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Internet Engineering Task Force (IETF)                 K. Bhargavan, Ed.Request for Comments: 7627                            A. Delignat-LavaudUpdates:5246                                                 A. PirontiCategory: Standards Track                       Inria Paris-RocquencourtISSN: 2070-1721                                               A. Langley                                                             Google Inc.                                                                  M. Ray                                                         Microsoft Corp.                                                          September 2015Transport Layer Security (TLS) Session Hash andExtended Master Secret ExtensionAbstract   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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7627.Bhargavan, et al.            Standards Track                    [Page 1]

RFC 7627               TLS Session Hash Extension         September 2015Copyright Notice   Copyright (c) 2015 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) 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 Contents1. Introduction ....................................................32. Requirements Notation ...........................................53. The TLS Session Hash ............................................54. The Extended Master Secret ......................................65. Extension Negotiation ...........................................65.1. Extension Definition .......................................65.2. Client and Server Behavior: Full Handshake .................75.3. Client and Server Behavior: Abbreviated Handshake ..........75.4. Interoperability Considerations ............................96. Security Considerations .........................................96.1. Triple Handshake Preconditions and Impact ..................96.2. Cryptographic Properties of the Hash Function .............116.3. Handshake Messages Included in the Session Hash ...........116.4. No SSL 3.0 Support ........................................127. IANA Considerations ............................................128. References .....................................................128.1. Normative References ......................................128.2. Informative References ....................................13   Acknowledgments ...................................................14   Authors' Addresses ................................................15Bhargavan, et al.            Standards Track                    [Page 2]

RFC 7627               TLS Session Hash Extension         September 20151.  Introduction   In TLS [RFC5246], 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 [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 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 forBhargavan, et al.            Standards Track                    [Page 3]

RFC 7627               TLS Session Hash Extension         September 2015   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 [VERIFIED-BINDINGS].   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 [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, called the "triple handshake", thatBhargavan, et al.            Standards Track                    [Page 4]

RFC 7627               TLS Session Hash Extension         September 2015   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 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 [TRIPLE-HS] and documented inSection 2.11 of [RFC7457].2.  Requirements Notation   This document uses the same notation and terminology used in the TLS   protocol specification [RFC5246].   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 inRFC2119 [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 inSection 7.4 of   [RFC5246].   For TLS 1.2, the "Hash" function is the one defined inSection 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.Bhargavan, et al.            Standards Track                    [Page 5]

RFC 7627               TLS Session Hash Extension         September 20154.  The Extended Master Secret   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   [RFC5246] 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 [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   Secure Shell (SSH) [RFC4251].   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   inSection 6.1.5.  Extension Negotiation5.1.  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 (in hexadecimal.)   Although this document refers only to TLS, the extension proposed   here can also be used with Datagram TLS (DTLS) [RFC6347].Bhargavan, et al.            Standards Track                    [Page 6]

RFC 7627               TLS Session Hash Extension         September 2015   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 inSection 4.  All other   cryptographic computations remain unchanged.5.2.  Client and Server Behavior: Full Handshake   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 inSection 5.4 to avoid dangerous usage   scenarios.  In particular, the master secret derived from the new   session should not be used for application-level authentication.5.3.  Client and Server Behavior: Abbreviated Handshake   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 inSection 5.4Bhargavan, et al.            Standards Track                    [Page 7]

RFC 7627               TLS Session Hash Extension         September 2015   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 [RFC5746].   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 inSection 5.2) 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 inSection 5.4.   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.Bhargavan, et al.            Standards Track                    [Page 8]

RFC 7627               TLS Session Hash Extension         September 20155.4.  Interoperability Considerations   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 inSection 1.  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 [RFC5705] and any Extensible Authentication Protocol (EAP)   relying on compound authentication [COMPOUND-AUTH].   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 inSection 1.   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 [RFC5929] 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.6.  Security Considerations6.1.  Triple Handshake Preconditions and Impact   One way to mount a triple handshake attack is described inSection 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.Bhargavan, et al.            Standards Track                    [Page 9]

RFC 7627               TLS Session Hash Extension         September 2015   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 [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'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 [RFC5077]).   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" [RFC5929] 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 [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; 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 securityBhargavan, et al.            Standards Track                   [Page 10]

