Subject: [AVT] SRTPv2: summary

From: "Elisabetta Carrara (ERA)" <[email-masked]>
Date: Wed, 5 Dec 2001 16:21:38 +0100

Hi
here is a summary of changes to the SRTP draft: 
<draft-ietf-avt-srtp-02.txt>

The draft has been restructured following a framework-like
approach, to allow upgrade to new cryptographic transforms. 
Pre-defined transforms are described, so that the majority 
of the original SRTP goals (bandwidth economy, small 
footprint, efficiency, etc) are achieved. 
 
The main changes:
 
1) following the comments in London:

	- TESLA discussion(for source origin authentication)  
		eliminated
	- clarification of how to retrieve the crypto context:
		via <SSRC, destination network address, 
		destination transport port number>
	- SRTCP coverage better explained (for compound packets)

2) others
 
	- optional SPI eliminated, as motivated in a note sent 
		to the mailing list on the 04 Oct 2001
	- new parameters in the crypto context, e.g. to allow 
		key derivation (new session keys without key exchange), 
		and re-keying
	- added a key derivation: it is used to derive all the 
		keys needed per RTP session (SRTP encryption key and 
		authentication key, and the corresponding ones for SRTCP) 
		from the same master key. It is also possible to iterate 
		such derivation (optionally), to get new session keys 
		without need of re-keying.
	- pre-defined encryption transforms: they are still 
		AES-CM (with new definition) and optional AES-f8. The 
		keystream is now split into 
		# a 'keystream prefix' (may be reserved for use 
		in a message authentication code, e.g. TMMH), and
		# a 'keystream suffix' (for the encryption)
 	- pre-defined authentication transforms : UMAC is 
 		replaced (lack of stable documentation). Default is now 
		HMAC/SHA1 (4 bytes of tag), and optional TMMH/16.
 	- test vectors for AES-CM and TMMH added

One open issue:
the current definition of AES-CM (the default encryption 
transform) does not allow multiple streams to share the same 
master key (AES-f8 allows it). 
In the previous versions of SRTP, this feature was a design goal and 
was achieved including the SSRC into the IV formation.
It is still an open issue mainly because we had lack of time 
for discussing it.

Thanks,
/Elisabetta

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