


Network Working Group                                        E. Rescorla
Internet-Draft                                         Network Resonance
Intended status:  Standards Track                      December 19, 2007
Expires:  June 21, 2008


     Keying Material Extractors for Transport Layer Security (TLS)
                    draft-ietf-tls-extractor-00.txt

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Copyright Notice

   Copyright (C) The IETF Trust (2007).

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.







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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Conventions Used In This Document . . . . . . . . . . . . . . . 3
   3.  Signalling Extractors . . . . . . . . . . . . . . . . . . . . . 3
   4.  Extractor Definition  . . . . . . . . . . . . . . . . . . . . . 3
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 4
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 5
     8.2.  Informational References  . . . . . . . . . . . . . . . . . 5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 5
   Intellectual Property and Copyright Statements  . . . . . . . . . . 6





































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1.  Introduction

   A number of protocols wish to leverage Transport Layer Security (TLS)
   [4] or Datagram TLS (DTLS) [5] to perform key establishment but then
   use some of the keying material for their own purposes.  A typical
   example is DTLS-SRTP [6], which uses DTLS to perform a key exchange
   and negotiate the SRTP [3] protection suite and then uses the DTLS
   master_secret to generate the SRTP keys.

   These applications imply a need to be able to extract Exported Keying
   Material (EKM) from TLS/DTLS.  This mechanism has the following
   requirements:

   o  Both client and server need to be able to extract the same EKM
      value.
   o  EKM values should be indistinguishable from random by attackers
      who don't know the master_secret.
   o  It should be possible to extract 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 fill these
   requirements.


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 [1].


3.  Signalling Extractors

   Other protocols which wish to use extractors SHOULD have some way for
   the peers to signal that an extractor will be used.  An example is a
   TLS extension, as used in DTLS-SRTP.


4.  Extractor Definition

   An extractor takes as input two values:

   o  A disambiguating label string
   o  A length value

   It then computes:



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

   The output is a pseudorandom bit string of length bytes generated
   from the master_secret.

   Label values MUST be registered via Specification Required as
   described by RFC 2434 [2].  Note that extractor labels have the
   potential to collide with existing PRF labels.  In order to prevent
   this, labels SHOULD begin with "EXTRACTOR".  This is not a MUST
   because there are existing uses which have labels which do not begin
   with this prefix.


5.  Security Considerations

   Because an extractor 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 be used for two different
   purposes--hence the requirement for IANA registration.  However,
   because extractors 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.


6.  IANA Considerations

   IANA is requested to create (has created) a TLS Extractor Label
   registry for this purpose.  The initial contents of the registry are
   given below:

      Value                          Reference
      -----                          ------------
      client finished                [RFC4346]
      server finished                [RFC4346]
      master secret                  [RFC4346]
      key expansion                  [RFC4346]
      client EAP encryption          [RFC2716]
      ttls keying material           [draft-funk-eap-ttls-v0-01]

   Future values are allocated via RFC2434 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.





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7.  Acknowledgments

   Thanks to Pasi Eronen for valuable comments and the contents of the
   IANA section.


8.  References

8.1.  Normative References

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

   [2]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
        Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

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

   [4]  Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS)
        Protocol Version 1.1", RFC 4346, April 2006.

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

8.2.  Informational References

   [6]  McGrew, D. and E. Rescorla, "Datagram Transport Layer Security
        (DTLS) Extension to Establish Keys for  Secure Real-time
        Transport Protocol (SRTP)", draft-ietf-avt-dtls-srtp-01 (work in
        progress), November 2007.


Author's Address

   Eric Rescorla
   Network Resonance
   2064 Edgewood Drive
   Palo Alto, CA  94303
   USA

   Email:  ekr@networkresonance.com








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