Re: Fwd: Re: [AVT] IESG Review of draft-ietf-srtp-08.txt - another set of comments

[Mats Näslund <[e-mail]>]

Hi all,

David Mcgrew wrote:
> Mats, Colin, and others,
> 
> I belatedly realized that there is another advantage to using the RTP 
> padding over other schemes: compactness.  If someone implements CBC without 
> using the RTP padding, then the plaintext needs to be turned into a 
> prefix-free code so that the receiver can unambiguously remove any padding 
> that may be present.  Because of this, AES CBC ciphertext will expand by an 
> extra 16-octet block on messages for which no padding is needed, if the 
> conventional method is used (see Appendix A of 
> http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf for more 
> detail).  OTOH, if the RTP padding is used, then the padding bit indicates 
> the presence of padding, and there is never need to further expand the 
> ciphertext.  Also, the NIST document cited above makes it clear that any 
> suitable padding method is considered acceptable.   So this makes me think 
> that we should recommend RTP padding for use with modes that require padding.

Will someone concerned with compactness use a mode that requires
padding in the first place? Won't they rely on the pre-defined modes?

I honestly don't see how we can recommend a padding that we *know*
is "flawed", in particular when the flaw occurs in the absence
of integrity---something we had to fight allow at all!

I am fine with keeping the RTP padding as a sort of "default",
be honest about its pros and cons, but I would not go as far as
to *recommend* it.

> 
> As a side note, I would still prefer a CBC variant that doesn't use padding 

Agreed, but that is for a future extension of SRTP.

> as I'd mentioned before, but at any rate the compactness benefit of using 
> RTP padding deserves to be mentioned.

Absolutely, but so does the security problems.

/M


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