RE: [AVT] SRTP: question about MKI length

Hi Mark,

I am curious about using the MKI to convey "a variety of key management information."  Could you please elaborate?  I know of the MKI being used in 3GPP2 to send key management information also, but with the concern that the MKI field is not integrity protected.  If the MKI is used to send a key index, we know that no integrity protection is required, but if it is intended for sending arbitrary key management information, then perhaps integrity protecting that field would be necessary.

Thoughts?

thanks and regards,
Lakshminath

-----Original Message-----
From: avt-bounces@ietf.org on behalf of Mark Baugher
Sent: Thu 7/21/2005 7:04 AM
To: Usha Sharma
Cc: avt@ietf.org
Subject: Re: [AVT] SRTP: question about MKI length

hi
   RFC 3711 assumes that the key management system will set the maximum 
length for the MKI.  The use of an MKI function is common in video 
broadcasting where a key gets rotated at rates that may be less than 
one second.  TV conditional access vendors operate proprietary systems 
that rotate the key according to application needs (there is really no 
cryptographic need to rotate a 128-bit AES counter-mode key until 2^64 
packets have been encrypted using it - a very long time).  The MKI was 
added for this application - and vendors in this industry use various 
sizes for the key index, particularly to convey a variety of key 
management information over a broadcast channel.

   In general, there is no need to use an MKI.  If there is, I would 
expect that a small, one-byte MKI would suffice to handle cases where 
key rotation might be useful.

Mark
On Jul 20, 2005, at 11:18 PM, Usha Sharma wrote:

> There is no description in RFC 3711 for upper limit of MKI length and 
> range of MKI value. SDP (draft-ietf-mmusic-sdescriptions-11.txt) 
> defines that MKI value is a positive integer and MKI length could be 
> up to 128 byte. Is it worthwhile to use such big MKI value for voice 
> applications, considering the bandwidth overhead introduced by it. 
> What would be the optimal value of MKI length for most applications?
> _______________________________________________
> Audio/Video Transport Working Group
> avt@ietf.org
> https://www1.ietf.org/mailman/listinfo/avt

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