Network Working Group                                       K. Bhargavan
Internet-Draft                                        A. Delignat-Lavaud
Expires: May 16, 2015                                         A. Pironti
                                                Inria Paris-Rocquencourt
                                                              A. Langley
                                                             Google Inc.
                                                                  M. Ray
                                                         Microsoft Corp.
                                                       November 12, 2014

            Transport Layer Security (TLS) Session Hash and
                    Extended Master Secret Extension


   The Transport Layer Security (TLS) master secret is not
   cryptographically bound to important session parameters.
   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 Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 16, 2015.

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Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( 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

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Notation . . . . . . . . . . . . . . . . . . . .   4
   3.  The TLS Session Hash  . . . . . . . . . . . . . . . . . . . .   5
   4.  The Extended Master Secret  . . . . . . . . . . . . . . . . .   5
   5.  Extension Negotiation . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Extension Definition  . . . . . . . . . . . . . . . . . .   6
     5.2.  Client and Server Behavior  . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
     6.1.  Triple Handshake Preconditions and Impact . . . . . . . .   6
     6.2.  Cryptographic Properties of the Hash Function . . . . . .   8
     6.3.  Session Hash Handshake Coverage . . . . . . . . . . . . .   8
     6.4.  No SSL 3.0 Support  . . . . . . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   In TLS [RFC5246], every session has a "master_secret" computed as:

   master_secret = PRF(pre_master_secret, "master secret",
                       ClientHello.random + ServerHello.random)

   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

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   for DHE ciphersuites, it is the result of a Diffie-Hellman key

   As described in [TRIPLE-HS], 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 malicious server, A.  A then connects to a
   server, S, and completes both handshakes.  For simplicity, assume
   that C and S only use RSA ciphersuites.  (Note that C thinks it is
   connecting to A and is oblivious of S's involvement.)

   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
   the two sessions and it will be associated both at C and S with the
   same session ID, even though the server identities on the two
   connections are different.  Moreover, the record keys on the two
   connections will also be the same.

   Similar scenarios can be achieved when the handshake uses a DHE
   ciphersuite, or an ECDHE ciphersuite with an arbitrary explicit
   curve.  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.  Other key exchanges may also be
   vulnerable.  If client authentication is used, the attack still

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   works, except that the two sessions now differ on both client and
   server identities.

   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, on 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 [RFC5746] or TLS channel
   bindings [RFC5929].  [TRIPLE-HS] describes several exploits based on
   such session synchronization attacks.  In particular, it describes a
   man-in-the-middle attack that circumvents the protections of
   [RFC5746] to break client-authenticated TLS renegotiation after
   session resumption.  Similar attacks apply to application-level
   authentication mechanisms that rely on channel bindings [RFC5929] or
   on key material exported from TLS [RFC5705].

   The underlying protocol issue is that since the master secret is not
   guaranteed to be unique across sessions, it cannot be used on its own
   as an authentication credential.  This specification introduces a TLS
   extension that computes the "master_secret" value from the log of the
   handshake that computes it, so that different handshakes will, by
   construction, create different master secrets.

2.  Requirements Notation

   This document uses the same notation and terminology used in the TLS
   Protocol specification [RFC5246].

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   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  The TLS Session Hash

   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 Section 7.4 of

   For TLS 1.2, the "Hash" function is the one defined in Section 7.4.9
   of [RFC5246] 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.

4.  The Extended Master Secret

   When the extended master secret extension is negotiated in a TLS
   session, the "master_secret" is computed as

       master_secret = PRF(pre_master_secret, "extended master secret",

   The extended master secret computation differs from the [RFC5246] in
   the following ways:

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

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

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

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   This design reflects the recommendation that keys should be bound to
   the security contexts that compute them [sp800-108].  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 SSH

   Clients and servers SHOULD NOT resume sessions 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 Section 6.1.

5.  Extension Negotiation

5.1.  Extension Definition

   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.

   If 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 Section 4.

   If an abbreviated handshake takes place, the extension has no effect.
   The resumed session is protected by the extended master secret if the
   extension was negotiated in the full handshake that generated the

5.2.  Client and Server Behavior

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

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

6.  Security Considerations

6.1.  Triple Handshake Preconditions and Impact

   One way to mount a triple handshake attack has been described in
   Section 1, along with a mention of the security mechanisms that break
   due to the attack; more in-depth discussion and diagrams can be found
   in [TRIPLE-HS].  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 key exchange,
      but arbitrary DH groups or ECDH curves 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 [RFC5705] 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
   identity, 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 [RFC5077]).

