Network Working Group E. Rescorla
Internet-Draft RTFM, Inc.
Intended status: Standards Track March 07, 2009
Expires: September 8, 2009
Keying Material Exporters for Transport Layer Security (TLS)
draft-ietf-tls-extractor-05.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 . . . . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . . 6
8.2. Informational References . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7
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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],
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, 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 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.
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 exporting 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,
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identifier(s) for the session(s) where the keys will be used, and the
lifetime(s) for the context and/or keys. At 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 [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 from 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.
o A disambiguating label string
o A per-association context value provided by the application using
the exporter
o A length value
It then computes:
PRF(SecurityParameters.master_secret, label,
SecurityParameters.client_random +
SecurityParameters.server_random +
context_value_length + context_value
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)[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.
opaque context<0..2^16-1>;
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.
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
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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.
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
----- ------------
client finished [RFC5246]
server finished [RFC5246]
master secret [RFC5246]
key expansion [RFC5246]
client EAP encryption [RFC2716]
ttls keying material [RFC5281]
ttls challenge [RFC5281]
Future values are allocated via RFC5226 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 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.
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[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 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.
8.2. Informational References
[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)",
draft-ietf-avt-dtls-srtp-07 (work in progress),
February 2009.
Author's Address
Eric Rescorla
RTFM, Inc.
2064 Edgewood Drive
Palo Alto, CA 94303
USA
Email: ekr@rtfm.com
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