<pre>Internet Engineering Task Force (IETF)                 K. Bhargavan, Ed.
Request for Comments: 7627                            A. Delignat-Lavaud
Updates: <a href="./rfc5246">5246</a>                                                 A. Pironti
Category: Standards Track                       Inria Paris-Rocquencourt
ISSN: 2070-1721                                               A. Langley
                                                             Google Inc.
                                                                  M. Ray
                                                         Microsoft Corp.
                                                          September 2015


            <span class="h1">Transport Layer Security (TLS) Session Hash and</span>
                    <span class="h1">Extended Master Secret Extension</span>

Abstract

   The Transport Layer Security (TLS) master secret is not
   cryptographically bound to important session parameters such as the
   server certificate.  Consequently, it is possible for an active
   attacker to set up two sessions, one with a client and another with a
   server, such that the master secrets on the two sessions are the
   same.  Thereafter, any mechanism that relies on the master secret for
   authentication, including session resumption, becomes vulnerable to a
   man-in-the-middle attack, where the attacker can simply forward
   messages back and forth between the client and server.  This
   specification defines a TLS extension that contextually binds the
   master secret to a log of the full handshake that computes it, thus
   preventing such attacks.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in <a href="./rfc5741#section-2">Section&nbsp;2 of RFC 5741</a>.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   <a href="http://www.rfc-editor.org/info/rfc7627">http://www.rfc-editor.org/info/rfc7627</a>.









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Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   <a href="#section-1">1</a>. Introduction ....................................................<a href="#page-3">3</a>
   <a href="#section-2">2</a>. Requirements Notation ...........................................<a href="#page-5">5</a>
   <a href="#section-3">3</a>. The TLS Session Hash ............................................<a href="#page-5">5</a>
   <a href="#section-4">4</a>. The Extended Master Secret ......................................<a href="#page-6">6</a>
   <a href="#section-5">5</a>. Extension Negotiation ...........................................<a href="#page-6">6</a>
      <a href="#section-5.1">5.1</a>. Extension Definition .......................................<a href="#page-6">6</a>
      <a href="#section-5.2">5.2</a>. Client and Server Behavior: Full Handshake .................<a href="#page-7">7</a>
      <a href="#section-5.3">5.3</a>. Client and Server Behavior: Abbreviated Handshake ..........<a href="#page-7">7</a>
      <a href="#section-5.4">5.4</a>. Interoperability Considerations ............................<a href="#page-9">9</a>
   <a href="#section-6">6</a>. Security Considerations .........................................<a href="#page-9">9</a>
      <a href="#section-6.1">6.1</a>. Triple Handshake Preconditions and Impact ..................<a href="#page-9">9</a>
      <a href="#section-6.2">6.2</a>. Cryptographic Properties of the Hash Function .............<a href="#page-11">11</a>
      <a href="#section-6.3">6.3</a>. Handshake Messages Included in the Session Hash ...........<a href="#page-11">11</a>
      <a href="#section-6.4">6.4</a>. No SSL 3.0 Support ........................................<a href="#page-12">12</a>
   <a href="#section-7">7</a>. IANA Considerations ............................................<a href="#page-12">12</a>
   <a href="#section-8">8</a>. References .....................................................<a href="#page-12">12</a>
      <a href="#section-8.1">8.1</a>. Normative References ......................................<a href="#page-12">12</a>
      <a href="#section-8.2">8.2</a>. Informative References ....................................<a href="#page-13">13</a>
   Acknowledgments ...................................................<a href="#page-14">14</a>
   Authors' Addresses ................................................<a href="#page-15">15</a>














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<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   In TLS [<a href="./rfc5246" title="&quot;The Transport Layer Security (TLS) Protocol Version 1.2&quot;">RFC5246</a>], every session has a "master_secret" computed as:

   master_secret = PRF(pre_master_secret, "master secret",
                       ClientHello.random + ServerHello.random)
                       [0..47];

   where the "pre_master_secret" is the result of some key exchange
   protocol.  For example, when the handshake uses an RSA ciphersuite,
   this value is generated uniformly at random by the client, whereas
   for Ephemeral Diffie-Hellman (DHE) ciphersuites, it is the result of
   a Diffie-Hellman key agreement.

