Keying Material Exporters for Transport Layer Security (TLS)
draft-ietf-tls-extractor-06.txt

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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
2.  Conventions Used In This Document
3.  Binding to Application Contexts
4.  Exporter Definition
5.  Security Considerations
6.  IANA Considerations
7.  Acknowledgments
8.  References
    8.1.  Normative References
    8.2.  Informative References
§  Author's Address

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) [RFC5246] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.) or Datagram TLS (DTLS) [RFC4347] (Rescorla, E. and N. Modadugu, “Datagram Transport Layer Security,” April 2006.) 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] (McGrew, D. and E. Rescorla, “Datagram Transport Layer Security (DTLS) Extension to Establish Keys for Secure Real-time Transport Protocol (SRTP),” February 2009.), a key management scheme for SRTP which uses DTLS to perform a key exchange and negotiate the SRTP [RFC3711] (Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” March 2004.) 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 (later called Exported Keying Material or EKM) from TLS/DTLS, and securely agree on the upper-layer context where the keying material will be used. The mechanism for exporting the keying material has the following requirements:

• Both client and server need to be able to export the same EKM value.
• EKM values should be indistinguishable from random by attackers who don't know the master_secret.
• It should be possible to export multiple EKM values from the same TLS/DTLS association.
• Knowing one EKM value should not reveal any information about the master_secret or about other EKM values.

The mechanism described in this document is intended to fulfill these requirements.

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

3.  Binding to Application Contexts

In addition to using an exporter to obtain keying material, an application using the 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 a minimum, there should be some mechanism for signalling that an exporter 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:

• One important part of the context -- which application will use the exported keys -- is given by the disambiguating label string (see Section 4).
• Information about the upper-layer context can be included in the optional data after the exporter label (see Section 4).
• Information about the upper-layer context can be exchanged in TLS extensions included in the ClientHello and ServerHello messages. This approach is used in [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),” February 2009.). The handshake messages are protected by the Finished messages, so once the handshake completes, the peers will have the same view of the information. Extensions also allow a limited form of negotiation: for example, the TLS client could propose several alternatives for some context parameters, and the TLS server could select one of them.
• The upper-layer protocol can include its own handshake which can be protected using the keys exported by 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.  Exporter Definition

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 value.

The exporter takes three input values

• a disambiguating label string,
• a per-association context value provided by the application using the exporter, and
• a length value.

It then computes:

       PRF(SecurityParameters.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.

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 5226 [RFC5226] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.). 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 exporter to mix 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 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. 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

The prime security requirement for exporter 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 (Exporter Definition), where the attacker can control the context value; such an attacker MUST NOT be able to predict the output of the exporter. Similarly, an attacker who does not know the master secret should not be able to distinguish valid exporter 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 exporter 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, 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 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

IANA is requested to create (has created) a TLS Exporter Label registry for this purpose. The initial contents of the registry are given below:

     Value                          Reference  Note
     -----------------------------  ---------  ----
     client finished                [RFC5246]  (1)
     server finished                [RFC5246]  (1)
     master secret                  [RFC5246]  (1)
     key expansion                  [RFC5246]  (1)
     client EAP encryption          [RFC5216]
     ttls keying material           [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 RFC 5226 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 (Binding to Application Contexts). Thanks to David McGrew for helpful discussion of the security considerations and Alfred Hoenes for editorial comments.

8.  References

8.1. Normative References

8.2. Informative References

Author's Address