p id="section-9-2">
Assume that an authentication service has created the following PASSporT for a call to the telephone number 2.222.555.2222 (note that these are dummy values):<a href="#section-9-2" class="pilcrow">¶</a></p>
<div class="artwork art-text alignLeft" id="section-9-3">
<pre>
   eyJhbGciOiJFUzI1NiIsInR5cCI6InBhc3Nwb3J0IiwieDV1IjoiaHR0cHM6Ly9
   jZXJ0LmV4YW1wbGUub3JnL3Bhc3Nwb3J0LmNlciJ9.eyJkZXN0Ijp7InRuIjpbI
   jIyMjI1NTUyMjIyIl19LCJpYXQiOiIxNTgzMjUxODEwIiwib3JpZyI6eyJ0biI6
   IjExMTE1NTUxMTExIn19.pnij4IlLHoR4vxID0u3CT1e9Hq4xLngZUTv45Vbxmd
   3IVyZug4KOSa378yfP4x6twY0KTdiDypsereS438ZHaQ
</pre><a href="#section-9-3" class="pilcrow">¶</a>
</div>
<p id="section-9-4">
Through some discovery mechanism (see <a href="#cps" class="xref">Section 10</a>), the authentication service discovers the network location of a web service that acts as the CPS for 2.222.555.2222. Through the same mechanism, we will say that it has also discovered one public encryption key for that destination. It uses that encryption key to encrypt the PASSporT, resulting in the encrypted PASSporT:<a href="#section-9-4" class="pilcrow">¶</a></p>
<div class="artwork art-text alignLeft" id="section-9-5">
<pre>
   rlWuoTpvBvWSHmV1AvVfVaE5pPV6VaOup3Ajo3W0VvjvrQI1VwbvnUE0pUZ6Yl9w
   MKW0YzI4LJ1joTHho3WaY3Oup3Ajo3W0YzAypvW9rlWxMKA0Vwc7VaIlnFV6JlWm
   nKN6LJkcL2INMKuuoKOfMF5wo20vKK0fVzyuqPV6VwR0AQZlZQtmAQHvYPWipzyaV
   wc7VaEhVwbvZGVkAGH1AGRlZGVvsK0ed3cwG1ubEjnxRTwUPaJFjHafuq0-mW6S1
   IBtSJFwUOe8Dwcwyx-pcSLcSLfbwAPcGmB3DsCBypxTnF6uRpx7j
</pre><a href="#section-9-5" class="pilcrow">¶</a>
</div>
<p id="section-9-6">
Having concluded the numbered steps in <a href="#as" class="xref">Section 8.1</a>, including acquiring any token (per <a href="#stor" class="xref">Section 6.1</a>) needed to store the PASSporT at the CPS, the authentication service then stores the encrypted PASSporT:<a href="#section-9-6" class="pilcrow">¶</a></p>
<div class="artwork art-text alignLeft" id="section-9-7">
<pre>
   POST /cps/2.222.555.2222/ppts HTTP/1.1
   Host: cps.example.com
   Content-Type: application/passport

   rlWuoTpvBvWSHmV1AvVfVaE5pPV6VaOup3Ajo3W0VvjvrQI1VwbvnUE0pUZ6Yl9w
   MKW0YzI4LJ1joTHho3WaY3Oup3Ajo3W0YzAypvW9rlWxMKA0Vwc7VaIlnFV6JlWm
   nKN6LJkcL2INMKuuoKOfMF5wo20vKK0fVzyuqPV6VwR0AQZlZQtmAQHvYPWipzyaV
   wc7VaEhVwbvZGVkAGH1AGRlZGVvsK0ed3cwG1ubEjnxRTwUPaJFjHafuq0-mW6S1
   IBtSJFwUOe8Dwcwyx-pcSLcSLfbwAPcGmB3DsCBypxTnF6uRpx7j
</pre><a href="#section-9-7" class="pilcrow">¶</a>
</div>
<p id="section-9-8">
The web service assigns a new location for this encrypted PASSporT in the collection, returning a 201 OK with the location of /cps/2.222.222.2222/ppts/ppt1.
