Updates to RFC 4572

RFC 4572 specifies how to establish Transport Layer Security (TLS) connections using the Session Description Protocol (SDP) . RFC 4572 defines the SDP 'fingerprint' attribute, which is used to carry a secure hash value (fingerprint) associated with a certificate. However, RFC 4572 is currently unclear on whether multiple 'fingerprint' attributes can be associated with a single SDP media description ("m= line") , and the associated semantics. Multiple fingerprints are needed if an endpoints wants to provide fingerprints associated with multiple certificates. For example, with RTP-based media, an endpoint might use different certificates for RTP and RTCP. RFC 4572 also specifies a preferred cipher suite. However, the currently preferred cipher suite is considered outdated, and the preference needs to be updated. RFC 4572 mandates that the hash function used to calculate the fingerprint is the same hash function used to calculate the certificate signature. That requirement might prevent usage of newer, stronger and more collision-safe hash functions for calculating certificate fingerprints. This change also requires that multiple 'fingerprint' attributes can be associated with a single "m=" line, so that implementations are able to provide fingerprints calculated using updated hash functions alongside those that are needed to interoperate with existing implementations. This document updates RFC 4572 by clarifying the usage of multiple SDP 'fingerprint' attributes. It is clarified that multiple 'fingerprint' attributes can be used to carry fingerprints, calculated using different hash functions, associated with a given certificate, and to carry fingerprints associated with multiple certificates. The fingerprint matching procedure, when multiple fingerprints are provided, are also clarified. The document also updates the preferred cipher suite with a stronger cipher suite, and removes the requirement to use the same hash function for calculating a certificate fingerprint and certificate signature. NOTE: Even though this document updates the procedures in RFC 4572, it does not make existing implementations non-compliant with RFC 4572. The updated procedures in this document have been defined in order to be backward compatible with the procedures in RFC 4572.

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 .

This section updates section 5 of RFC 4572.

This document improves security. It updates the preferred hash function cipher suite from SHA-1 to SHA-256. By clarifying the usage and handling of multiple fingerprints, the document also enables hash agility, and incremental deployment of newer, and more secure, cipher suites.

IANA is requested to add a reference to this document for the att-field (both session and media level) registration "fingerprint" in Session Description Protocol (SDP) Parameters registry.

Martin Thomson, Paul Kyzivat, Jonathan Lennox and Roman Shpount provided valuable comments and input on this document.

[RFC EDITOR NOTE: Please remove this section when publishing] Changes from draft-ietf-mmusic-4572-update-05 Added a requirement to generate a fingerprint that matches the signature. Added text clarifying that updates do not make existing implementations non-compliant with RFC 4572. IANA Considerations text added. Changes from draft-ietf-mmusic-4572-update-04 Removed prevously added requirement that endpoint must calcuate at least one fingerprint using a hash function that was also used by the peer. Changes from draft-ietf-mmusic-4572-update-03 Mandatory (except in specific situations) to provide a fingerprint calculated using SHA-256. When an endpoint receives fingerprints from its peer, the endpoint must (except in specific situations) calculate at least one fingerpint using a hash function that was also used by the peer. Changes from draft-ietf-mmusic-4572-update-02 Editorial fixes based on comments from Martin Thomson. Non-used references removed. Changes from draft-ietf-mmusic-4572-update-01 Changes based on comments from Martin Thomson. - Editorial fixes Changes in handling of multiple fingerprints. - Sender must send same set of hash functions for each offered certificate. - Receiver must check the hash function it considers most secure for a match. It may check other hash functions. Changes from draft-ietf-mmusic-4572-update-00 Changes in handling of multiple fingerprints. - Number of fingerprints calculated for each certificate does not have to match. - Clarified that receiver shall check check fingerprints using hash algorithms it considers safe. - Additional text added to security considerations section. Changes from draft-holmberg-mmusic-4572-update-01 Adopted WG document (draft-ietf-mmusic-4572-update-00) submitted. IANA considerations section added.