Review (Re: [TLS] I-D ACTION:draft-ietf-tls-ctr-00.txt)

[Lakshminath Dondeti <[e-mail]>]

Hi Nagendra,

I have a few questions and thoughts on the TLS-CTR mode I-D:

On the IV construction:

I see that the nonce is 48 bits in length.  My recollection of David 
McGrew's analysis of CTR mode is that 64 bits of nonce would provide 
128-bit security (for a 128bit key and CTR mode).  My question is on 
the level of security 48-bit nonces provide.  I see that 3686 uses a 
32-bit nonce.  I am confused at what is the right length for the 
nonce.  Some discussion on that might be appropriate in the SEC 
considerations section.  Perhaps David might help clarify (I may have 
misread his paper).

On the block level counter, 3686 specifies a 32-bit counter citing 
IPv6 Jumbograms' requirement.  SRTP makes a note that for multimedia 
apps, that might be safely ignored.  Could you also make a similar 
note in this document, or perhaps you might also want to support 
jumbograms?  (Perhaps IPv6 historians might tell us whether there are 
practical uses for jumbograms).

I don't think it is sufficient to refer to 3686's security 
considerations.  Those notes are written for a different protocol and 
context.  Perhaps you might copy and paste from there and edit the 
text, if you want to reuse text.

best,
Lakshminath

At 03:50 PM 2/28/2006, you wrote:
>A New Internet-Draft is available from the on-line Internet-Drafts 
>directories.
>This draft is a work item of the Transport Layer Security Working 
>Group of the IETF.
>
>         Title           : AES Counter Mode Cipher Suites for TLS and DTLS
>         Author(s)       : N. Modadugu, E. Rescorla
>         Filename        : draft-ietf-tls-ctr-00.txt
>         Pages           : 9
>         Date            : 2006-2-28
>
>    This document describes the use of the Advanced Encryption Standard
>    (AES) Counter Mode for use as a Transport Layer Security (TLS) and
>    Datagram Transport Layer Security (DTLS) confidentiality mechanism.
>
>A URL for this Internet-Draft is:
>http://www.ietf.org/internet-drafts/draft-ietf-tls-ctr-00.txt
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