Now the authentication service can place the call, which may be signaled by various protocols. Once the call arrives at the terminating side, a verification service
contacts its CPS to ask for the set of incoming calls for its telephone number (2.222.222.2222).<a href="#section-9-8" class="pilcrow">¶</a></p>
<div class="artwork art-text alignLeft" id="section-9-9">
<pre>
   GET /cps/2.222.555.2222/ppts
   Host: cps.example.com
</pre><a href="#section-9-9" class="pilcrow">¶</a>
</div>
<p id="section-9-10">
 This returns to the verification service a list of the PASSporTs currently in the collection, which currently consists of only /cps/2.222.222.2222/ppts/ppt1. The verification
 service then sends a new GET for /cps/2.222.555.2222/ppts/ppt1/ which yields:<a href="#section-9-10" class="pilcrow">¶</a></p>
<div class="artwork art-text alignLeft" id="section-9-11">
<pre>
   HTTP/1.1 200 OK
   Content-Type: application/passport
   Link: &lt;https://cps.example.com/cps/2.222.555.2222/ppts&gt;

   rlWuoTpvBvWSHmV1AvVfVaE5pPV6VaOup3Ajo3W0VvjvrQI1VwbvnUE0pUZ6Yl9w
   MKW0YzI4LJ1joTHho3WaY3Oup3Ajo3W0YzAypvW9rlWxMKA0Vwc7VaIlnFV6JlWm
   nKN6LJkcL2INMKuuoKOfMF5wo20vKK0fVzyuqPV6VwR0AQZlZQtmAQHvYPWipzyaV
   wc7VaEhVwbvZGVkAGH1AGRlZGVvsK0ed3cwG1ubEjnxRTwUPaJFjHafuq0-mW6S1
   IBtSJFwUOe8Dwcwyx-pcSLcSLfbwAPcGmB3DsCBypxTnF6uRpx7j
</pre><a href="#section-9-11" class="pilcrow">¶</a>
</div>
<p id="section-9-12">
 That concludes <a href="#vs-step1" class="xref">Step 1</a> of <a href="#vs" class="xref">Section 8.2</a>; the verification service then goes on to the next step, processing that PASSporT through its various checks. A complete protocol description for CPS interactions is left to future work.<a href="#section-9-12" class="pilcrow">¶</a></p>
</section>
</div>
<div id="cps">
<section id="section-10">
      <h2 id="name-cps-discovery">
<a href="#section-10" class="section-number selfRef">10. </a><a href="#name-cps-discovery" class="section-name selfRef">CPS Discovery</a>
      </h2>
<p id="section-10-1">
In order for the two ends of the out-of-band dataflow to coordinate, they must agree on a way to discover a CPS and retrieve PASSporT objects from it
based solely on the rendezvous information available: the calling party number and the called number. Because the storage of PASSporTs in this architecture is indexed
by the called party number, it makes sense to discover a CPS based on the called party number as well.
There are a number of potential service discovery mechanisms that could be used for
this purpose. The means of service discovery may vary by use case.<a href="#section-10-1" class="pilcrow">¶</a></p>
<p id="section-10-2">
 Although the discussion above is written largely in terms of a single CPS, having a significant fraction of all telephone calls result in storing and retrieving PASSporTs at a single monolithic CPS
    has obvious scaling problems, and would as well allow the CPS to
    gather metadata about a very wide set of callers and callees.  These issues can be alleviated by operational models with a
   federated CPS; any service discovery mechanism for out-of-band STIR
should enable federation of the CPS function. Likely models include ones
   where a carrier operates one or more CPS instances on behalf of its customers,
an enterprise runs a CPS instance on behalf of its PBX users, or a third-party service provider
offers a CPS as a cloud service.<a href="#section-10-2" class="pilcrow">¶</a></p>
<p id="section-10-3">
   Some service discovery possibilities under consideration include the following:<a href="#section-10-3" class="pilcrow">¶</a></p>
<ul class="ulEmpty normal">
<li class="ulEmpty normal" id="section-10-4.1">
   For some deployments in closed (e.g., intra-network) environments, the CPS location can simply
   be provisioned in implementations, obviating the need for a discovery protocol.<a href="#section-10-4.1" class="pilcrow">¶</a>
</li>
        <li class="ulEmpty normal" id="section-10-4.2">
   If a credential lookup service is already available (see <a href="#lookup" class="xref">Section 11</a>),
   the CPS location can also be recorded in the callee's credentials;
an extension to <span>[<a href="#RFC8226" class="xref">RFC8226</a>]</span> could, for example,
provide a link to the location of the CPS
   where PASSporTs should be stored for a destination.<a href="#section-10-4.2" class="pilcrow">¶</a>
</li>
        <li class="ulEmpty normal" id="section-10-4.3">
There exist a number of common directory systems that might be used to translate
telephone numbers into the URIs of a CPS.
