draft-ietf-tls-extractor-03.txt   draft-ietf-tls-extractor-04.txt 
Network Working Group E. Rescorla Network Working Group E. Rescorla
Internet-Draft Network Resonance Internet-Draft RTFM, Inc.
Intended status: Standards Track November 01, 2008 Intended status: Standards Track February 28, 2009
Expires: May 5, 2009 Expires: September 1, 2009
Keying Material Extractors for Transport Layer Security (TLS) Keying Material Exporters for Transport Layer Security (TLS)
draft-ietf-tls-extractor-03.txt draft-ietf-tls-extractor-04.txt
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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. Exporter Definition . . . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5 5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
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 . . . . . . . . . . . . . . . . . 7 8.2. Informational References . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 6
Intellectual Property and Copyright Statements . . . . . . . . . . 8
1. Introduction 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) A number of protocols wish to leverage Transport Layer Security (TLS)
[RFC4346] 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],
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.
These applications imply a need to be able to extract 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 extracting 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 extract 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
who don't know the master_secret. who don't know the master_secret.
o It should be possible to extract 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 o Knowing one EKM value should not reveal any information about the
master_secret or about other EKM values. master_secret or about other EKM values.
The mechanism described in this document is intended to fill these The mechanism described in this document is intended to fill these
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 extracting keying material, an application using the In addition to exporting keying material, an application using the
keying material has to securely establish the upper-layer context keying material has to securely establish the upper-layer layer
where the keying material will be used. The details of this context context where the keying material will be used. The details of this
depend on the application, but it could include things such as context depend on the application, but it could include things such
algorithms and parameters that will be used with the keys, 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 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 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 extracted 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 extractor 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 [DTLS-SRTP]. The handshake messages are
protected by the Finished messages, so once the handshake protected by the Finished messages, so once the handshake
completes, the peers will have the same view of the information. completes, the peers will have the same view of the information.
Extensions also allow a limited form of negotiation: for example, Extensions also allow a limited form of negotiation: for example,
the TLS client could propose several alternatives for some context the TLS client could propose several alternatives for some context
parameters, and TLS server could select one of them. parameters, and 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 extracted from TLS. be protected using the keys exported 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. Exporter 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 exporter 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 exporter 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 The output is a pseudorandom bit string of length bytes generated
pseudorandom bit string of length bytes generated from the from the master_secret.
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 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.
opaque context<0..2^16-1>; 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 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.
The context value allows the application using the extractor to mix The context value allows the application using the exporter to mix
its own data with the TLS PRF for the extractor output. One example its own data with the TLS PRF for the exporter output. The context
of where this might be useful is an authentication setting where the value length is encoded as an unsigned 16-bit quantity (uint16)
client credentials are valid for more than one identity; the context representing the length of the context value.
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 Because an exporter produces the same value if applied twice with the
independent. More formally, after a particular TLS session, if an same label to the same master_secret, it is critical that two EKM
adversary is allowed to choose multiple (label, context value) pairs values generated with the same label be used for two different
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
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, purposes--hence the requirement for IANA registration. However,
because extractors 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.
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 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
----- ------------ ----- ------------
client finished [RFC4346] client finished [RFC5246]
server finished [RFC4346] server finished [RFC5246]
master secret [RFC4346] master secret [RFC5246]
key expansion [RFC4346] key expansion [RFC5246]
client EAP encryption [RFC2716] client EAP encryption [RFC2716]
ttls keying material [draft-funk-eap-ttls-v0-01] ttls keying material [draft-funk-eap-ttls-v0-01]
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. Thanks to David McGrew for helpful IANA section and Section 3.
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,
October 1998. October 1998.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006. (TLS) Protocol Version 1.2", RFC 5246, August 2008.
8.2. Informational References 8.2. Informational References
[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.
[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-06 (work in progress), draft-ietf-avt-dtls-srtp-06 (work in progress),
October 2008. October 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
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