RFC 7627               TLS Session Hash Extension         September 2015   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 [VERIFIED-BINDINGS].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 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 [RFC5746] to work, because a   meaningful collision on the handshake messages (and hence on the   "verify_data") may re-enable the renegotiation attack [Ray09].   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.6.3.  Handshake Messages Included in the Session Hash   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.Bhargavan, et al.            Standards Track                   [Page 11]

RFC 7627               TLS Session Hash Extension         September 2015   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 [RFC5077], 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.6.4.  No SSL 3.0 Support   The Secure Sockets Layer (SSL) protocol version 3.0 [RFC6101] 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 [RFC7568].   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 ofSection 5.4 apply.7.  IANA Considerations   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 [RFC5246].8.  References8.1.  Normative References   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate               Requirement Levels",BCP 14,RFC 2119,               DOI 10.17487/RFC2119, March 1997,               <http://www.rfc-editor.org/info/rfc2119>.   [RFC5246]   Dierks, T. and E. Rescorla, "The Transport Layer Security               (TLS) Protocol Version 1.2",RFC 5246,               DOI 10.17487/RFC5246, August 2008,               <http://www.rfc-editor.org/info/rfc5246>.Bhargavan, et al.            Standards Track                   [Page 12]

RFC 7627               TLS Session Hash Extension         September 20158.2.  Informative References   [COMPOUND-AUTH]               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.   [Ray09]     Ray, M., "Authentication Gap in TLS Renegotiation", 2009.   [RFC4251]   Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)               Protocol Architecture",RFC 4251, DOI 10.17487/RFC4251,               January 2006, <http://www.rfc-editor.org/info/rfc4251>.   [RFC5077]   Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,               "Transport Layer Security (TLS) Session Resumption               without Server-Side State",RFC 5077,               DOI 10.17487/RFC5077, January 2008,               <http://www.rfc-editor.org/info/rfc5077>.   [RFC5705]   Rescorla, E., "Keying Material Exporters for Transport               Layer Security (TLS)",RFC 5705, DOI 10.17487/RFC5705,               March 2010, <http://www.rfc-editor.org/info/rfc5705>.   [RFC5746]   Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,               "Transport Layer Security (TLS) Renegotiation Indication               Extension",RFC 5746, DOI 10.17487/RFC5746, February               2010, <http://www.rfc-editor.org/info/rfc5746>.   [RFC5929]   Altman, J., Williams, N., and L. Zhu, "Channel Bindings               for TLS",RFC 5929, DOI 10.17487/RFC5929, July 2010,               <http://www.rfc-editor.org/info/rfc5929>.   [RFC6101]   Freier, A., Karlton, P., and P. Kocher, "The Secure               Sockets Layer (SSL) Protocol Version 3.0",RFC 6101,               DOI 10.17487/RFC6101, August 2011,               <http://www.rfc-editor.org/info/rfc6101>.   [RFC6347]   Rescorla, E. and N. Modadugu, "Datagram Transport Layer               Security Version 1.2",RFC 6347, DOI 10.17487/RFC6347,               January 2012, <http://www.rfc-editor.org/info/rfc6347>.   [RFC7457]   Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing               Known Attacks on Transport Layer Security (TLS) and               Datagram TLS (DTLS)",RFC 7457, DOI 10.17487/RFC7457,               February 2015, <http://www.rfc-editor.org/info/rfc7457>.Bhargavan, et al.            Standards Track                   [Page 13]

RFC 7627               TLS Session Hash Extension         September 2015   [RFC7568]   Barnes, R., Thomson, M., Pironti, A., and A. Langley,               "Deprecating Secure Sockets Layer Version 3.0",RFC 7568,               DOI 10.17487/RFC7568, June 2015,               <http://www.rfc-editor.org/info/rfc7568>.   [SP800-108] Chen, L., "Recommendation for Key Derivation Using               Pseudorandom Functions (Revised)", NIST Special               Publication 800-108, 2009.   [TRIPLE-HS] 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.   [VERIFIED-BINDINGS]               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.Bhargavan, et al.            Standards Track                   [Page 14]

RFC 7627               TLS Session Hash Extension         September 2015Authors' 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.comBhargavan, et al.            Standards Track                   [Page 15]