<span><a href="#RFC6116" class="xref">ENUM</a> [<a href="#RFC6116" class="xref">RFC6116</a>]</span> is commonly implemented,
though no "golden root" central ENUM administration exists that could be easily
reused today to help the endpoints discover a common CPS. Other protocols associated
with queries for telephone numbers, such as the
<span><a href="#I-D.ietf-modern-teri" class="xref">Telephone-Related Information (TeRI) protocol</a> [<a href="#I-D.ietf-modern-teri" class="xref">MODERN-TERI</a>]</span>,
could also serve for this application.<a href="#section-10-4.3" class="pilcrow">¶</a>
</li>
        <li class="ulEmpty normal" id="section-10-4.4">
Another possibility is to use a single distributed service for this function.
<span><a href="#I-D.jennings-vipr-overview" class="xref">Verification Involving PSTN Reachability (VIPR)</a> [<a href="#I-D.jennings-vipr-overview" class="xref">VIPR-OVERVIEW</a>]</span> proposed a
<span><a href="#RFC6940" class="xref">REsource LOcation And Discovery (RELOAD)</a> [<a href="#RFC6940" class="xref">RFC6940</a>]</span> usage for telephone numbers to help direct calls to enterprises on the Internet. It would be possible to describe a similar RELOAD usage
to identify the CPS where calls for a particular telephone number should be stored.
One advantage that the STIR architecture has over VIPR is that it assumes a credential system
that proves authority over telephone numbers; those credentials could be used to determine
whether or not a CPS could legitimately claim to be the proper store for a given telephone
number.<a href="#section-10-4.4" class="pilcrow">¶</a>
</li>
      </ul>
<p id="section-10-5">
This document does not prescribe any single way to do service discovery for a CPS;
it is envisioned that initial deployments will provision the location of the CPS
at the authentication service and verification service.<a href="#section-10-5" class="pilcrow">¶</a></p>
</section>
</div>
<div id="lookup">
<section id="section-11">
      <h2 id="name-encryption-key-lookup">
<a href="#section-11" class="section-number selfRef">11. </a><a href="#name-encryption-key-lookup" class="section-name selfRef">Encryption Key Lookup</a>
      </h2>
<p id="section-11-1">
   In order to encrypt a PASSporT (see <a href="#stor" class="xref">Section 6.1</a>), the caller needs access to the callee's
   public encryption key. Note that because STIR uses the Elliptic Curve Digital Signature Algorithm (ECDSA)
   for signing PASSporTs, the public key used to
   verify PASSporTs is not suitable for this function, and thus the encryption key
   must be discovered separately. This requires some sort
   of directory/lookup system.<a href="#section-11-1" class="pilcrow">¶</a></p>
<p id="section-11-2">
   Some initial STIR deployments have fielded certificate repositories so that verification services
can acquire the signing credentials for PASSporTs, which are linked through a URI in the "x5u" element of the PASSporT.
These certificate repositories could clearly be repurposed for allowing authentication services to download
the public encryption key for the called party -- provided they can be discovered by calling parties.
   This document does not specify any
   particular discovery scheme, but instead offers some general guidance about potential approaches.<a href="#section-11-2" class="pilcrow">¶</a></p>
<p id="section-11-3">
   It is a desirable property that the public encryption key for a given party
be linked to their STIR credential. An <span><a href="#RFC7748" class="xref">Elliptic Curve Diffie-Hellman (ECDH)</a> [<a href="#RFC7748" class="xref">RFC7748</a>]</span>
public-private key pair might be generated for a
<span><a href="#I-D.ietf-tls-subcerts" class="xref">subcert</a> [<a href="#I-D.ietf-tls-subcerts" class="xref">TLS-SUBCERTS</a>]</span> of the STIR credential.
That subcert could be looked up along with the STIR credential of the called party.
