Network Working Group C. Holmberg
Internet-Draft Ericsson
Updates: 4572 (if approved) September 23, 2016
Intended status: Standards Track
Expires: March 27, 2017
Updates to RFC 4572
draft-ietf-mmusic-4572-update-06.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 with a stronger
cipher suite, and removes the requirement to use the same hash
function for calculating a certificate fingerprint that is used to
calculate the certificate signature.
Status of This Memo
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This Internet-Draft will expire on March 27, 2017.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Update to RFC 4572 . . . . . . . . . . . . . . . . . . . . . 3
3.1. Update to the sixth paragraph of section 5 . . . . . . . 3
3.2. New paragraphs to the end of section 5 . . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
7. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 6
8. Normative References . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
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 (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") [RFC4566], 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currently preferred cipher suite is considered outdated, and the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.
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This document updates RFC 4572 [RFC4572] 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.
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
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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
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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 functions. It can also
occur if one or more fingerprints associated with multiple
certificates have been calculated. This might be needed if multiple
certificates will be used for media associated with an m- line
(e.g. if separate certificates are used for RTP and RTCP), or where
it is not known which certificate will be used when the
fingerprints are exchanged. In such cases, one or more fingerprints
MUST be calculated for each possible certificate. An endpoint
MUST, as a minimum, calculate a fingerprint using both the 'SHA-256'
hash function algorithm and the hash function used to generate the
signature on the certificate for each possible certificate.
Including the hash from the signature algorithm ensures
interoperability with strict implementations of RFC 4572.
Either of these fingerprints MAY be omitted if the endpoint includes
a hash with a stronger hash algorithm that it knows that the peer
supports, if it is known that the peer does not support the hash
algorithm, or if local policy mandates use of stronger algorithms.
If fingerprints associated with multiple certificates are
calculated, the same set of hash functions MUST be used to
calculate fingerprints for each certificate associated with the
m- line.
For each used certificate, an endpoint MUST be able to match at
least one fingerprint, calculated using the hash function that the
endpoint supports and considers most secure, with the used
certificate. If the checked fingerprint does not match the used
certificate, the endpoint MUST NOT establish the TLS connection. In
addition, the endpoint MAY also check fingerprints calculated using
other hash functions that it has received for a match. For each
hash function checked, one of the received fingerprints calculated
using the hash function MUST match the used certificate.
NOTE: The SDP fingerprint attribute does not contain a reference to
a specific certificate. Endpoints need to compare the fingerprint
with a certificate hash in order to look for a match.
4. Security Considerations
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
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agility, and incremental deployment of newer, and more secure, cipher
suites.
5. IANA Considerations
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.
6. Acknowledgements
Martin Thomson, Paul Kyzivat, Jonathan Lennox and Roman Shpount
provided valuable comments and input on this document.
7. Change Log
[RFC EDITOR NOTE: Please remove this section when publishing]
Changes from draft-ietf-mmusic-4572-update-05
o Added a requirement to generate a fingerprint that matches the
signature.
o Added text clarifying that updates do not make existing
implementations non-compliant with RFC 4572.
o IANA Considerations text added.
Changes from draft-ietf-mmusic-4572-update-04
o 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
o Mandatory (except in specific situations) to provide a fingerprint
calculated using SHA-256.
o 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
o Editorial fixes based on comments from Martin Thomson.
o Non-used references removed.
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Changes from draft-ietf-mmusic-4572-update-01
o Changes based on comments from Martin Thomson.
o - Editorial fixes
o Changes in handling of multiple fingerprints.
o - Sender must send same set of hash functions for each offered
certificate.
o - 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
o Changes in handling of multiple fingerprints.
o - Number of fingerprints calculated for each certificate does not
have to match.
o - Clarified that receiver shall check check fingerprints using
hash algorithms it considers safe.
o - Additional text added to security considerations section.
Changes from draft-holmberg-mmusic-4572-update-01
o Adopted WG document (draft-ietf-mmusic-4572-update-00) submitted.
o IANA considerations section added.
8. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <http://www.rfc-editor.org/info/rfc4566>.
[RFC4572] Lennox, J., "Connection-Oriented Media Transport over the
Transport Layer Security (TLS) Protocol in the Session
Description Protocol (SDP)", RFC 4572,
DOI 10.17487/RFC4572, July 2006,
<http://www.rfc-editor.org/info/rfc4572>.
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Author's Address
Christer Holmberg
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: christer.holmberg@ericsson.com
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