--- 1/draft-ietf-tls-extractor-02.txt 2008-11-02 20:12:05.000000000 +0100 +++ 2/draft-ietf-tls-extractor-03.txt 2008-11-02 20:12:05.000000000 +0100 @@ -1,18 +1,18 @@ Network Working Group E. Rescorla Internet-Draft Network Resonance -Intended status: Standards Track September 11, 2008 -Expires: March 15, 2009 +Intended status: Standards Track November 01, 2008 +Expires: May 5, 2009 Keying Material Extractors for Transport Layer Security (TLS) - draft-ietf-tls-extractor-02.txt + draft-ietf-tls-extractor-03.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that @@ -23,43 +23,43 @@ 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 http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. - This Internet-Draft will expire on March 15, 2009. + This Internet-Draft will expire on May 5, 2009. 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions Used In This Document . . . . . . . . . . . . . . . 3 3. Binding to Application Contexts . . . . . . . . . . . . . . . . 3 4. Extractor Definition . . . . . . . . . . . . . . . . . . . . . 4 5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5 - 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5 + 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 6 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6 8.1. Normative References . . . . . . . . . . . . . . . . . . . 6 - 8.2. Informational References . . . . . . . . . . . . . . . . . 6 - Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 6 - Intellectual Property and Copyright Statements . . . . . . . . . . 7 + 8.2. Informational References . . . . . . . . . . . . . . . . . 7 + Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7 + Intellectual Property and Copyright Statements . . . . . . . . . . 8 1. Introduction A number of protocols wish to leverage Transport Layer Security (TLS) [RFC4346] 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 [I-D.ietf-avt-dtls-srtp], which 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. @@ -84,24 +84,24 @@ 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 extracting keying material, an application using the - keying material has to securely establish the upper-layer 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, + 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 minimum, there should be some mechanism for signalling that an extractor 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 One important part of the context -- which application will use @@ -120,56 +120,83 @@ o The upper-layer protocol can include its own handshake which can be protected using the keys extracted from TLS. 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. 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: o A disambiguating label string o A per-association context value provided by the extractor using application o A length value It then computes: PRF(master_secret, label, SecurityParameters.client_random + SecurityParameters.server_random + context_value_length + context_value )[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 - from the 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. - 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>; The context value allows the application using the extractor to mix - its own data with the TLS PRF for the extractor output. The context - value length is encoded as an unsigned 16-bit quantity (uint16) - representing the length of the context value. + its own data with the TLS PRF for the extractor 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) + representing the length of the context value. The context MAY be + zero length. 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 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, 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 generated from another label. 6. IANA Considerations IANA is requested to create (has created) a TLS Extractor Label registry for this purpose. The initial contents of the registry are given below: @@ -185,21 +212,22 @@ Future values are allocated via RFC2434 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 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 8.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, @@ -214,22 +242,22 @@ Security", RFC 4347, April 2006. [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004. [I-D.ietf-avt-dtls-srtp] McGrew, D. and E. Rescorla, "Datagram Transport Layer Security (DTLS) Extension to Establish Keys for Secure Real-time Transport Protocol (SRTP)", - draft-ietf-avt-dtls-srtp-04 (work in progress), - August 2008. + draft-ietf-avt-dtls-srtp-06 (work in progress), + October 2008. Author's Address Eric Rescorla Network Resonance 2064 Edgewood Drive Palo Alto, CA 94303 USA Email: ekr@networkresonance.com