   As described in [<a href="#ref-TRIPLE-HS" title="&quot;Triple Handshakes and Cookie Cutters: Breaking and Fixing Authentication over TLS&quot;">TRIPLE-HS</a>], in both the RSA and DHE key exchanges,
   an active attacker can synchronize two TLS sessions so that they
   share the same "master_secret".  For an RSA key exchange where the
   client is unauthenticated, this is achieved as follows.  Suppose a
   client C connects to a server A.  C does not realize that A is
   malicious and that A connects in the background to an honest server S
   and completes both handshakes.  For simplicity, assume that C and S
   only use RSA ciphersuites.

   1.  C sends a "ClientHello" to A, and A forwards it to S.

   2.  S sends a "ServerHello" to A, and A forwards it to C.

   3.  S sends a "Certificate", containing its certificate chain, to A.
       A replaces it with its own certificate chain and sends it to C.

   4.  S sends a "ServerHelloDone" to A, and A forwards it to C.

   5.  C sends a "ClientKeyExchange" to A, containing the
       "pre_master_secret", encrypted with A's public key.  A decrypts
       the "pre_master_secret", re-encrypts it with S's public key, and
       sends it on to S.

   6.  C sends a "Finished" to A.  A computes a "Finished" for its
       connection with S and sends it to S.

   7.  S sends a "Finished" to A.  A computes a "Finished" for its
       connection with C and sends it to C.

   At this point, both connections (between C and A, and between A and
   S) have new sessions that share the same "pre_master_secret",
   "ClientHello.random", "ServerHello.random", as well as other session
   parameters, including the session identifier and, optionally, the
   session ticket.  Hence, the "master_secret" value will be equal for



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   the two sessions and will be associated both at C and S with the same
   session ID, even though the server identities on the two connections
   are different.  Recall that C only sees A's certificate and is
   unaware of A's connection with S.  Moreover, the record keys on the
   two connections will also be the same.

   The scenario above shows that TLS does not guarantee that the master
   secrets and keys used on different connections will be different.
   Even if client authentication is used, the scenario still works,
   except that the two sessions now differ on both client and server
   identities.

   A similar scenario can be achieved when the handshake uses a DHE
   ciphersuite.  Note that even if the client or server does not prefer
   using RSA or DHE, the attacker can force them to use it by offering
   only RSA or DHE in its hello messages.  Handshakes using Ephemeral
   Elliptic Curve Diffie-Hellman (ECDHE) ciphersuites are also
   vulnerable if they allow arbitrary explicit curves or use curves with
   small subgroups.  Against more powerful adversaries, other key
   exchanges, such as Secure Remote Password (SRP) and Pre-Shared Key
   (PSK), have also been shown to be vulnerable [<a href="#ref-VERIFIED-BINDINGS">VERIFIED-BINDINGS</a>].

   Once A has synchronized the two connections, since the keys are the
   same on the two sides, it can step away and transparently forward
   messages between C and S, reading and modifying when it desires.  In
   the key exchange literature, such occurrences are called unknown key-
   share attacks, since C and S share a secret but they both think that
   their secret is shared only with A.  In themselves, these attacks do
   not break integrity or confidentiality between honest parties, but
   they offer a useful starting point from which to mount impersonation
   attacks on C and S.

   Suppose C tries to resume its session on a new connection with A.  A
   can then resume its session with S on a new connection and forward
   the abbreviated handshake messages unchanged between C and S.  Since
   the abbreviated handshake only relies on the master secret for
   authentication and does not mention client or server identities, both
   handshakes complete successfully, resulting in the same session keys
   and the same handshake log.  A still knows the connection keys and
   can send messages to both C and S.

   Critically, at the new connection, even the handshake log is the same
   on C and S, thus defeating any man-in-the-middle protection scheme
   that relies on the uniqueness of finished messages, such as the
   secure renegotiation indication extension [<a href="./rfc5746" title="&quot;Transport Layer Security (TLS) Renegotiation Indication Extension&quot;">RFC5746</a>] or TLS channel
   bindings [<a href="./rfc5929" title="&quot;Channel Bindings for TLS&quot;">RFC5929</a>].  [<a href="#ref-TRIPLE-HS" title="&quot;Triple Handshakes and Cookie Cutters: Breaking and Fixing Authentication over TLS&quot;">TRIPLE-HS</a>] describes several exploits based on
   such session synchronization attacks.  In particular, it describes a
   man-in-the-middle attack, called the "triple handshake", that



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   circumvents the protections of [<a href="./rfc5746" title="&quot;Transport Layer Security (TLS) Renegotiation Indication Extension&quot;">RFC5746</a>] to break client-
   authenticated TLS renegotiation after session resumption.  Similar
   attacks apply to application-level authentication mechanisms that
   rely on channel bindings [<a href="./rfc5929" title="&quot;Channel Bindings for TLS&quot;">RFC5929</a>] or on key material exported from
   TLS [<a href="./rfc5705" title="&quot;Keying Material Exporters for Transport Layer Security (TLS)&quot;">RFC5705</a>].

