TLS Exporters: Keying Material Exporters for Transport Layer Security (TLS)

Internet Engineering Task Force (IETF)                       E. Rescorla
Request for Comments: 5705                                    RTFM, Inc.
Category: Standards Track                                     March 2010
ISSN: 2070-1721


      Keying Material Exporters for Transport Layer Security (TLS)

Abstract

   A number of protocols wish to leverage Transport Layer Security (TLS)
   to perform key establishment but then use some of the keying material
   for their own purposes.  This document describes a general mechanism
   for allowing that.

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 Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/5705.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
   2.  Conventions Used In This Document . . . . . . . . . . . . . . . 3
   3.  Binding to Application Contexts . . . . . . . . . . . . . . . . 3
   4.  Exporter Definition . . . . . . . . . . . . . . . . . . . . . . 4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 7

1.  Introduction

   Note:  The mechanism described in this document was previously known
          as "TLS Extractors" but was changed to avoid a name conflict
          with the use of the term "Extractor" in the cryptographic
          community.

   A number of protocols wish to leverage Transport Layer Security (TLS)
   [RFC5246] or Datagram TLS (DTLS) [RFC4347] to perform key
   establishment but then use some of the keying material for their own
   purposes.  A typical example is DTLS-SRTP [DTLS-SRTP], a key
   management scheme for the Secure Real-time Transport Protocol (SRTP)
   that uses DTLS to perform a key exchange and negotiate the SRTP
   [RFC3711] protection suite and then uses the DTLS master_secret to
   generate the SRTP keys.

   These applications imply a need to be able to export keying material
   (later called Exported Keying Material or EKM) from TLS/DTLS to an
   application or protocol residing at an upper layer, and to securely
   agree on the upper-layer context where the keying material will be
   used.  The mechanism for exporting the keying material has the
   following requirements:

   o  Both client and server need to be able to export the same EKM
      value.

   o  EKM values should be indistinguishable from random data to
      attackers who don't know the master_secret.

   o  It should be possible to export multiple EKM values from the same
      TLS/DTLS association.

   o  Knowing one EKM value should not reveal any useful information
      about the master_secret or about other EKM values.

   The mechanism described in this document is intended to fulfill these
   requirements.  This mechanism is compatible with all versions of TLS.

2.  Conventions Used In This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.  Binding to Application Contexts

   In addition to using an exporter to obtain keying material, an
   application using the keying material has to securely establish the
   upper-layer context where the keying material will be used.  The
   details of this context depend on the application, but it could
   include things such as algorithms and parameters that will be used
   with the keys, identifier(s) for the endpoint(s) who will use the
   keys, identifier(s) for the session(s) where the keys will be used,
   and the lifetime(s) for the context and/or keys.  At a minimum, there
   should be some mechanism for signaling that an exporter will be used.

   This specification does not mandate a single mechanism for agreeing
   on such context; instead, there are several possibilities that can be
   used (and can complement each other).  For example:

   o  Information about the upper-layer context can be included in the
      optional data after the exporter label (see Section 4).

   o  Information about the upper-layer context can be exchanged in TLS
      extensions included in the ClientHello and ServerHello messages.
      This approach is used in [DTLS-SRTP].  The handshake messages are
      protected by the Finished messages, so once the handshake
      completes, the peers will have the same view of the information.
      Extensions also allow a limited form of negotiation: for example,
      the TLS client could propose several alternatives for some context
      parameters, and the TLS server could select one of them.

   o  The upper-layer protocol can include its own handshake, which can
      be protected using the keys exported by TLS.

   No matter how the context is agreed, it is required that it has one
   part that indicates which application will use the exported keys.
   This part is the disambiguating label string (see Section 4).

   It is important to note that just embedding TLS messages in the
   upper-layer protocol may not automatically secure all the important
   context information, since the upper-layer messages are not covered
   by TLS Finished messages.

4.  Exporter Definition

   The output of the exporter is intended to be used in a single scope,
   which is associated with the TLS session, the label, and the context
   value.

   The exporter takes three input values:

   o  a disambiguating label string,

   o  a per-association context value provided by the application using
      the exporter, and

   o  a length value.

   If no context is provided, it then computes:

           PRF(SecurityParameters.master_secret, label,
               SecurityParameters.client_random +
               SecurityParameters.server_random
               )[length]

   If context is provided, it computes:

           PRF(SecurityParameters.master_secret, label,
               SecurityParameters.client_random +
               SecurityParameters.server_random +
               context_value_length + context_value
               )[length]

   Where PRF is the TLS Pseudorandom Function in use for the session.
   The output is a pseudorandom bit string of length bytes generated
   from the master_secret.  (This construction allows for
   interoperability with older exporter-type constructions which do not
   use context values, e.g., [RFC5281]).

   Labels here have the same definition as in TLS, i.e., an ASCII string
   with no terminating NULL.  Label values beginning with "EXPERIMENTAL"
   MAY be used for private use without registration.  All other label
   values MUST be registered via Specification Required as described by
   RFC 5226 [RFC5226].  Note that exporter labels have the potential to
   collide with existing PRF labels.  In order to prevent this, labels
   SHOULD begin with "EXPORTER".  This is not a MUST because there are
   existing uses that have labels which do not begin with this prefix.