Further details of this subcert, and the exact lookup mechanism involved, are deferred for future protocol work.<a href="#section-11-3" class="pilcrow">¶</a></p>
<p id="section-11-4">
   Obviously, if there is a single central database that the caller and
   callee each access in real time to download the other's
   keys, then this represents a real privacy risk, as the central
   key database learns about each call.  A number of mechanisms are
   potentially available to mitigate this:<a href="#section-11-4" class="pilcrow">¶</a></p>
<ul class="ulEmpty normal">
<li class="ulEmpty normal" id="section-11-5.1">
 Have endpoints pre-fetch keys for potential counterparties
      (e.g., their address book or the entire database).<a href="#section-11-5.1" class="pilcrow">¶</a>
</li>
        <li class="ulEmpty normal" id="section-11-5.2">
  Have caching servers in the user's network that proxy their
      fetches and thus conceal the relationship between the user and the
      keys they are fetching.<a href="#section-11-5.2" class="pilcrow">¶</a>
</li>
      </ul>
<p id="section-11-6">
  Clearly, there is a privacy/timeliness trade-off in that getting
    up-to-date knowledge about credential validity requires
   contacting the credential directory in real-time (e.g., via the
   <span><a href="#RFC6960" class="xref">Online Certificate Status Protocol (OCSP)</a> [<a href="#RFC6960" class="xref">RFC6960</a>]</span>).
   This is somewhat mitigated for the caller's credentials in that he
   can get short-term credentials right before placing a call which only
   reveals his calling rate, but not who he is calling.  Alternately,
   the CPS can verify the caller's credentials via OCSP, though of
   course this requires the callee to trust the CPS's verification.
   This approach does not work as well for the callee's credentials, but
   the risk there is more modest since an attacker would need to both
   have the callee's credentials and regularly poll the database for
   every potential caller.<a href="#section-11-6" class="pilcrow">¶</a></p>
<p id="section-11-7">
   We consider the exact best point in the trade-off space to be an open
   issue.<a href="#section-11-7" class="pilcrow">¶</a></p>
</section>
</div>
<div id="IANA">
<section id="section-12">
      <h2 id="name-iana-considerations">
<a href="#section-12" class="section-number selfRef">12. </a><a href="#name-iana-considerations" class="section-name selfRef">IANA Considerations</a>
      </h2>
<p id="section-12-1">This document has no IANA actions.<a href="#section-12-1" class="pilcrow">¶</a></p>
</section>
</div>
<div id="priv">
<section id="section-13">
      <h2 id="name-privacy-considerations">
<a href="#section-13" class="section-number selfRef">13. </a><a href="#name-privacy-considerations" class="section-name selfRef">Privacy Considerations</a>
      </h2>
<p id="section-13-1">
    Delivering PASSporTs out-of-band offers a different set of privacy properties
than traditional in-band STIR. In-band operations convey
PASSporTs as headers in SIP messages in cleartext, which any
forwarding intermediaries can potentially inspect. By contrast, out-of-band
    STIR stores these PASSporTs at a service after encrypting them
as described in <a href="#authz" class="xref">Section 6</a>, effectively creating a path
    between the authentication and verification service in which the CPS
is the sole intermediary, but the CPS cannot read the PASSporTs.
    Potentially, out-of-band PASSporT delivery could thus improve on the privacy story of STIR.<a href="#section-13-1" class="pilcrow">¶</a></p>
<p id="section-13-2">
    The principle actors in the operation of out-of-band are the
AS, VS, and CPS. The
AS and VS functions differ from baseline behavior <span>[<a href="#RFC8224" class="xref">RFC8224</a>]</span>,
in that they interact with a CPS over a non-SIP interface,
of which the REST interface in <a href="#web" class="xref">Section 9</a> serves as an example.
Some out-of-band deployments may also require a discovery service for the
CPS itself (<a href="#cps" class="xref">Section 10</a>) and/or encryption keys
(<a href="#lookup" class="xref">Section 11</a>). Even with encrypted PASSporTs,
the network interactions by which the AS and VS interact with the CPS, and
to a lesser extent any discovery services, thus create potential opportunities
for data leakage about calling and called parties.<a href="#section-13-2" class="pilcrow">¶</a></p>
<p id="section-13-3">
    The process of storing and retrieving PASSporTs at a CPS can itself
reveal information about calls being placed. The mechanism takes
    care not to require that the AS authenticate itself to the CPS,
relying instead on a blind signature mechanism for flood control prevention.
<a href="#sub" class="xref">Section 7.4</a>
    discusses the practice of storing "dummy" PASSporTs at random intervals
to thwart traffic analysis, and as <a href="#vs" class="xref">Section 8.2</a> notes, a CPS is required to
    return a dummy PASSporT even if there is no PASSporT indexed for
that calling number, which similarly enables the retrieval side to
randomly request PASSporTs when there are no calls in progress.