   The underlying protocol issue leading to these attacks is that the
   TLS master secret is not guaranteed to be unique across sessions,
   since it is not context-bound to the full handshake that generated
   it.  If we fix this problem in the initial master secret computation,
   then all these attacks can be prevented.  This specification
   introduces a TLS extension that changes the way the "master_secret"
   value is computed in a full handshake by including the log of the
   handshake messages, so that different sessions will, by construction,
   have different master secrets.  This prevents the attacks described
   in [<a href="#ref-TRIPLE-HS" title="&quot;Triple Handshakes and Cookie Cutters: Breaking and Fixing Authentication over TLS&quot;">TRIPLE-HS</a>] and documented in <a href="./rfc7457#section-2.11">Section&nbsp;2.11 of [RFC7457]</a>.

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Requirements Notation</span>

   This document uses the same notation and terminology used in the TLS
   protocol specification [<a href="./rfc5246" title="&quot;The Transport Layer Security (TLS) Protocol Version 1.2&quot;">RFC5246</a>].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in <a href="./rfc2119">RFC</a>
   <a href="./rfc2119">2119</a> [<a href="./rfc2119" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">RFC2119</a>].

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  The TLS Session Hash</span>

   When a full TLS handshake takes place, we define

         session_hash = Hash(handshake_messages)

   where "handshake_messages" refers to all handshake messages sent or
   received, starting at the ClientHello up to and including the
   ClientKeyExchange message, including the type and length fields of
   the handshake messages.  This is the concatenation of all the
   exchanged Handshake structures, as defined in <a href="./rfc5246#section-7.4">Section&nbsp;7.4 of
   [RFC5246]</a>.

   For TLS 1.2, the "Hash" function is the one defined in <a href="./rfc5246#section-7.4.9">Section&nbsp;7.4.9
   of [RFC5246]</a> for the Finished message computation.  For all previous
   versions of TLS, the "Hash" function computes the concatenation of
   MD5 and SHA1.

   There is no "session_hash" for resumed handshakes, as they do not
   lead to the creation of a new session.




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<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  The Extended Master Secret</span>

   When the extended master secret extension is negotiated in a full
   handshake, the "master_secret" is computed as

   master_secret = PRF(pre_master_secret, "extended master secret",
                       session_hash)
                       [0..47];

   The extended master secret computation differs from that described in
   [<a href="./rfc5246" title="&quot;The Transport Layer Security (TLS) Protocol Version 1.2&quot;">RFC5246</a>] in the following ways:

   o  The "extended master secret" label is used instead of "master
      secret".

   o  The "session_hash" is used instead of the "ClientHello.random" and
      "ServerHello.random".

   The "session_hash" depends upon a handshake log that includes
   "ClientHello.random" and "ServerHello.random", in addition to
   ciphersuites, key exchange information, and certificates (if any)
   from the client and server.  Consequently, the extended master secret
   depends upon the choice of all these session parameters.

   This design reflects the recommendation that keys should be bound to
   the security contexts that compute them [<a href="#ref-SP800-108" title="&quot;Recommendation for Key Derivation Using Pseudorandom Functions (Revised)&quot;">SP800-108</a>].  The technique
   of mixing a hash of the key exchange messages into master key
   derivation is already used in other well-known protocols such as
   Secure Shell (SSH) [<a href="./rfc4251" title="&quot;The Secure Shell (SSH) Protocol Architecture&quot;">RFC4251</a>].

   Clients and servers SHOULD NOT accept handshakes that do not use the
   extended master secret, especially if they rely on features like
   compound authentication that fall into the vulnerable cases described
   in <a href="#section-6.1">Section 6.1</a>.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Extension Negotiation</span>

<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>.  Extension Definition</span>

   This document defines a new TLS extension, "extended_master_secret"
   (with extension type 0x0017), which is used to signal both client and
   server to use the extended master secret computation.  The
   "extension_data" field of this extension is empty.  Thus, the entire
   encoding of the extension is 00 17 00 00 (in hexadecimal.)

