Re: [AVT] FEC and SRTP, some clarifications

["vsathe at san.rr.com" <vsathe at san.rr.com>]

Please also clarify whether the FEC stream and the RTP stream can be
decrypted independent of each other and can possibly use different
encryption/key management algorithms. 

Thank you.
Vinay


-----Original Message-----
From: avt-admin@ietf.org [mailto:avt-admin@ietf.org] On Behalf Of
Elisabetta Carrara (KI/EAB)
Sent: Monday, April 26, 2004 9:37 AM
To: 'avt@ietf.org'
Cc: Mats Näslund (KI/EAB); Karl Norrman (KI/EAB); 'Mark Baugher' (E-mail);
David A. McGrew (E-mail)
Subject: [AVT] FEC and SRTP, some clarifications

Hi,
we have lately received some questions about the combined use of SRTP and
FEC. We thought of bringing them to the mailing list.

SRTP only defines the default order of the combined processing (first FEC,
then SRTP, at sender side).

First of all, the SRTP spec misses to specify normative language about the
case when the FEC stream is separated from the original RTP stream (one
possibility in RFC2733) and the latter uses encryption. 
For this case, the normative language needed is:
the separate FEC stream MUST be encrypted if the original RTP
stream(s) is encrypted.
This text is needed because if the FEC stream (related to a separate
encrypted RTP stream) is sent unencrypted, there is a security leakage.
RFC2733 discusses the issue and uses a SHOULD. It has to be a MUST, and
should have been present explicitly in SRTP.

Second, there are constraints on the key usage according to how the
"separate stream" is implemented. We were asked on this, so here is a
summary.
SRTP mandates that different RTP sessions must use separate master keys,
and the sharing of the master key within the same RTP session requires
unique SSRC.
RFC2733 is very open in the definition of what the "separate stream"
is. Possible key settings:

C1) if the RTP stream and the FEC stream have different SSRCs, and are in
the same RTP session, then they MAY share the same master key.
Note, in case they share it, the FEC stream and the original RTP stream
simply use the same session keys.

C2) if the RTP stream and the FEC stream have the same SSRC, and are in
different RTP sessions, they use different master keys.

We hope this has answered some of the questions we got.

Cheers
/the SRTP authors



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