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Re: IV sizes for AES candidates

   From: "Steven M. Bellovin" <smb@research.att.com>
   Date: Thu, 10 Aug 2000 14:29:13 -0400

   In message <392A357CE6FFD111AC3E00A0C99848B002FE990F@hdsmsx31.hd.intel.com>, "W
   alker, Jesse" writes:
   >I share Helger's desire to at least consider counter mode for AES. Counter
   >mode is an opportunity to gain better data privacy than CBC mode offers and
   >perhaps better performance as well. The WG can fall back to CBC mode if
   >scrutiny reveals counter mode is somehow inapplicable within ESP.

   Counter mode appears to be one instance of a "seekable stream cipher", 
   per draft-mcgrew-ipsec-scesp-00.txt.  As was discussed in Pittsburgh, 
   there are a number of limitations, including the very strong 
   requirement for authentication and the need for a flat-out ban on using 
   it with manual keying -- if you don't use IKE, there's just too much 
   risk of seeing two streams encrypted with the same key and counter.

When we talked how IV's for CBC mode, we were told that using a counter
for the IV was a bad idea, because if the first eight bytes were known
and fixed (as they commonly were), you could end up with known
plaintext/ciphertext pairs that were a low hamming distance apart, and
this could be used as a wedge to attack a cipher.

If you use counter mode, doesn't this guarantee that in the face of
known plaintext, you'll have a *lot* of known plaintext/cipgertext that
have a low haming distance, and so would be subject to the same attacks
as mentioned in the previous paragraph, right?

So wouldn't the argument that says that we shouldn't use a counter to
generate IV's also mean that using counter mode would also be a bad
idea?  (This is in addition to all of the issues of an absolute
requirement for using a MAC if you care about message integrity at
all, and the dangers of resusing a keystream if you're not careful.)

							- Ted

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