   Although this document refers only to TLS, the extension proposed
   here can also be used with Datagram TLS (DTLS) [<a href="./rfc6347" title="&quot;Datagram Transport Layer Security Version 1.2&quot;">RFC6347</a>].




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   If the client and server agree on this extension and a full handshake
   takes place, both client and server MUST use the extended master
   secret derivation algorithm, as defined in <a href="#section-4">Section 4</a>.  All other
   cryptographic computations remain unchanged.

<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>.  Client and Server Behavior: Full Handshake</span>

   In the following, we use the phrase "abort the handshake" as
   shorthand for terminating the handshake by sending a fatal
   "handshake_failure" alert.

   In all handshakes, a client implementing this document MUST send the
   "extended_master_secret" extension in its ClientHello.

   If a server implementing this document receives the
   "extended_master_secret" extension, it MUST include the extension in
   its ServerHello message.

   If both the ClientHello and ServerHello contain the extension, the
   new session uses the extended master secret computation.

   If the server receives a ClientHello without the extension, it SHOULD
   abort the handshake if it does not wish to interoperate with legacy
   clients.  If it chooses to continue the handshake, then it MUST NOT
   include the extension in the ServerHello.

   If a client receives a ServerHello without the extension, it SHOULD
   abort the handshake if it does not wish to interoperate with legacy
   servers.

   If the client and server choose to continue a full handshake without
   the extension, they MUST use the standard master secret derivation
   for the new session.  In this case, the new session is not protected
   by the mechanisms described in this document.  So, implementers
   should follow the guidelines in <a href="#section-5.4">Section 5.4</a> to avoid dangerous usage
   scenarios.  In particular, the master secret derived from the new
   session should not be used for application-level authentication.

<span class="h3"><a class="selflink" id="section-5.3" href="#section-5.3">5.3</a>.  Client and Server Behavior: Abbreviated Handshake</span>

   The client SHOULD NOT offer an abbreviated handshake to resume a
   session that does not use an extended master secret.  Instead, it
   SHOULD offer a full handshake.

   If the client chooses to offer an abbreviated handshake even for such
   sessions in order to support legacy insecure resumption, then the
   current connection is not protected by the mechanisms in this
   document.  So, the client should follow the guidelines in <a href="#section-5.4">Section 5.4</a>



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   to avoid dangerous usage scenarios.  In particular, renegotiation is
   no longer secure on this connection, even if the client and server
   support the renegotiation indication extension [<a href="./rfc5746" title="&quot;Transport Layer Security (TLS) Renegotiation Indication Extension&quot;">RFC5746</a>].

   When offering an abbreviated handshake, the client MUST send the
   "extended_master_secret" extension in its ClientHello.

   If a server receives a ClientHello for an abbreviated handshake
   offering to resume a known previous session, it behaves as follows:

   o  If the original session did not use the "extended_master_secret"
      extension but the new ClientHello contains the extension, then the
      server MUST NOT perform the abbreviated handshake.  Instead, it
      SHOULD continue with a full handshake (as described in
      <a href="#section-5.2">Section 5.2</a>) to negotiate a new session.

   o  If the original session used the "extended_master_secret"
      extension but the new ClientHello does not contain it, the server
      MUST abort the abbreviated handshake.

   o  If neither the original session nor the new ClientHello uses the
      extension, the server SHOULD abort the handshake.  If it continues
      with an abbreviated handshake in order to support legacy insecure
      resumption, the connection is no longer protected by the
      mechanisms in this document, and the server should follow the
      guidelines in <a href="#section-5.4">Section 5.4</a>.

   o  If the new ClientHello contains the extension and the server
      chooses to continue the handshake, then the server MUST include
      the "extended_master_secret" extension in its ServerHello message.

   If a client receives a ServerHello that accepts an abbreviated
   handshake, it behaves as follows:

   o  If the original session did not use the "extended_master_secret"
      extension but the new ServerHello contains the extension, the
      client MUST abort the handshake.

   o  If the original session used the extension but the new ServerHello
      does not contain the extension, the client MUST abort the
      handshake.

   If the client and server continue the abbreviated handshake, they
   derive the connection keys for the new session as usual from the
   master secret of the original session.






