Subject: [AVT] SRTP MAC question

From: Stephen Casner <[email-masked]>
Date: Wed, 18 Dec 2002 15:43:07 -0800 (PST)

Allison and Steve,

In the minutes of the AVT meeting in Atlanta, which were prepared from
notes taken during the meeting and a review of the audio tape, we
noted the following statements from you as part of the discussion of
SRTP security considerations:

   Mark Baugher asked if changing the default mandatory transforms,
   adding CBC mode as an option, would satisfy concerns?  Steve Bellovin
   answered that, assuming you meet requirements for safely using counter
   mode, there is no strong need for CBC mode; the MAC is much more
   critical.

   Allison commented that the draft is intended to be general purpose,
   but is optimized for cellular use.  The default transform needs to be
   suitable for the general case, with a non-optional MAC if counter mode
   is used, and justification why weaker options are present for cellular
   operation.

I would like to get a clarification of your position:  Independent of
what transforms are chosen as the defaults, are you saying that if
counter mode is selected as the cypher for a particular application,
then a MAC MUST also be used?  And if so, do you mean to impose a
minimum size for the MAC?

Note that if your answer is yes, we have a conflict with what a number
of people hold as the requirements for some applications.

                                                        -- Steve

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