Note that the caller's IP address itself leaks information about the caller.
Proxying the storage of the CPS through some third party could help prevent
this attack. It might also be possible to use a more sophisticated system
such as Riposte <span>[<a href="#RIPOSTE" class="xref">RIPOSTE</a>]</span>.
These measures can help to mitigate information disclosure in the system.
In implementations that require service discovery
(see <a href="#cps" class="xref">Section 10</a>), perhaps through key discovery
(<a href="#lookup" class="xref">Section 11</a>), similar measures could be used
to make sure that service discovery does not itself disclose information about calls.<a href="#section-13-3" class="pilcrow">¶</a></p>
<p id="section-13-4">
    Ultimately, this document only provides a framework for future implementation
of out-of-band systems, and the privacy properties of a given implementation will
depend on architectural assumptions made in those environments. More closed
systems for intranet operations may adopt a weaker security posture but
otherwise mitigate the risks of information disclosure, whereas more open environments
will require careful implementation of the practices described here.<a href="#section-13-4" class="pilcrow">¶</a></p>
<p id="section-13-5">
    For general privacy risks associated with the operations of STIR,
also see the privacy considerations covered in <span><a href="https://www.rfc-editor.org/rfc/rfc8224#section-11" class="relref">Section 11</a> of [<a href="#RFC8224" class="xref">RFC8224</a>]</span>.<a href="#section-13-5" class="pilcrow">¶</a></p>
</section>
</div>
<div id="Security">
<section id="section-14">
      <h2 id="name-security-considerations">
<a href="#section-14" class="section-number selfRef">14. </a><a href="#name-security-considerations" class="section-name selfRef">Security Considerations</a>
      </h2>
<p id="section-14-1">This entire document is about security, but the detailed security
   properties will vary depending on how the framework is applied and deployed. General guidance for dealing
      with the most obvious security challenges posed by this framework is given in
Sections <a href="#sec" class="xref">7.3</a> and <a href="#sub" class="xref">7.4</a>,
along proposed solutions for problems like denial-of-service attacks or traffic analysis against the CPS.<a href="#section-14-1" class="pilcrow">¶</a></p>
<p id="section-14-2">Although there are considerable security challenges associated with
widespread deployment of a public CPS, those must be weighed against the
potential usefulness of a service that delivers a STIR assurance without
requiring the passage of end-to-end SIP. Ultimately, the security properties
of this mechanism are at least comparable to in-band
  STIR: the substitution attack documented in <a href="#sub" class="xref">Section 7.4</a>
could be implemented by any in-band SIP intermediary or eavesdropper who
happened to see the PASSporT in transit, say, and launched its own call with a
copy of that PASSporT to race against the original to the destination.<a href="#section-14-2" class="pilcrow">¶</a></p>
</section>
</div>
<section id="section-15">
      <h2 id="name-informative-references">
<a href="#section-15" class="section-number selfRef">15. </a><a href="#name-informative-references" class="section-name selfRef">Informative References</a>
      </h2>
<dl class="references">
<dt id="I-D.ietf-modern-teri">[MODERN-TERI]</dt>
      <dd>
<span class="refAuthor">Peterson, J.</span>, <span class="refTitle">"An Architecture and Information Model for Telephone-Related Information (TeRI)"</span>, <span class="refContent">Work in Progress</span>, <span class="seriesInfo">Internet-Draft, draft-ietf-modern-teri-00</span>, <time datetime="2018-07-02" class="refDate">2 July 2018</time>, <span>&lt;<a href="https://tools.ietf.org/html/draft-ietf-modern-teri-00">https://tools.ietf.org/html/draft-ietf-modern-teri-00</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="I-D.ietf-stir-passport-divert">[PASSPORT-DIVERT]</dt>
      <dd>
<span class="refAuthor">Peterson, J.</span>, <span class="refTitle">"PASSporT Extension for Diverted Calls"</span>, <span class="refContent">Work in Progress</span>, <span class="seriesInfo">Internet-Draft, draft-ietf-stir-passport-divert-09</span>, <time datetime="2020-07-13" class="refDate">13 July 2020</time>, <span>&lt;<a href="https://tools.ietf.org/html/draft-ietf-stir-passport-divert-09">https://tools.ietf.org/html/draft-ietf-stir-passport-divert-09</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="I-D.privacy-pass">[PRIVACY-PASS]</dt>
