draft-ietf-tls-extractor-02.txt   draft-ietf-tls-extractor-03.txt 
Network Working Group E. Rescorla Network Working Group E. Rescorla
Internet-Draft Network Resonance Internet-Draft Network Resonance
Intended status: Standards Track September 11, 2008 Intended status: Standards Track November 01, 2008
Expires: March 15, 2009 Expires: May 5, 2009
Keying Material Extractors for Transport Layer Security (TLS) Keying Material Extractors for Transport Layer Security (TLS)
draft-ietf-tls-extractor-02.txt draft-ietf-tls-extractor-03.txt
Status of this Memo Status of this Memo
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Abstract Abstract
A number of protocols wish to leverage Transport Layer Security (TLS) A number of protocols wish to leverage Transport Layer Security (TLS)
to perform key establishment but then use some of the keying material to perform key establishment but then use some of the keying material
for their own purposes. This document describes a general mechanism for their own purposes. This document describes a general mechanism
for allowing that. for allowing that.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used In This Document . . . . . . . . . . . . . . . 3 2. Conventions Used In This Document . . . . . . . . . . . . . . . 3
3. Binding to Application Contexts . . . . . . . . . . . . . . . . 3 3. Binding to Application Contexts . . . . . . . . . . . . . . . . 3
4. Extractor Definition . . . . . . . . . . . . . . . . . . . . . 4 4. Extractor Definition . . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5 5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 6 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . . 6 8.1. Normative References . . . . . . . . . . . . . . . . . . . 6
8.2. Informational References . . . . . . . . . . . . . . . . . 6 8.2. Informational References . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 6 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7
Intellectual Property and Copyright Statements . . . . . . . . . . 7 Intellectual Property and Copyright Statements . . . . . . . . . . 8
1. Introduction 1. Introduction
A number of protocols wish to leverage Transport Layer Security (TLS) A number of protocols wish to leverage Transport Layer Security (TLS)
[RFC4346] or Datagram TLS (DTLS) [RFC4347] to perform key [RFC4346] or Datagram TLS (DTLS) [RFC4347] to perform key
establishment but then use some of the keying material for their own 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], 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 which uses DTLS to perform a key exchange and negotiate the SRTP
[RFC3711] protection suite and then uses the DTLS master_secret to [RFC3711] protection suite and then uses the DTLS master_secret to
generate the SRTP keys. generate the SRTP keys.
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2. Conventions Used In This Document 2. Conventions Used In This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
3. Binding to Application Contexts 3. Binding to Application Contexts
In addition to extracting keying material, an application using the In addition to extracting keying material, an application using the
keying material has to securely establish the upper-layer layer keying material has to securely establish the upper-layer context
context where the keying material will be used. The details of this where the keying material will be used. The details of this context
context depend on the application, but it could include things such depend on the application, but it could include things such as
as algorithms and parameters that will be used with the keys, 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 endpoint(s) who will use the keys,
identifier(s) for the session(s) where the keys will be used, and the 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 lifetime(s) for the context and/or keys. At minimum, there should be
some mechanism for signalling that an extractor will be used. some mechanism for signalling that an extractor will be used.
This specification does not mandate a single mechanism for agreeing This specification does not mandate a single mechanism for agreeing
on such context; instead, there are several possibilities that can be on such context; instead, there are several possibilities that can be
used (and can complement each other). For example: used (and can complement each other). For example:
o One important part of the context -- which application will use o One important part of the context -- which application will use
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o The upper-layer protocol can include its own handshake which can o The upper-layer protocol can include its own handshake which can
be protected using the keys extracted from TLS. be protected using the keys extracted from TLS.
It is important to note that just embedding TLS messages in the It is important to note that just embedding TLS messages in the
upper-layer protocol may not automatically secure all the important upper-layer protocol may not automatically secure all the important
context information, since the upper-layer messages are not covered context information, since the upper-layer messages are not covered
by TLS Finished messages. by TLS Finished messages.