   The context value allows the application using the exporter to mix
   its own data with the TLS PRF for the exporter output.  One example
   of where this might be useful is an authentication setting where the
   client credentials are valid for more than one identity; the context
   value could then be used to mix the expected identity into the keying
   material, thus preventing substitution attacks.  The context value
   length is encoded as an unsigned, 16-bit quantity (uint16; see
   [RFC5246], Section 4.4) representing the length of the context value.
   The context MAY be zero length.  Because the context value is mixed
   with the master_secret via the PRF, it is safe to mix confidential
   information into the exporter, provided that the master_secret will
   not be known to the attacker.

5.  Security Considerations

   The prime security requirement for exporter outputs is that they be
   independent.  More formally, after a particular TLS session, if an
   adversary is allowed to choose multiple (label, context value) pairs
   and is given the output of the PRF for those values, the attacker is
   still unable to distinguish between the output of the PRF for a
   (label, context value) pair (different from the ones that it
   submitted) and a random value of the same length.  In particular,
   there may be settings, such as the one described in Section 4, where
   the attacker can control the context value; such an attacker MUST NOT
   be able to predict the output of the exporter.  Similarly, an
   attacker who does not know the master secret should not be able to
   distinguish valid exporter outputs from random values.  The current
   set of TLS PRFs is believed to meet this objective, provided the
   master secret is randomly generated.

   Because an exporter produces the same value if applied twice with the
   same label to the same master_secret, it is critical that two EKM
   values generated with the same label not be used for two different
   purposes -- hence, the requirement for IANA registration.  However,
   because exporters depend on the TLS PRF, it is not a threat to the
   use of an EKM value generated from one label to reveal an EKM value
   generated from another label.

   With certain TLS cipher suites, the TLS master secret is not
   necessarily unique to a single TLS session.  In particular, with RSA
   key exchange, a malicious party acting as TLS server in one session
   and as TLS client in another session can cause those two sessions to
   have the same TLS master secret (though the sessions must be
   established simultaneously to get adequate control of the Random
   values).  Applications using the EKM need to consider this in how
   they use the EKM; in some cases, requiring the use of other cipher
   suites (such as those using a Diffie-Hellman key exchange) may be
   advisable.

   Designing a secure mechanism that uses exporters is not necessarily
   straightforward.  This document only provides the exporter mechanism,
   but the problem of agreeing on the surrounding context and the
   meaning of the information passed to and from the exporter remains.
   Any new uses of the exporter mechanism should be subject to careful
   review.

6.  IANA Considerations

   IANA has created a TLS Exporter Label registry for this purpose.  The
   initial contents of the registry are given below:

        Value                          Reference  Note
        -----------------------------  ---------  ----
        client finished                [RFC5246]  (1)
        server finished                [RFC5246]  (1)
        master secret                  [RFC5246]  (1)
        key expansion                  [RFC5246]  (1)
        client EAP encryption          [RFC5216]
        ttls keying material           [RFC5281]
        ttls challenge                 [RFC5281]

   Note: (1) These entries are reserved and MUST NOT be used for the
   purpose described in RFC 5705, in order to avoid confusion with
   similar, but distinct, use in RFC 5246.

   Future values are allocated via the RFC 5226 Specification Required
   policy.  The label is a string consisting of printable ASCII
   characters.  IANA MUST also verify that one label is not a prefix of
   any other label.  For example, labels "key" or "master secretary" are
   forbidden.

7.  Acknowledgments

   Thanks to Pasi Eronen for valuable comments and for the contents of
   the IANA section and Section 3.  Thanks to David McGrew for helpful
   discussion of the security considerations and to Vijay Gurbani and
   Alfred Hoenes for editorial comments.

8.  References

8.1.  Normative References

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

   [RFC5226]    Narten, T. and H. Alvestrand, "Guidelines for Writing an
                IANA Considerations Section in RFCs", BCP 26, RFC 5226,
                May 2008.

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

8.2.  Informative References

   [DTLS-SRTP]  McGrew, D. and E. Rescorla, "Datagram Transport Layer
                Security (DTLS) Extension to Establish Keys for Secure
                Real-time Transport Protocol (SRTP)", Work in Progress,
                February 2009.

   [RFC3711]    Baugher, M., McGrew, D., Naslund, M., Carrara, E., and
                K. Norrman, "The Secure Real-time Transport Protocol
                (SRTP)", RFC 3711, March 2004.

   [RFC4347]    Rescorla, E. and N. Modadugu, "Datagram Transport Layer
                Security", RFC 4347, April 2006.

   [RFC5216]    Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS
                Authentication Protocol", RFC 5216, March 2008.

   [RFC5281]    Funk, P. and S. Blake-Wilson, "Extensible Authentication
                Protocol Tunneled Transport Layer Security Authenticated
                Protocol Version 0 (EAP-TTLSv0)", RFC 5281, August 2008.

Author's Address

   Eric Rescorla
   RTFM, Inc.
   2064 Edgewood Drive
   Palo Alto, CA  94303
   USA

   EMail: ekr@rtfm.com