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<span class="h3"><a class="selflink" id="section-5.4" href="#section-5.4">5.4</a>.  Interoperability Considerations</span>

   To allow interoperability with legacy clients and servers, a TLS peer
   may decide to accept full handshakes that use the legacy master
   secret computation.  If so, they need to differentiate between
   sessions that use legacy and extended master secrets by adding a flag
   to the session state.

   If a client or server chooses to continue with a full handshake
   without the extended master secret extension, then the new session
   becomes vulnerable to the man-in-the-middle key synchronization
   attack described in <a href="#section-1">Section 1</a>.  Hence, the client or server MUST NOT
   export any key material based on the new master secret for any
   subsequent application-level authentication.  In particular, it MUST
   disable [<a href="./rfc5705" title="&quot;Keying Material Exporters for Transport Layer Security (TLS)&quot;">RFC5705</a>] and any Extensible Authentication Protocol (EAP)
   relying on compound authentication [<a href="#ref-COMPOUND-AUTH">COMPOUND-AUTH</a>].

   If a client or server chooses to continue an abbreviated handshake to
   resume a session that does not use the extended master secret, then
   the current connection becomes vulnerable to a man-in-the-middle
   handshake log synchronization attack as described in <a href="#section-1">Section 1</a>.
   Hence, the client or server MUST NOT use the current handshake's
   "verify_data" for application-level authentication.  In particular,
   the client MUST disable renegotiation and any use of the "tls-unique"
   channel binding [<a href="./rfc5929" title="&quot;Channel Bindings for TLS&quot;">RFC5929</a>] on the current connection.

   If the original session uses an extended master secret but the
   ClientHello or ServerHello in the abbreviated handshake does not
   include the extension, it MAY be safe to continue the abbreviated
   handshake since it is protected by the extended master secret of the
   original session.  This scenario may occur, for example, when a
   server that implements this extension establishes a session but the
   session is subsequently resumed at a different server that does not
   support the extension.  Since such situations are unusual and likely
   to be the result of transient or inadvertent misconfigurations, this
   document recommends that the client and server MUST abort such
   handshakes.

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  Security Considerations</span>

<span class="h3"><a class="selflink" id="section-6.1" href="#section-6.1">6.1</a>.  Triple Handshake Preconditions and Impact</span>

   One way to mount a triple handshake attack is described in <a href="#section-1">Section 1</a>,
   along with a mention of the security mechanisms that break due to the
   attack; more in-depth discussion and diagrams can be found in
   [<a href="#ref-TRIPLE-HS" title="&quot;Triple Handshakes and Cookie Cutters: Breaking and Fixing Authentication over TLS&quot;">TRIPLE-HS</a>].  Here, some further discussion is presented about attack
   preconditions and impact.




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   To mount a triple handshake attack, it must be possible to force the
   same master secret on two different sessions.  For this to happen,
   two preconditions must be met:

   o  The client, C, must be willing to connect to a malicious server,
      A.  In certain contexts, like the web, this can be easily
      achieved, since a browser can be instructed to load content from
      an untrusted origin.

   o  The pre-master secret must be synchronized on the two sessions.
      This is particularly easy to achieve with the RSA and DHE key
      exchanges, but under some conditions, ECDHE, SRP, and PSK key
      exchanges can be exploited to this effect as well.

   Once the master secret is synchronized on two sessions, any security
   property that relies on the uniqueness of the master secret is
   compromised.  For example, a TLS exporter [<a href="./rfc5705" title="&quot;Keying Material Exporters for Transport Layer Security (TLS)&quot;">RFC5705</a>] no longer
   provides a unique key bound to the current session.

   TLS session resumption also relies on the uniqueness of the master
   secret to authenticate the resuming peers.  Hence, if a synchronized
   session is resumed, the peers cannot be sure about each other's
   identities, and the attacker knows the connection keys.  Clearly, a
   precondition to this step of the attack is that both client and
   server support session resumption (either via session identifier or
   session tickets [<a href="./rfc5077" title="&quot;Transport Layer Security (TLS) Session Resumption without Server-Side State&quot;">RFC5077</a>]).

   Additionally, in a synchronized abbreviated handshake, the whole
   transcript (which includes the "verify_data" values) is synchronized.
   So, after an abbreviated handshake, channel bindings like
   "tls-unique" [<a href="./rfc5929" title="&quot;Channel Bindings for TLS&quot;">RFC5929</a>] will not uniquely identify the connection
   anymore.