   Additionally, in a synchronized abbreviated handshake, the whole
   transcript is synchronized, which includes the "verify_data" values.
   So, after an abbreviated handshake, channel bindings like "tls-
   unique" [RFC5929] will not identify uniquely the connection anymore.

   Synchronization of the "verify_data" in abbreviated handshakes also
   undermines the security guarantees of the renegotiation indication
   extension [RFC5746], re-enabling a prefix-injection flaw similar to
   the renegotiation attack [Ray09].  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, that where 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
   properties relying on the uniqueness of the master secret are now

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   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.

6.2.  Cryptographic Properties of the Hash Function

   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

   We observe that the "Hash" function used in the Finished message
   computation already needs to be collision resistant, for the
   renegotiation indication extension [RFC5746] to work: a collision on
   the verify_data (and hence on the hash function computing the
   handshake messages hash) defeats the renegotiation indication

   As a matter of fact, all current ciphersuites defined for TLS 1.2 use
   SHA256 or better.  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.

6.3.  Session Hash Handshake Coverage

   The "session_hash" is designed to encompass all relevant session
   information, including ciphersuite negotiation, key exchange messages
   and client and server identities.

   This document sets the "session_hash" to cover all handshake messages
   up to and including the ClientKeyExchange.  In this way, on one hand,
   all the relevant session information is included; on the other hand,
   the master secret can be computed right after the ClientKeyExchange
   message, allowing implementations to shred the pre-master secret from
   memory as soon as possible.

   It is crucial that any message sent after the ClientKeyExchange does
   not alter the session information.  This is the case for the Finished
   messages, as well as for the client CertificateVerify in client-
   authenticated sessions.  This also applies to session ticket messages
   [RFC5077].  Any protocol extension that adds protocol messages after
   the Client Key Exchange MUST either ensure that such messages do not
   alter the session information, or it MUST analyze the impact of the
   protocol changes with respect to the handshake coverage of the
   session hash.

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6.4.  No SSL 3.0 Support

   SSL 3.0 [RFC6101] is a predecessor of the TLS protocol, and it is
   equally vulnerable to the triple handshake attacks.

   The use of extensions precludes use of the extended master secret
   with SSL 3.0.  Yet, this protocol uses encryption schemes and
   algorithms that are now considered weak.  Furthermore, it seems
   likely that any system that did not upgrate from SSL 3.0 to any later
   version of TLS will be exposed to several other vulnerabilties
   anyway.  As a consequence, this document does not provide workarounds
   to accommodate SSL 3.0.

7.  IANA Considerations

   IANA has added the extension code point 23 (0x0017), which has been
   used for prototype implementations, for the "extended_master_secret"
   extension to the TLS ExtensionType values registry as specified in
   TLS [RFC5246].

8.  Acknowledgments

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

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

9.2.  Informative References

   [RFC5746]  Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
              "Transport Layer Security (TLS) Renegotiation Indication
              Extension", RFC 5746, February 2010.

   [RFC5705]  Rescorla, E., "Keying Material Exporters for Transport
              Layer Security (TLS)", RFC 5705, March 2010.

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   [RFC5929]  Altman, J., Williams, N., and L. Zhu, "Channel Bindings
              for TLS", RFC 5929, July 2010.

   [RFC4251]  Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
              Protocol Architecture", RFC 4251, January 2006.

   [RFC5077]  Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
              "Transport Layer Security (TLS) Session Resumption without
              Server-Side State", RFC 5077, January 2008.

   [RFC6101]  Freier, A., Karlton, P., and P. Kocher, "The Secure
              Sockets Layer (SSL) Protocol Version 3.0", RFC 6101,
              August 2011.

              Bhargavan, K., Delignat-Lavaud, A., Fournet, C., Pironti,
              A., and P. Strub, "Triple Handshakes and Cookie Cutters:
              Breaking and Fixing Authentication over TLS", IEEE
              Symposium on Security and Privacy, pages 98-113 , 2014.

              Chen, L., "NIST Special Publication 800-108:
              Recommendation for Key Derivation Using Pseudorandom
              Functions", Unpublished draft , 2009.

   [Ray09]    Ray, M., "Authentication Gap in TLS Renegotiation", 2009.

Authors' Addresses

   Karthikeyan Bhargavan
   Inria Paris-Rocquencourt
   23, Avenue d'Italie
   Paris  75214 CEDEX 13


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


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


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


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


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