Columbia crypto box

[Steven M. Bellovin]

In message <Pine.LNX.4.40.0302101144420.13432-100000 at bolt.sonic.net>, bear writ
es:
>

>>It's one of those things, like re-using a pad.
>
>Actually, it is re-using a pad, exactly.  It's just a pseudorandom
>pad (stream cipher) instead of a one-time pad.
>
>And while WEP had problems, it didn't have that particular problem.
>New messages with the "same" key would use a later chunk of the
>cipherstream pad under WEP.

That's not correct.  Each packet is encrypted with a key consisting of
<basekey,IV>, where "IV" is a 24-bit counter.  It does not use a later 
part of the stream; each packet starts from the beginning.

Note that with a 24-bit key, plus the difficulty of changing the key, 
there *will* be reuse.  It's compounded because (a) everyone has the 
same key, so there's lots of traffic; (b) both directions use the same 
key; and (c) some units, when power-cycled, always start the IV at 0, 
making collisions in that space more likely.

Read the Borisov et al. paper for more details on all of these points 
and more.

		--Steve Bellovin, http://www.research.att.com/~smb (me)
		http://www.wilyhacker.com (2nd edition of "Firewalls" book)



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