   Synchronization of the "verify_data" in abbreviated handshakes also
   undermines the security guarantees of the renegotiation indication
   extension [<a href="./rfc5746" title="&quot;Transport Layer Security (TLS) Renegotiation Indication Extension&quot;">RFC5746</a>], re-enabling a prefix-injection flaw similar to
   the renegotiation attack [<a href="#ref-Ray09" title="&quot;Authentication Gap in TLS Renegotiation&quot;">Ray09</a>].  However, in a triple handshake
   attack, the client sees the server certificate changing across
   different full handshakes.  Hence, a precondition to mount this stage
   of the attack is that the client accepts different certificates at
   each handshake, even if their common names do not match.  Before the
   triple handshake attack was discovered, this used to be widespread
   behavior, at least among some web browsers; such browsers were hence
   vulnerable to the attack.

   The extended master secret extension thwarts triple handshake attacks
   at their first stage by ensuring that different sessions necessarily
   end up with different master secret values.  Hence, all security



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   properties relying on the uniqueness of the master secret are now
   expected to hold.  In particular, if a TLS session is protected by
   the extended master secret extension, it is safe to resume it, to use
   its channel bindings, and to allow for certificate changes across
   renegotiation, meaning that all certificates are controlled by the
   same peer.  A symbolic cryptographic protocol analysis justifying the
   extended master secret extension appears in [<a href="#ref-VERIFIED-BINDINGS">VERIFIED-BINDINGS</a>].

<span class="h3"><a class="selflink" id="section-6.2" href="#section-6.2">6.2</a>.  Cryptographic Properties of the Hash Function</span>

   The session hashes of two different sessions need to be distinct;
   hence, the "Hash" function used to compute the "session_hash" needs
   to be collision resistant.  As such, hash functions such as MD5 or
   SHA1 are NOT RECOMMENDED.

   We observe that the "Hash" function used in the Finished message
   computation already needs to be collision resistant for the
   renegotiation indication extension [<a href="./rfc5746" title="&quot;Transport Layer Security (TLS) Renegotiation Indication Extension&quot;">RFC5746</a>] to work, because a
   meaningful collision on the handshake messages (and hence on the
   "verify_data") may re-enable the renegotiation attack [<a href="#ref-Ray09" title="&quot;Authentication Gap in TLS Renegotiation&quot;">Ray09</a>].

   The hash function used to compute the session hash depends on the TLS
   protocol version.  All current ciphersuites defined for TLS 1.2 use
   SHA256 or better, and so does the session hash.  For earlier versions
   of the protocol, only MD5 and SHA1 can be assumed to be supported,
   and this document does not require legacy implementations to add
   support for new hash functions.  In these versions, the session hash
   uses the concatenation of MD5 and SHA1, as in the Finished message.

<span class="h3"><a class="selflink" id="section-6.3" href="#section-6.3">6.3</a>.  Handshake Messages Included in the Session Hash</span>

   The "session_hash" is intended to encompass all relevant session
   information, including ciphersuite negotiation, key exchange
   messages, and client and server identities.  The hash is needed to
   compute the extended master secret and hence must be available before
   the Finished messages.

   This document sets the "session_hash" to cover all handshake messages
   up to and including the ClientKeyExchange.  For existing TLS
   ciphersuites, these messages include all the significant contents of
   the new session -- CertificateVerify does not change the session
   content.  At the same time, this allows the extended master secret to
   be computed immediately after the pre-master secret, so that
   implementations can shred the temporary pre-master secret from memory
   as early as possible.






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   It is possible that new ciphersuites or TLS extensions may include
   additional messages between ClientKeyExchange and Finished that add
   important session context.  In such cases, some of the security
   guarantees of this specification may no longer apply, and new man-in-
   the-middle attacks may be possible.  For example, if the client and
   server support the session ticket extension [<a href="./rfc5077" title="&quot;Transport Layer Security (TLS) Session Resumption without Server-Side State&quot;">RFC5077</a>], the session
   hash does not cover the new session ticket sent by the server.
   Hence, a man-in-the-middle may be able to cause a client to store a
   session ticket that was not meant for the current session.  Attacks
   based on this vector are not yet known, but applications that store
   additional information in session tickets beyond those covered in the
   session hash require careful analysis.

