draft-ietf-tls-extractor-07.txt   rfc5705.txt 
Network Working Group E. Rescorla Internet Engineering Task Force (IETF) E. Rescorla
Internet-Draft Network Resonance Request for Comments: 5705 RTFM, Inc.
Intended status: Standards Track September 07, 2009 Category: Standards Track March 2010
Expires: March 11, 2010 ISSN: 2070-1721
Keying Material Exporters for Transport Layer Security (TLS) Keying Material Exporters for Transport Layer Security (TLS)
draft-ietf-tls-extractor-07.txt
Status of this Memo Abstract
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. This document may contain material
from IETF Documents or IETF Contributions published or made publicly
available before November 10, 2008. The person(s) controlling the
copyright in some of this material may not have granted the IETF
Trust the right to allow 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 not be created outside the IETF Standards
Process, except to format it for publication as an RFC or to
translate it into languages other than English.
Internet-Drafts are working documents of the Internet Engineering A number of protocols wish to leverage Transport Layer Security (TLS)
Task Force (IETF), its areas, and its working groups. Note that to perform key establishment but then use some of the keying material
other groups may also distribute working documents as Internet- for their own purposes. This document describes a general mechanism
Drafts. for allowing that.
Internet-Drafts are draft documents valid for a maximum of six months Status of This Memo
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at This is an Internet Standards Track document.
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at This document is a product of the Internet Engineering Task Force
http://www.ietf.org/shadow.html. (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.
This Internet-Draft will expire on March 11, 2010. 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 Notice
Copyright (c) 2009 IETF Trust and the persons identified as the Copyright (c) 2010 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
publication of this document (http://trustee.ietf.org/license-info). (http://trustee.ietf.org/license-info) in effect on the date of
Please review these documents carefully, as they describe your rights publication of this document. Please review these documents
and restrictions with respect to this document. carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
Abstract include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
A number of protocols wish to leverage Transport Layer Security (TLS) This document may contain material from IETF Documents or IETF
to perform key establishment but then use some of the keying material Contributions published or made publicly available before November
for their own purposes. This document describes a general mechanism 10, 2008. The person(s) controlling the copyright in some of this
for allowing that. material may not have granted the IETF Trust the right to allow
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
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
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 . . . . . . . . . . . . . . . . . . . . . . . . 7 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . . 7 8.1. Normative References . . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . . 7 8.2. Informative References . . . . . . . . . . . . . . . . . . 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
the use of the term "Extractor" in the cryptographic community. with 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], a purposes. A typical example is DTLS-SRTP [DTLS-SRTP], a key
key management scheme for SRTP which uses DTLS to perform a key management scheme for the Secure Real-time Transport Protocol (SRTP)
exchange and negotiate the SRTP [RFC3711] protection suite and then that uses DTLS to perform a key exchange and negotiate the SRTP
uses the DTLS master_secret to generate the SRTP keys. [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 These applications imply a need to be able to export keying material
(later called Exported Keying Material or EKM) from TLS/DTLS to an (later called Exported Keying Material or EKM) from TLS/DTLS to an
application or protocol residing at an upper-layer, and securely application or protocol residing at an upper layer, and to securely
agree on the upper-layer context where the keying material will be agree on the upper-layer context where the keying material will be
used. The mechanism for exporting the keying material has the used. The mechanism for exporting the keying material has the
following requirements: 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 data by
o EKM values should be indistinguishable from random data to
attackers who don't know the master_secret. attackers who don't know the master_secret.
o It should be possible to export multiple EKM values from the same o It should be possible to export multiple EKM values from the same
TLS/DTLS association. TLS/DTLS association.
o Knowing one EKM value should not reveal any information about the
master_secret or about other EKM values. 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 The mechanism described in this document is intended to fulfill these
requirements. This mechanism is compatible with all versions of TLS. requirements. This mechanism is compatible with all versions of TLS.
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 using an exporter to obtain keying material, an In addition to using an exporter to obtain keying material, an
application using the keying material has to securely establish the application using the keying material has to securely establish the
upper-layer context where the keying material will be used. The upper-layer context where the keying material will be used. The
details of this context depend on the application, but it could details of this context depend on the application, but it could
include things such as algorithms and parameters that will be used include things such as algorithms and parameters that will be used
with the keys, identifier(s) for the endpoint(s) who will use the 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, 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 and the lifetime(s) for the context and/or keys. At a minimum, there
should be some mechanism for signalling that an exporter will be should be some mechanism for signaling that an exporter will be used.
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
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 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 [I-D.ietf-avt-dtls-srtp]. The handshake This approach is used in [DTLS-SRTP]. The handshake messages are
messages are protected by the Finished messages, so once the protected by the Finished messages, so once the handshake
handshake completes, the peers will have the same view of the completes, the peers will have the same view of the information.
information. Extensions also allow a limited form of negotiation: Extensions also allow a limited form of negotiation: for example,
for example, the TLS client could propose several alternatives for the TLS client could propose several alternatives for some context
some context parameters, and the TLS server could select one of parameters, and the TLS server could select one of them.
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 by TLS. be protected using the keys exported by TLS.
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 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.
The exporter takes three input values The exporter takes three input values:
o a disambiguating label string, o a disambiguating label string,
o a per-association context value provided by the application using o a per-association context value provided by the application using
the exporter, and the exporter, and
o a length value. o a length value.
