Keying Material Exporters for Transport Layer Security (TLS)
draft-ietf-tls-extractor-07

Network Working Group                                        E. Rescorla
Internet-Draft                                         Network Resonance
Intended status:  Standards Track                     September 07, 2009
Expires:  March 11, 2010

      Keying Material Exporters for Transport Layer Security (TLS)
                    draft-ietf-tls-extractor-07.txt

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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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   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 . . . . . . . . . . . . . . . . . . . . . . . . 7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 8

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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 [I-D.ietf-avt-dtls-srtp], a
   key management scheme for SRTP which 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 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 by
      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 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

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   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 signalling 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  One important part of the context -- which application will use
      the exported keys -- is given by the disambiguating label string
      (see Section 4).
   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 [I-D.ietf-avt-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.

   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.

   It then computes:

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

   Where PRF is the TLS PRF in use for the session.  The output is a
   pseudorandom bit string of length bytes generated from the
   master_secret.

   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 which 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)
   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 extractor 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

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   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 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 Diffie-Hellman key exchange) may be
   advisable.

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

6.  IANA Considerations

   IANA is requested to create (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 XXXX, in order to avoid confusion with
   similar, but distinct use in RFC 5246.

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   [ RFC Editor:  Please replace 'XXXX' above by the RFC number assigned
   to this document and delete this remark. ]

   Future values are allocated via 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 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

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

   [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.

   [I-D.ietf-avt-dtls-srtp]
              McGrew, D. and E. Rescorla, "Datagram Transport Layer
              Security (DTLS) Extension to Establish Keys for  Secure
              Real-time Transport Protocol (SRTP)",

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              draft-ietf-avt-dtls-srtp-07 (work in progress),
              February 2009.

   [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
   Network Resonance
   2064 Edgewood Drive
   Palo Alto, CA  94303
   USA

   Email:  ekr@networkresonance.com

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