<span class="h3"><a class="selflink" id="section-6.4" href="#section-6.4">6.4</a>.  No SSL 3.0 Support</span>

   The Secure Sockets Layer (SSL) protocol version 3.0 [<a href="./rfc6101" title="&quot;The Secure Sockets Layer (SSL) Protocol Version 3.0&quot;">RFC6101</a>] is a
   predecessor of the TLS protocol, and it is equally vulnerable to
   triple handshake attacks, alongside other vulnerabilities stemming
   from its use of obsolete cryptographic constructions that are now
   considered weak.  SSL 3.0 has been deprecated [<a href="./rfc7568" title="&quot;Deprecating Secure Sockets Layer Version 3.0&quot;">RFC7568</a>].

   The countermeasure described in this document relies on a TLS
   extension and hence cannot be used with SSL 3.0.  Clients and servers
   implementing this document SHOULD refuse SSL 3.0 handshakes.  If they
   choose to support SSL 3.0, the resulting sessions MUST use the legacy
   master secret computation, and the interoperability considerations of
   <a href="#section-5.4">Section 5.4</a> apply.

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  IANA Considerations</span>

   IANA has added the extension code point 23 (0x0017), which has been
   used by prototype implementations, for the "extended_master_secret"
   extension to the "ExtensionType Values" registry specified in the TLS
   specification [<a href="./rfc5246" title="&quot;The Transport Layer Security (TLS) Protocol Version 1.2&quot;">RFC5246</a>].

<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  References</span>

<span class="h3"><a class="selflink" id="section-8.1" href="#section-8.1">8.1</a>.  Normative References</span>

   [<a id="ref-RFC2119">RFC2119</a>]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>,
               DOI 10.17487/RFC2119, March 1997,
               &lt;<a href="http://www.rfc-editor.org/info/rfc2119">http://www.rfc-editor.org/info/rfc2119</a>&gt;.

   [<a id="ref-RFC5246">RFC5246</a>]   Dierks, T. and E. Rescorla, "The Transport Layer Security
               (TLS) Protocol Version 1.2", <a href="./rfc5246">RFC 5246</a>,
               DOI 10.17487/RFC5246, August 2008,
               &lt;<a href="http://www.rfc-editor.org/info/rfc5246">http://www.rfc-editor.org/info/rfc5246</a>&gt;.



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<span class="h3"><a class="selflink" id="section-8.2" href="#section-8.2">8.2</a>.  Informative References</span>

   [<a id="ref-COMPOUND-AUTH">COMPOUND-AUTH</a>]
               Asokan, N., Valtteri, N., and K. Nyberg, "Man-in-the-
               Middle in Tunnelled Authentication Protocols", Security
               Protocols, LNCS, Volume 3364, DOI 10.1007/11542322_6,
               2005.

   [<a id="ref-Ray09">Ray09</a>]     Ray, M., "Authentication Gap in TLS Renegotiation", 2009.

   [<a id="ref-RFC4251">RFC4251</a>]   Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
               Protocol Architecture", <a href="./rfc4251">RFC 4251</a>, DOI 10.17487/RFC4251,
               January 2006, &lt;<a href="http://www.rfc-editor.org/info/rfc4251">http://www.rfc-editor.org/info/rfc4251</a>&gt;.

   [<a id="ref-RFC5077">RFC5077</a>]   Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
               "Transport Layer Security (TLS) Session Resumption
               without Server-Side State", <a href="./rfc5077">RFC 5077</a>,
               DOI 10.17487/RFC5077, January 2008,
               &lt;<a href="http://www.rfc-editor.org/info/rfc5077">http://www.rfc-editor.org/info/rfc5077</a>&gt;.

   [<a id="ref-RFC5705">RFC5705</a>]   Rescorla, E., "Keying Material Exporters for Transport
               Layer Security (TLS)", <a href="./rfc5705">RFC 5705</a>, DOI 10.17487/RFC5705,
               March 2010, &lt;<a href="http://www.rfc-editor.org/info/rfc5705">http://www.rfc-editor.org/info/rfc5705</a>&gt;.

   [<a id="ref-RFC5746">RFC5746</a>]   Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
               "Transport Layer Security (TLS) Renegotiation Indication
               Extension", <a href="./rfc5746">RFC 5746</a>, DOI 10.17487/RFC5746, February
               2010, &lt;<a href="http://www.rfc-editor.org/info/rfc5746">http://www.rfc-editor.org/info/rfc5746</a>&gt;.