      <dd>
<span class="refAuthor">Davidson, A.</span><span class="refAuthor"> and N. Sullivan</span>, <span class="refTitle">"The Privacy Pass Protocol"</span>, <span class="refContent">Work in Progress</span>, <span class="seriesInfo">Internet-Draft, draft-privacy-pass-00</span>, <time datetime="2019-11-03" class="refDate">3 November 2019</time>, <span>&lt;<a href="https://tools.ietf.org/html/draft-privacy-pass-00">https://tools.ietf.org/html/draft-privacy-pass-00</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="REST">[REST]</dt>
      <dd>
<span class="refAuthor">Fielding, R.</span>, <span class="refTitle">"Architectural Styles and the Design of Network-based Software Architectures, Chapter 5: Representational State Transfer"</span>, <span class="seriesInfo">Ph.D. Dissertation, University of California, Irvine</span>, <time datetime="2010" class="refDate">2010</time>. </dd>
<dd class="break"></dd>
<dt id="RFC2119">[RFC2119]</dt>
      <dd>
<span class="refAuthor">Bradner, S.</span>, <span class="refTitle">"Key words for use in RFCs to Indicate Requirement Levels"</span>, <span class="seriesInfo">BCP 14</span>, <span class="seriesInfo">RFC 2119</span>, <span class="seriesInfo">DOI 10.17487/RFC2119</span>, <time datetime="1997-03" class="refDate">March 1997</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc2119">https://www.rfc-editor.org/info/rfc2119</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC3261">[RFC3261]</dt>
      <dd>
<span class="refAuthor">Rosenberg, J.</span><span class="refAuthor">, Schulzrinne, H.</span><span class="refAuthor">, Camarillo, G.</span><span class="refAuthor">, Johnston, A.</span><span class="refAuthor">, Peterson, J.</span><span class="refAuthor">, Sparks, R.</span><span class="refAuthor">, Handley, M.</span><span class="refAuthor">, and E. Schooler</span>, <span class="refTitle">"SIP: Session Initiation Protocol"</span>, <span class="seriesInfo">RFC 3261</span>, <span class="seriesInfo">DOI 10.17487/RFC3261</span>, <time datetime="2002-06" class="refDate">June 2002</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc3261">https://www.rfc-editor.org/info/rfc3261</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC5636">[RFC5636]</dt>
      <dd>
<span class="refAuthor">Park, S.</span><span class="refAuthor">, Park, H.</span><span class="refAuthor">, Won, Y.</span><span class="refAuthor">, Lee, J.</span><span class="refAuthor">, and S. Kent</span>, <span class="refTitle">"Traceable Anonymous Certificate"</span>, <span class="seriesInfo">RFC 5636</span>, <span class="seriesInfo">DOI 10.17487/RFC5636</span>, <time datetime="2009-08" class="refDate">August 2009</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc5636">https://www.rfc-editor.org/info/rfc5636</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC6116">[RFC6116]</dt>
      <dd>
<span class="refAuthor">Bradner, S.</span><span class="refAuthor">, Conroy, L.</span><span class="refAuthor">, and K. Fujiwara</span>, <span class="refTitle">"The E.164 to Uniform Resource Identifiers (URI) Dynamic Delegation Discovery System (DDDS) Application (ENUM)"</span>, <span class="seriesInfo">RFC 6116</span>, <span class="seriesInfo">DOI 10.17487/RFC6116</span>, <time datetime="2011-03" class="refDate">March 2011</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc6116">https://www.rfc-editor.org/info/rfc6116</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC6940">[RFC6940]</dt>
      <dd>
<span class="refAuthor">Jennings, C.</span><span class="refAuthor">, Lowekamp, B., Ed.</span><span class="refAuthor">, Rescorla, E.</span><span class="refAuthor">, Baset, S.</span><span class="refAuthor">, and H. Schulzrinne</span>, <span class="refTitle">"REsource LOcation And Discovery (RELOAD) Base Protocol"</span>, <span class="seriesInfo">RFC 6940</span>, <span class="seriesInfo">DOI 10.17487/RFC6940</span>, <time datetime="2014-01" class="refDate">January 2014</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc6940">https://www.rfc-editor.org/info/rfc6940</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC6960">[RFC6960]</dt>
      <dd>
