RFC 5479, "Requirements and Analysis of Media Security Management Protocols", April 2009Source of RFC: sip (rai)
Errata ID: 2602
Publication Format(s) : TEXT
Reported By: Fabio Pietrosanti
Date Reported: 2010-11-04
Verifier Name: Robert Sparks
Date Verified: 2011-02-21
Section A.5.2 says:
SDP Security Descriptions with SIPS Not applicable; SDP Security Descriptions does not have a long- term secret.
It should say:
SDP Security Descriptions with SIPS The PFS feature of SDP Security Description with SIPS rely on TLS and the availability or not of PFS for SRTP calls depends on the negotiated TLS key negotiation algorithm. If the selected TLS key negotiation algorithm of SIPS provide PFS feature, then the underlying SRTP encryption will support PFS. For example TLS_DHE_RSA_WITH_AES_256_CBC_SHA provde PFS feature as described in RFC5246. If the selected TLS key negotiation algorithm of SIPS does not provide PFS feature, then the underlying SRTP encryption will not support PFS. For example TLS_RSA_WITH_AES_256_CBC_SHA does not provide PFS feature as described in RFC5246.
It's not true that SDP Security Descriptions with SIPS have PFS "Not applicable" because the SDES rely on TLS that is part of the security scheme.
Practically if the long terms keys (the x509v3 RSA key of SIPS server) is compromised, the TLS sessions can be decrypted, the SDES key extracted and SRTP calls deciphered.
TLS support key exchange methods that provide PFS trough the use of Ephemeral Diffie Hellman keys.
When SIPS use TLS with DHE key negotiation, then SDES acquire PFS feature because even in case of long-term key compromise (the server x509v3 RSA key), the short term keys (the SDES keys exchanged) will be safe.
From reviewer Dale Worley:
It seems that the entry for "SDP Security Descriptions with S/MIME" is
also incorrect, as revelation of the private keys of the participants
will render the SDES readable. I think better phrasing of the revised
SDP Security Descriptions with SIPS
PFS if the selected TLS cipher suites for the SIPS hops provide PFS.
SDP Security Descriptions with S/MIME