It then computes: If no context is provided, 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 )[length]
)[length]
Where PRF is the TLS PRF in use for the session. The output is a If context is provided, it computes:
pseudorandom bit string of length bytes generated from the
master_secret. 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 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 that have labels which do not begin with this prefix.
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; see
representing the length of the context value. The context MAY be [RFC5246], Section 4.4) representing the length of the context value.
zero length. Because the context value is mixed with the The context MAY be zero length. Because the context value is mixed
master_secret via the PRF, it is safe to mix confidential information with the master_secret via the PRF, it is safe to mix confidential
into the extractor provided that the master_secret will not be known information into the exporter, provided that the master_secret will
to the attacker. 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,
skipping to change at page 6, line 9 skipping to change at page 5, line 43
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 With certain TLS cipher suites, the TLS master secret is not
necessarily unique to a single TLS session. In particular, with RSA necessarily unique to a single TLS session. In particular, with RSA
key exchange, a malicious party acting as TLS server in one session key exchange, a malicious party acting as TLS server in one session
and TLS client in another session can cause those two sessions to and as TLS client in another session can cause those two sessions to
have the same TLS master secret (though the sessions must be have the same TLS master secret (though the sessions must be
established simultaneously to get adequate control of the Random established simultaneously to get adequate control of the Random
values). Applications using the EKM need to consider this in how values). Applications using the EKM need to consider this in how
they use the EKM; in some cases, requiring the use of other cipher they use the EKM; in some cases, requiring the use of other cipher
suites (such as those using Diffie-Hellman key exchange) may be suites (such as those using a Diffie-Hellman key exchange) may be
advisable. advisable.
Designing a secure mechanism that uses extractors is not necessarily Designing a secure mechanism that uses exporters is not necessarily
straightforward. This document only provides the extractor straightforward. This document only provides the exporter mechanism,
mechanism, but the problem of agreeing on the surrounding context and but the problem of agreeing on the surrounding context and the
the meaning of the information passed to and from the extractor meaning of the information passed to and from the exporter remains.
remains. Any new uses of the extractor mechanism should be subject Any new uses of the exporter mechanism should be subject to careful
to careful review. review.
6. IANA Considerations 6. IANA Considerations
IANA is requested to create (has created) a TLS Exporter Label IANA has created a TLS Exporter Label registry for this purpose. The
registry for this purpose. The initial contents of the registry are initial contents of the registry are given below:
given below:
Value Reference Note Value Reference Note
----------------------------- --------- ---- ----------------------------- --------- ----
client finished [RFC5246] (1) client finished [RFC5246] (1)
server finished [RFC5246] (1) server finished [RFC5246] (1)
master secret [RFC5246] (1) master secret [RFC5246] (1)
key expansion [RFC5246] (1) key expansion [RFC5246] (1)
client EAP encryption [RFC5216] 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 Note: (1) These entries are reserved and MUST NOT be used for the
purpose described in RFC XXXX, in order to avoid confusion with purpose described in RFC 5705, in order to avoid confusion with
similar, but distinct use in RFC 5246. 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 RFC 5226 Specification Required Future values are allocated via the RFC 5226 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 for the contents of
IANA section and Section 3. Thanks to David McGrew for helpful the IANA section and Section 3. Thanks to David McGrew for helpful
discussion of the security considerations and to Vijay Gurbani and discussion of the security considerations and to Vijay Gurbani and
Alfred Hoenes for editorial comments. Alfred Hoenes for editorial comments.
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.
[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
(TLS) Protocol Version 1.2", RFC 5246, August 2008. Security (TLS) Protocol Version 1.2", RFC 5246,
August 2008.
8.2. Informative References 8.2. Informative References
[RFC5216] Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS [DTLS-SRTP] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Authentication Protocol", RFC 5216, March 2008. 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. [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and
Norrman, "The Secure Real-time Transport Protocol (SRTP)", K. Norrman, "The Secure Real-time Transport Protocol
RFC 3711, March 2004. (SRTP)", 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] [RFC5216] Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS
McGrew, D. and E. Rescorla, "Datagram Transport Layer Authentication Protocol", RFC 5216, March 2008.
Security (DTLS) Extension to Establish Keys for Secure
Real-time Transport Protocol (SRTP)",
draft-ietf-avt-dtls-srtp-07 (work in progress),
February 2009.
[RFC5281] Funk, P. and S. Blake-Wilson, "Extensible Authentication [RFC5281] Funk, P. and S. Blake-Wilson, "Extensible Authentication
Protocol Tunneled Transport Layer Security Authenticated Protocol Tunneled Transport Layer Security Authenticated
Protocol Version 0 (EAP-TTLSv0)", RFC 5281, August 2008. Protocol Version 0 (EAP-TTLSv0)", RFC 5281, August 2008.
Author's Address Author's Address
Eric Rescorla Eric Rescorla
Network Resonance RTFM, Inc.
2064 Edgewood Drive 2064 Edgewood Drive
Palo Alto, CA 94303 Palo Alto, CA 94303
USA USA
Email: ekr@networkresonance.com EMail: ekr@rtfm.com
 End of changes. 51 change blocks. 
131 lines changed or deleted 138 lines changed or added

This html diff was produced by rfcdiff 1.38. The latest version is available from http://tools.ietf.org/tools/rfcdiff/