4. Extractor Definition 4. Extractor Definition
The output of the extractor is intended to be used in a single scope,
which is associated with the TLS session, the label, and the context
value.
An extractor takes as input three values: An extractor takes as input three values:
o A disambiguating label string o A disambiguating label string
o A per-association context value provided by the extractor using o A per-association context value provided by the extractor using
application application
o A length value o A length value
It then computes: It then computes:
PRF(master_secret, label, PRF(master_secret, label,
SecurityParameters.client_random + SecurityParameters.client_random +
SecurityParameters.server_random + SecurityParameters.server_random +
context_value_length + context_value context_value_length + context_value
)[length] )[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.
The output is a pseudorandom bit string of length bytes generated Labels here have the same definition as in TLS, i.e., an ASCII string
from the master_secret. 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 2434 [RFC2434]. 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.
Label values beginning with "EXPERIMENTAL" MAY be used for private opaque context<0..2^16-1>;
use without registration. All other label values MUST be registered
via Specification Required as described by RFC 2434 [RFC2434]. 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.
The context value allows the application using the extractor to mix The context value allows the application using the extractor to mix
its own data with the TLS PRF for the extractor output. The context its own data with the TLS PRF for the extractor output. One example
value length is encoded as an unsigned 16-bit quantity (uint16) of where this might be useful is an authentication setting where the
representing the length of the context value. 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 5. Security Considerations
The prime security requirement for extractor 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 extractor. Similarly, an
attacker who does not know the master secret should not be able to
distinguish valid extractor 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 extractor produces the same value if applied twice with 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 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 values generated with the same label not be used for two different
purposes--hence the requirement for IANA registration. However, purposes--hence the requirement for IANA registration. However,
because extractors depend on the TLS PRF, it is not a threat to the 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 use of an EKM value generated from one label to reveal an EKM value
generated from another label. generated from another label.
6. IANA Considerations 6. IANA Considerations
IANA is requested to create (has created) a TLS Extractor Label IANA is requested to create (has created) a TLS Extractor Label
registry for this purpose. The initial contents of the registry are registry for this purpose. The initial contents of the registry are
given below: given below:
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Future values are allocated via RFC2434 Specification Required Future values are allocated via RFC2434 Specification Required
policy. The label is a string consisting of printable ASCII policy. The label is a string consisting of printable ASCII
characters. IANA MUST also verify that one label is not a prefix of characters. IANA MUST also verify that one label is not a prefix of
any other label. For example, labels "key" or "master secretary" are any other label. For example, labels "key" or "master secretary" are
forbidden. forbidden.
7. Acknowledgments 7. Acknowledgments
Thanks to Pasi Eronen for valuable comments and the contents of the Thanks to Pasi Eronen for valuable comments and the contents of the
IANA section and Section 3. IANA section and Section 3. Thanks to David McGrew for helpful
discussion of the security considerations.
8. References 8. References
8.1. Normative References 8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434, IANA Considerations Section in RFCs", BCP 26, RFC 2434,
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Security", RFC 4347, April 2006. Security", RFC 4347, April 2006.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)", Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004. RFC 3711, March 2004.
[I-D.ietf-avt-dtls-srtp] [I-D.ietf-avt-dtls-srtp]
McGrew, D. and E. Rescorla, "Datagram Transport Layer McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for Secure Security (DTLS) Extension to Establish Keys for Secure
Real-time Transport Protocol (SRTP)", Real-time Transport Protocol (SRTP)",
draft-ietf-avt-dtls-srtp-04 (work in progress), draft-ietf-avt-dtls-srtp-06 (work in progress),
August 2008. October 2008.
Author's Address Author's Address
Eric Rescorla Eric Rescorla
Network Resonance Network Resonance
2064 Edgewood Drive 2064 Edgewood Drive
Palo Alto, CA 94303 Palo Alto, CA 94303
USA USA
Email: ekr@networkresonance.com Email: ekr@networkresonance.com
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