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.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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material may not have granted the IETF Trust the right to allow
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RFC 5705 TLS Exporters March 2010
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
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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.
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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.
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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
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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
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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.
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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
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RFC, FYI, BCP