draft-ietf-tls-extractor-05.txt   draft-ietf-tls-extractor-06.txt 
Network Working Group E. Rescorla Network Working Group E. Rescorla
Internet-Draft RTFM, Inc. Internet-Draft Network Resonance
Intended status: Standards Track March 07, 2009 Intended status: Standards Track July 12, 2009
Expires: September 8, 2009 Expires: January 13, 2010
Keying Material Exporters for Transport Layer Security (TLS) Keying Material Exporters for Transport Layer Security (TLS)
draft-ietf-tls-extractor-05.txt draft-ietf-tls-extractor-06.txt
Status of this Memo Status of this Memo
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This Internet-Draft will expire on September 8, 2009. This Internet-Draft will expire on January 13, 2010.
Copyright Notice Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info). publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights Please review these documents carefully, as they describe your rights
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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. Exporter Definition . . . . . . . . . . . . . . . . . . . . . . 4 4. Exporter Definition . . . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5 5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 6 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . . 6 8.1. Normative References . . . . . . . . . . . . . . . . . . . 7
8.2. Informational References . . . . . . . . . . . . . . . . . 7 8.2. Informative References . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction 1. Introduction
Note: The mechanism described in this document was previously known Note: The mechanism described in this document was previously known
as "TLS Extractors" but was changed to avoid a name conflict with as "TLS Extractors" but was changed to avoid a name conflict with
the use of the term "Extractor" in the cryptographic community. the use of the term "Extractor" in the cryptographic community.
A number of protocols wish to leverage Transport Layer Security (TLS) A number of protocols wish to leverage Transport Layer Security (TLS)
[RFC5246] or Datagram TLS (DTLS) [RFC4347] to perform key [RFC5246] 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], a
which uses DTLS to perform a key exchange and negotiate the SRTP key management scheme for SRTP which uses DTLS to perform a key
[RFC3711] protection suite and then uses the DTLS master_secret to exchange and negotiate the SRTP [RFC3711] protection suite and then
generate the SRTP keys. uses the DTLS master_secret to generate the SRTP keys.
These applications imply a need to be able to export keying material These applications imply a need to be able to export keying material
(later called Exported Keying Material or EKM) from TLS/DTLS, and (later called Exported Keying Material or EKM) from TLS/DTLS, and
securely agree on the upper-layer context where the keying material securely agree on the upper-layer context where the keying material
will be used. The mechanism for exporting the keying material has will be used. The mechanism for exporting the keying material has
the following requirements: the following requirements:
o Both client and server need to be able to export the same EKM o Both client and server need to be able to export the same EKM
value. value.
o EKM values should be indistinguishable from random by attackers o EKM values should be indistinguishable from random by attackers
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requirements. requirements.
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 exporting keying material, an application using the In addition to using an exporter to obtain keying material, an
keying material has to securely establish the upper-layer context application using the keying material has to securely establish the
where the keying material will be used. The details of this context upper-layer context where the keying material will be used. The
depend on the application, but it could include things such as details of this context depend on the application, but it could
algorithms and parameters that will be used with the keys, include things such as algorithms and parameters that will be used
identifier(s) for the endpoint(s) who will use the keys, with the keys, identifier(s) for the endpoint(s) who will use the
identifier(s) for the session(s) where the keys will be used, and the keys, identifier(s) for the session(s) where the keys will be used,
lifetime(s) for the context and/or keys. At minimum, there should be and the lifetime(s) for the context and/or keys. At a minimum, there
some mechanism for signalling that an exporter will be used. should be some mechanism for signalling that an exporter 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
the exported keys -- is given by the disambiguating label string the exported keys -- is given by the disambiguating label string
(see Section 4). (see Section 4).
o Information about the upper-layer context can be included in the o Information about the upper-layer context can be included in the
optional data after the exporter label (see Section 4). optional data after the exporter label (see Section 4).
o Information about the upper-layer context can be exchanged in TLS o Information about the upper-layer context can be exchanged in TLS
extensions included in the ClientHello and ServerHello messages. extensions included in the ClientHello and ServerHello messages.
This approach is used in [DTLS-SRTP]. The handshake messages are This approach is used in [I-D.ietf-avt-dtls-srtp]. The handshake
protected by the Finished messages, so once the handshake messages are protected by the Finished messages, so once the
completes, the peers will have the same view of the information. handshake completes, the peers will have the same view of the
Extensions also allow a limited form of negotiation: for example, information. Extensions also allow a limited form of negotiation:
the TLS client could propose several alternatives for some context for example, the TLS client could propose several alternatives for
parameters, and the TLS server could select one of them. some context parameters, and the TLS server could select one of
them.
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 exported from TLS. be protected using the keys exported by 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. Exporter Definition 4. Exporter Definition
The output of the exporter is intended to be used in a single scope, 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 which is associated with the TLS session, the label, and the context
value. value.
o A disambiguating label string The exporter takes three input values
o A per-association context value provided by the application using
the exporter o a disambiguating label string,
o A length value o a per-association context value provided by the application using
the exporter, and
o a length value.