<span class="refAuthor">Santesson, S.</span><span class="refAuthor">, Myers, M.</span><span class="refAuthor">, Ankney, R.</span><span class="refAuthor">, Malpani, A.</span><span class="refAuthor">, Galperin, S.</span><span class="refAuthor">, and C. Adams</span>, <span class="refTitle">"X.509 Internet Public Key Infrastructure Online Certificate Status Protocol - OCSP"</span>, <span class="seriesInfo">RFC 6960</span>, <span class="seriesInfo">DOI 10.17487/RFC6960</span>, <time datetime="2013-06" class="refDate">June 2013</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc6960">https://www.rfc-editor.org/info/rfc6960</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC7258">[RFC7258]</dt>
      <dd>
<span class="refAuthor">Farrell, S.</span><span class="refAuthor"> and H. Tschofenig</span>, <span class="refTitle">"Pervasive Monitoring Is an Attack"</span>, <span class="seriesInfo">BCP 188</span>, <span class="seriesInfo">RFC 7258</span>, <span class="seriesInfo">DOI 10.17487/RFC7258</span>, <time datetime="2014-05" class="refDate">May 2014</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc7258">https://www.rfc-editor.org/info/rfc7258</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC7340">[RFC7340]</dt>
      <dd>
<span class="refAuthor">Peterson, J.</span><span class="refAuthor">, Schulzrinne, H.</span><span class="refAuthor">, and H. Tschofenig</span>, <span class="refTitle">"Secure Telephone Identity Problem Statement and Requirements"</span>, <span class="seriesInfo">RFC 7340</span>, <span class="seriesInfo">DOI 10.17487/RFC7340</span>, <time datetime="2014-09" class="refDate">September 2014</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc7340">https://www.rfc-editor.org/info/rfc7340</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC7748">[RFC7748]</dt>
      <dd>
<span class="refAuthor">Langley, A.</span><span class="refAuthor">, Hamburg, M.</span><span class="refAuthor">, and S. Turner</span>, <span class="refTitle">"Elliptic Curves for Security"</span>, <span class="seriesInfo">RFC 7748</span>, <span class="seriesInfo">DOI 10.17487/RFC7748</span>, <time datetime="2016-01" class="refDate">January 2016</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc7748">https://www.rfc-editor.org/info/rfc7748</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC8174">[RFC8174]</dt>
      <dd>
<span class="refAuthor">Leiba, B.</span>, <span class="refTitle">"Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words"</span>, <span class="seriesInfo">BCP 14</span>, <span class="seriesInfo">RFC 8174</span>, <span class="seriesInfo">DOI 10.17487/RFC8174</span>, <time datetime="2017-05" class="refDate">May 2017</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8174">https://www.rfc-editor.org/info/rfc8174</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC8224">[RFC8224]</dt>
      <dd>
<span class="refAuthor">Peterson, J.</span><span class="refAuthor">, Jennings, C.</span><span class="refAuthor">, Rescorla, E.</span><span class="refAuthor">, and C. Wendt</span>, <span class="refTitle">"Authenticated Identity Management in the Session Initiation Protocol (SIP)"</span>, <span class="seriesInfo">RFC 8224</span>, <span class="seriesInfo">DOI 10.17487/RFC8224</span>, <time datetime="2018-02" class="refDate">February 2018</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8224">https://www.rfc-editor.org/info/rfc8224</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC8225">[RFC8225]</dt>
      <dd>
<span class="refAuthor">Wendt, C.</span><span class="refAuthor"> and J. Peterson</span>, <span class="refTitle">"PASSporT: Personal Assertion Token"</span>, <span class="seriesInfo">RFC 8225</span>, <span class="seriesInfo">DOI 10.17487/RFC8225</span>, <time datetime="2018-02" class="refDate">February 2018</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8225">https://www.rfc-editor.org/info/rfc8225</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC8226">[RFC8226]</dt>
      <dd>
<span class="refAuthor">Peterson, J.</span><span class="refAuthor"> and S. Turner</span>, <span class="refTitle">"Secure Telephone Identity Credentials: Certificates"</span>, <span class="seriesInfo">RFC 8226</span>, <span class="seriesInfo">DOI 10.17487/RFC8226</span>, <time datetime="2018-02" class="refDate">February 2018</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8226">https://www.rfc-editor.org/info/rfc8226</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC8446">[RFC8446]</dt>
      <dd>