   [<a id="ref-RFC5929">RFC5929</a>]   Altman, J., Williams, N., and L. Zhu, "Channel Bindings
               for TLS", <a href="./rfc5929">RFC 5929</a>, DOI 10.17487/RFC5929, July 2010,
               &lt;<a href="http://www.rfc-editor.org/info/rfc5929">http://www.rfc-editor.org/info/rfc5929</a>&gt;.

   [<a id="ref-RFC6101">RFC6101</a>]   Freier, A., Karlton, P., and P. Kocher, "The Secure
               Sockets Layer (SSL) Protocol Version 3.0", <a href="./rfc6101">RFC 6101</a>,
               DOI 10.17487/RFC6101, August 2011,
               &lt;<a href="http://www.rfc-editor.org/info/rfc6101">http://www.rfc-editor.org/info/rfc6101</a>&gt;.

   [<a id="ref-RFC6347">RFC6347</a>]   Rescorla, E. and N. Modadugu, "Datagram Transport Layer
               Security Version 1.2", <a href="./rfc6347">RFC 6347</a>, DOI 10.17487/RFC6347,
               January 2012, &lt;<a href="http://www.rfc-editor.org/info/rfc6347">http://www.rfc-editor.org/info/rfc6347</a>&gt;.

   [<a id="ref-RFC7457">RFC7457</a>]   Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing
               Known Attacks on Transport Layer Security (TLS) and
               Datagram TLS (DTLS)", <a href="./rfc7457">RFC 7457</a>, DOI 10.17487/RFC7457,
               February 2015, &lt;<a href="http://www.rfc-editor.org/info/rfc7457">http://www.rfc-editor.org/info/rfc7457</a>&gt;.





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   [<a id="ref-RFC7568">RFC7568</a>]   Barnes, R., Thomson, M., Pironti, A., and A. Langley,
               "Deprecating Secure Sockets Layer Version 3.0", <a href="./rfc7568">RFC 7568</a>,
               DOI 10.17487/RFC7568, June 2015,
               &lt;<a href="http://www.rfc-editor.org/info/rfc7568">http://www.rfc-editor.org/info/rfc7568</a>&gt;.

   [<a id="ref-SP800-108">SP800-108</a>] Chen, L., "Recommendation for Key Derivation Using
               Pseudorandom Functions (Revised)", NIST Special
               Publication 800-108, 2009.

   [<a id="ref-TRIPLE-HS">TRIPLE-HS</a>] Bhargavan, K., Delignat-Lavaud, A., Fournet, C., Pironti,
               A., and P-Y. Strub, "Triple Handshakes and Cookie
               Cutters: Breaking and Fixing Authentication over TLS",
               IEEE Symposium on Security and Privacy,
               DOI 10.1109/SP.2014.14, 2014.

   [<a id="ref-VERIFIED-BINDINGS">VERIFIED-BINDINGS</a>]
               Bhargavan, K., Delignat-Lavaud, A., and A. Pironti,
               "Verified Contributive Channel Bindings for Compound
               Authentication", Network and Distributed System Security
               Symposium (NDSS), 2015.

Acknowledgments

   Triple handshake attacks were originally discovered by Antoine
   Delignat-Lavaud, Karthikeyan Bhargavan, and Alfredo Pironti.  They
   were further developed by the miTLS team: Cedric Fournet, Pierre-Yves
   Strub, Markulf Kohlweiss, and Santiago Zanella-Beguelin.  Many of the
   ideas in this document emerged from discussions with Martin Abadi,
   Ben Laurie, Nikos Mavrogiannopoulos, Manuel Pegourie-Gonnard, Eric
   Rescorla, Martin Rex, and Brian Smith.





















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Authors' Addresses

   Karthikeyan Bhargavan (editor)
   Inria Paris-Rocquencourt
   23, Avenue d'Italie
   Paris  75214 CEDEX 13
   France

   Email: karthikeyan.bhargavan@inria.fr


   Antoine Delignat-Lavaud
   Inria Paris-Rocquencourt
   23, Avenue d'Italie
   Paris  75214 CEDEX 13
   France

   Email: antoine.delignat-lavaud@inria.fr


   Alfredo Pironti
   Inria Paris-Rocquencourt
   23, Avenue d'Italie
   Paris  75214 CEDEX 13
   France

   Email: alfredo.pironti@inria.fr


   Adam Langley
   Google Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA  94043
   United States

   Email: agl@google.com


   Marsh Ray
   Microsoft Corp.
   1 Microsoft Way
   Redmond, WA  98052
   United States

   Email: maray@microsoft.com






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