It then computes: It then computes:
PRF(SecurityParameters.master_secret, label, PRF(SecurityParameters.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 Where PRF is the TLS PRF in use for the session. The output is a
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Labels here have the same definition as in TLS, i.e., an ASCII string Labels here have the same definition as in TLS, i.e., an ASCII string
with no terminating NULL. Label values beginning with "EXPERIMENTAL" with no terminating NULL. Label values beginning with "EXPERIMENTAL"
MAY be used for private use without registration. All other label MAY be used for private use without registration. All other label
values MUST be registered via Specification Required as described by values MUST be registered via Specification Required as described by
RFC 5226 [RFC5226]. Note that exporter labels have the potential to RFC 5226 [RFC5226]. Note that exporter labels have the potential to
collide with existing PRF labels. In order to prevent this, labels collide with existing PRF labels. In order to prevent this, labels
SHOULD begin with "EXPORTER". This is not a MUST because there are SHOULD begin with "EXPORTER". This is not a MUST because there are
existing uses which have labels which do not begin with this prefix. 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 The context value allows the application using the exporter to mix
its own data with the TLS PRF for the exporter output. One example 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 of where this might be useful is an authentication setting where the
client credentials are valid for more than one identity; the context client credentials are valid for more than one identity; the context
value could then be used to mix the expected identity into the keying value could then be used to mix the expected identity into the keying
material, thus preventing substitution attacks. The context value material, thus preventing substitution attacks. The context value
length is encoded as an unsigned 16-bit quantity (uint16) length is encoded as an unsigned 16-bit quantity (uint16)
representing the length of the context value. The context MAY be representing the length of the context value. The context MAY be
zero length. zero length. Because the context value is mixed with the
master_secret via the PRF, it is safe to mix confidential information
into the extractor provided that the master_secret will not be known
to the attacker.
5. Security Considerations 5. Security Considerations
The prime security requirement for exporter outputs is that they be The prime security requirement for exporter outputs is that they be
independent. More formally, after a particular TLS session, if an independent. More formally, after a particular TLS session, if an
adversary is allowed to choose multiple (label, context value) pairs adversary is allowed to choose multiple (label, context value) pairs
and is given the output of the PRF for those values, the attacker is 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 still unable to distinguish between the output of the PRF for a
(label, context value) pair (different from the ones that it (label, context value) pair (different from the ones that it
submitted) and a random value of the same length. In particular, submitted) and a random value of the same length. In particular,
there may be settings, such as the one described in Section 4, where 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 the attacker can control the context value; such an attacker MUST NOT
be able to predict the output of the exporter. Similarly, an be able to predict the output of the exporter. Similarly, an
attacker who does not know the master secret should not be able to attacker who does not know the master secret should not be able to
distinguish valid exporter outputs from random values. The current distinguish valid exporter outputs from random values. The current
set of TLS PRFs is believed to meet this objective, provided the set of TLS PRFs is believed to meet this objective, provided the
master secret is randomly generated. master secret is randomly generated.
Because an exporter produces the same value if applied twice with the 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 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 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 exporters depend on the TLS PRF, it is not a threat to the 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 use of an EKM value generated from one label to reveal an EKM value
generated from another label. 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 TLS client in another session can cause those two sessions to
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 Diffie-Hellman key exchange) may be
advisable.
Designing a secure mechanism that uses extractors is not necessarily
straightforward. This document only provides the extractor
mechanism, but the problem of agreeing on the surrounding context and
the meaning of the information passed to and from the extractor
remains. Any new uses of the extractor mechanism should be subject
to careful review.
6. IANA Considerations 6. IANA Considerations
IANA is requested to create (has created) a TLS Exporter Label IANA is requested to create (has created) a TLS Exporter 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:
Value Reference Value Reference Note
----- ------------ ----------------------------- --------- ----
client finished [RFC5246] client finished [RFC5246] (1)
server finished [RFC5246] server finished [RFC5246] (1)
master secret [RFC5246] master secret [RFC5246] (1)
key expansion [RFC5246] key expansion [RFC5246] (1)
client EAP encryption [RFC2716] client EAP encryption [RFC5216]
ttls keying material [RFC5281] ttls keying material [RFC5281]
ttls challenge [RFC5281] ttls challenge [RFC5281]
Note(1): These entries are reserved and MUST NOT be used for the
purpose described in RFC XXXX, in order to avoid confusion with
similar, but distinct use in RFC 5246.
[ RFC Editor: Please replace 'XXXX' above by the RFC number assigned
to this document and delete this remark. ]
Future values are allocated via RFC5226 Specification Required Future values are allocated via RFC5226 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. Thanks to David McGrew for helpful IANA section and Section 3. Thanks to David McGrew for helpful
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[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.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226, IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008. May 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008. (TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5281] Funk, P. and S. Blake-Wilson, "Extensible Authentication 8.2. Informative References
Protocol Tunneled Transport Layer Security Authenticated
Protocol Version 0 (EAP-TTLSv0)", RFC 5281, August 2008.
8.2. Informational References [RFC5216] Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS
Authentication Protocol", RFC 5216, March 2008.
[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.
[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer [RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006. Security", RFC 4347, April 2006.
[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-07 (work in progress), draft-ietf-avt-dtls-srtp-07 (work in progress),
February 2009. February 2009.
[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 Author's Address
Eric Rescorla Eric Rescorla
RTFM, Inc. Network Resonance
2064 Edgewood Drive 2064 Edgewood Drive
Palo Alto, CA 94303 Palo Alto, CA 94303
USA USA
Email: ekr@rtfm.com Email: ekr@networkresonance.com
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