Subject: [AVT] draft-perkins-avt-srtp-vbr-audio-00

From: David McGrew <[email-masked]>
Date: Sun, 8 Mar 2009 17:19:47 -0700

Hi Colin,

thanks for writing this draft, it provides guidance on an important  
issue.

I think we should consider using the RTP padding facility with SRTP  
and VBR codecs.  It is "legal" according to RFC 3711, and it could  
provide significant security advantages.   The security concerns with  
the interaction between padding and encryption that are mentioned in  
RFC 3711 would not apply if message authentication were in use, and we  
could include this in the recommendation.    (Here I'm revisiting  
Section 7 of [V02] from RFC 3711, which is online at http://lasecwww.epfl.ch/pub/lasec/doc/Vau02a.ps) 
    I offer to help look into the security more if it seems desirable  
to use SRTP with VBRs.   (It might be desirable for interoperability  
reasons, even if padding was used to hide VBR lengths; at least that's  
my thinking.)

One consideration is that the maximum padding that can be added with  
the RTP padding facility is 255 bytes, and depending on the VBR codec,  
that might not be enough for security.

I think that we're up against some theoretical limits here, in that we  
have to choose between compression efficiency, latency, and  
confidentiality.

best regards,

David