Updates to RFC 4572
draft-holmberg-mmusic-4572-update-01.txt

Abstract

This document updates RFC 4572 by clarifying the usage of multiple SDP 'fingerprint' attributes with a single TLS connection. The document also updates the preferred cipher suite to be used, and removes the requirement to use the same hash function for calculating the certificate fingerprint that is used to calculate the certificate signature.

Status of This Memo

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Table of Contents

• 1. Introduction
• 2. Conventions
• 3. Update to RFC 4572
• 3.1. Update to the sixth paragraph of section 5
• 3.2. New paragraphs to the end of section 5
• 4. Security Considerations
• 5. Acknowledgements
• 6. Change Log
• 7. Normative References
• Author's Address

1. Introduction

RFC 4572 [RFC4572] specifies how to establish Transport Layer Security (TLS) connections using the Session Description Protocol (SDP) [RFC4566].

RFC 4572 defines the SDP 'fingerprint' attribute, which is used to carry a secure hash value associated with a certificate. However, RFC 4572 is currently unclear on whether multiple 'fingerprint' can be associated with a single SDP media description ("m= line") [RFC4566], and the associated semantics. Multiple 'fingerprint' attributes are needed when an endpoint wants to provide multiple fingerprint, using different hash functions, for a certificate. Multiple 'fingerprint' attributes are also needed if an endpoint 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 However, the currently preferred cipher suite is considered outdated, and the preference needs to be updated.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 document updates RFC 4572 [RFC4572] by clarifying the usage of multiple SDP 'fingerprint' attributes with a single TLS connection. The document also updates the preferred cipher suite to be used, and removes the requirement to use the same hash function for calculating the certificiate fingerprint and certificate signature.

2. Conventions

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. Update to RFC 4572

This section updates section 5 of RFC 4572.

3.1. Update to the sixth paragraph of section 5

OLD TEXT:

   A certificate fingerprint MUST be computed using the same one-way
   hash function as is used in the certificate's signature algorithm.
   (This ensures that the security properties required for the
   certificate also apply for the fingerprint.  It also guarantees that
   the fingerprint will be usable by the other endpoint, so long as the
   certificate itself is.)  Following RFC 3279 [7] as updated by RFC
   4055 [9], therefore, the defined hash functions are 'SHA-1' [11]
   [19], 'SHA-224' [11], 'SHA-256' [11], 'SHA-384' [11], 'SHA-512' [11],
   'MD5' [12], and 'MD2' [13], with 'SHA-1' preferred.  A new IANA
   registry of Hash Function Textual Names, specified in Section 8,
   allows for addition of future tokens, but they may only be added if
   they are included in RFCs that update or obsolete RFC 3279 [7].
   Self-signed certificates (for which legacy certificates are not a
   consideration) MUST use one of the FIPS 180 algorithms (SHA-1,
   SHA-224, SHA-256, SHA-384, or SHA-512) as their signature algorithm,
   and thus also MUST use it to calculate certificate fingerprints.

   
NEW TEXT:

   Following RFC 3279 [7] as updated by RFC 4055 [9], therefore, the 
   defined hash functions are 'SHA-1' [11] [19], 'SHA-224' [11], 
   'SHA-256' [11], 'SHA-384' [11], 'SHA-512' [11], 'MD5' [12], and 
   'MD2' [13], with 'SHA-256' preferred.  A new IANA registry of Hash 
   Function Textual Names, specified in Section 8, allows for addition 
   of future tokens, but they may only be added if they are included 
   in RFCs that update or obsolete RFC 3279 [7].
      
                

3.2. New paragraphs to the end of section 5

                
NEW TEXT:

    Multiple SDP fingerprint attributes can be associated with an m- line. 
    This can occur if multiple fingerprints have been calculated for a 
    certificate, using different hash algorithms. It can also occur if 
    multiple certificates might be used (e.g. separate certificates for
    RTP and RTCP). In such cases, the same number of fingerprints MUST be 
    calculated for each certificate, and for each certificate the same set 
    of hash algorithms MUST be used.

    An endpoint MUST be able to match at least one of the received 
    fingerprints with the cerficiate(s) to be used. If there is no match,
    the endpoint MUST NOT establish the TLS connection.
    
    NOTE: The SDP fingerprint attribute does not contain a reference to
    a specific certificate. Endpoints need to compare all fingerprints with
    the cerficiate hash when looking for a match.
    
                

4. Security Considerations

This document improves security.

5. Acknowledgements

Martin Thompson, Paul Kyzivat and Jonathan Lennox provided valuable comments and input on this document.

6. Change Log

[RFC EDITOR NOTE: Please remove this section when publishing]

Changes from draft-holmberg-mmusic-4572-update-xx

• Add text

7. Normative References

Author's Address