<span class="refAuthor">Rescorla, E.</span>, <span class="refTitle">"The Transport Layer Security (TLS) Protocol Version 1.3"</span>, <span class="seriesInfo">RFC 8446</span>, <span class="seriesInfo">DOI 10.17487/RFC8446</span>, <time datetime="2018-08" class="refDate">August 2018</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8446">https://www.rfc-editor.org/info/rfc8446</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RIPOSTE">[RIPOSTE]</dt>
      <dd>
<span class="refAuthor">Corrigan-Gibbs, H.</span><span class="refAuthor">, Boneh, D.</span><span class="refAuthor">, and D. Mazières</span>, <span class="refTitle">"Riposte: An Anonymous Messaging System Handling Millions of Users"</span>, <time datetime="2015-05" class="refDate">May 2015</time>, <span>&lt;<a href="https://people.csail.mit.edu/henrycg/pubs/oakland15riposte/">https://people.csail.mit.edu/henrycg/pubs/oakland15riposte/</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="I-D.ietf-tls-subcerts">[TLS-SUBCERTS]</dt>
      <dd>
<span class="refAuthor">Barnes, R.</span><span class="refAuthor">, Iyengar, S.</span><span class="refAuthor">, Sullivan, N.</span><span class="refAuthor">, and E. Rescorla</span>, <span class="refTitle">"Delegated Credentials for TLS"</span>, <span class="refContent">Work in Progress</span>, <span class="seriesInfo">Internet-Draft, draft-ietf-tls-subcerts-10</span>, <time datetime="2021-01-24" class="refDate">24 January 2021</time>, <span>&lt;<a href="https://tools.ietf.org/html/draft-ietf-tls-subcerts-10">https://tools.ietf.org/html/draft-ietf-tls-subcerts-10</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="I-D.jennings-vipr-overview">[VIPR-OVERVIEW]</dt>
    <dd>
<span class="refAuthor">Barnes, M.</span><span class="refAuthor">, Jennings, C.</span><span class="refAuthor">, Rosenberg, J.</span><span class="refAuthor">, and M. Petit-Huguenin</span>, <span class="refTitle">"Verification Involving PSTN Reachability: Requirements and Architecture Overview"</span>, <span class="refContent">Work in Progress</span>, <span class="seriesInfo">Internet-Draft, draft-jennings-vipr-overview-06</span>, <time datetime="2013-12-09" class="refDate">9 December 2013</time>, <span>&lt;<a href="https://tools.ietf.org/html/draft-jennings-vipr-overview-06">https://tools.ietf.org/html/draft-jennings-vipr-overview-06</a>&gt;</span>. </dd>
<dd class="break"></dd>
</dl>
</section>
<div id="Acknowledgments">
<section id="section-appendix.a">
      <h2 id="name-acknowledgments">
<a href="#name-acknowledgments" class="section-name selfRef">Acknowledgments</a>
      </h2>
<p id="section-appendix.a-1">The ideas
   in this document came out of discussions with <span class="contact-name">Richard Barnes</span> and
   <span class="contact-name">Cullen Jennings</span>. We'd also like to thank
   <span class="contact-name">Russ Housley</span>, <span class="contact-name">Chris Wendt</span>,
   <span class="contact-name">Eric Burger</span>, <span class="contact-name">Mary Barnes</span>,
   <span class="contact-name">Ben Campbell</span>, <span class="contact-name">Ted Huang</span>,
   <span class="contact-name">Jonathan Rosenberg</span>, and <span class="contact-name">Robert Sparks</span>
   for helpful suggestions.<a href="#section-appendix.a-1" class="pilcrow">¶</a></p>
</section>
</div>
<div id="authors-addresses">
<section id="section-appendix.b">
      <h2 id="name-authors-addresses">
<a href="#name-authors-addresses" class="section-name selfRef">Authors' Addresses</a>
      </h2>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Eric Rescorla</span></div>
<div dir="auto" class="left"><span class="org">Mozilla</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:ekr@rtfm.com" class="email">ekr@rtfm.com</a>
</div>
</address>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Jon Peterson</span></div>
<div dir="auto" class="left"><span class="org">Neustar, Inc.</span></div>
<div dir="auto" class="left"><span class="street-address">1800 Sutter St Suite 570</span></div>
<div dir="auto" class="left">
<span class="locality">Concord</span>, <span class="region">CA</span> <span class="postal-code">94520</span>
</div>
<div dir="auto" class="left"><span class="country-name">United States of America</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:jon.peterson@team.neustar" class="email">jon.peterson@team.neustar</a>
</div>
</address>
</section>
</div>
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