Network Working Group E. Rescorla
Internet-Draft Network Resonance
Intended status: Standards Track November 17, 2007
Expires: May 20, 2008
Keying Material Extractors for Transport Layer Security (TLS)
draft-rescorla-tls-extractor-01.txt
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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-00 (work in
progress), July 2007.
Author's Address
Eric Rescorla
Network Resonance
2064 Edgewood Drive
Palo Alto, CA 94303
USA
Email: ekr@networkresonance.com
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