Subject: [AVT] SRTP : initial sequence number at receiver side

From: "Usha Sharma" <[email-masked]>
Date: Wed, 16 Nov 2005 16:17:48 -0800

Hi all.

I have a question about using the s_1 for initial sequence number in
SRTP receiver. We generate srtp index based on locally maintained ROC
and sequence number of the RTP packet. The key derivation uses the srtp
index to derive keys. 

At the beginning of the session, the sender will use the randomly
generated RTP sequence number (say 1000) and ROC=0 to generate the
session keys for the first time and use these keys for encrypting
packets. 

At the receiver side, if we lose a few initial packets and the first RTP
packet received is say 1003. Now the receiver uses sequence number 1003
and ROC=0 to generate the session keys and tries to decrypt packets
using these keys. The derived keys will be different from those used by
the sender and the authenticate will fail at receiver.

How do we get around this problem. I guess one of the ways is to signal
the initial sequence number in signaling. That way even if we lose a few
packets, key derivation will not be affected. But if such a provision is
not available, is there a way to get a reliable initial sequence number
at receiver! 

Thanks

Usha 

_______________________________________________
Audio/Video Transport Working Group
[email-masked]
https://www1.ietf.org/mailman/listinfo/avt

---

From: Mats Näslund <[email-masked]>
Date: Thu, 17 Nov 2005 09:36:39 +0100

Hi Usha,

There is no problem. Please refer to

	http://srtp.sourceforge.net/faq.html

e.g. Q1 and Q14. The correct key is uniquely determined
(both at sender and receiver) by the correct ROC and the
SEQ of the *current* packet. Put differently,

   current_packet_key = f(master_key, ROC, SEQ, derivation_rate)

where f is a given function.

Best,

/Mats

Usha Sharma wrote:

> session keys for the first time and use these keys for encrypting packets.
> 
>  
>