the ROC) is not communicated but derived, and if we
allow multiple entities to use the same SSRC
(sequentially), the ROC can be wrong.
- Problem 4: All crypto contexts that share a master key need to
maintain a shared set of counters (master key
lifetime), and if we allow for multiple entities on
different platforms to share a master key, we would
need a mechanism to synchronize these counters.
Problem 1 could be addressed by using the MKI as proposed
separately; however, it would result in using extra bandwidth for
each SRTP media packet. Solving problem 2 implies a need for
being able to synchronize SSRC values with the answerer (or
abandon the session when SSRC reuse or SSRC collisions occur).
Problem 3 implies a need for being able to synchronize ROC values
on a per SSRC basis (or abandon the session when SSRC reuse
occurs). Problem 4 could be solved by having the offerer (Alice,
i.e., the entity receiving media) determine how many packets have
actually been generated by the total set of senders to Alice and,
hence, be the one to initiate the rekeying. In the case of packet
losses, etc. this is not foolproof, but in practice it could
probably be addressed by use of a reasonable safety margin.
In conclusion, it would be expected from an offer/answer and SIP
point of view to have the offer (and answer) keying material be
the receive keying material; however, doing so would trade
security for SIP friendliness, e.g., two-time pad and master key
lifetime issues, and violate the RFC 3711 rule for sharing an SRTP
master key across SRTP sessions.
Authors’ Addresses
Flemming Andreasen
Cisco Systems, Inc.
499 Thornall Street, 8th Floor
Edison, New Jersey 08837 USA
EMail: fandreas@cisco.com
Mark Baugher
5510 SW Orchid Street
Portland, Oregon 97219 USA
EMail: mbaugher@cisco.com
Dan Wing
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134 USA
EMail: dwing@cisco.com
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