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Re: CISCO PIX Vulnerability
From: daw () CS BERKELEY EDU (David Wagner)
Date: Wed, 3 Jun 1998 19:25:30 -0700


In article <v03007801b19b0b9bd8f5 () [194 82 141 242]> you write:
CISCO PIX Private Link feature uses DES key that is only 48 bits in length.
[...]
Apparently, knowing what bits are fixed will not bring attacker
any additional 'gain' in breaking a DES. At least I was told that by
people from sci.crypt group.

Either the sci.crypt folks were confused, or I am.  With only 48
unknown bits in the DES key, you can break the encryption 2^8 = 256
times faster than you can break DES.  This is a serious weakness.

Another thing is that PIX is using DES in ECB mode.

My god, that's atrocious!  This is ``kindergarten crypto'' (to
steal a quote from Bruce Schneier).

You can probably break a fair amount of traffic with classical
frequency analysis (roughly like solving a simple substitution cipher
like in the back of the daily newspapers, only trickier).
Stereotyped text and headers should be easily recovered.

What's worse is that this has a nasty interaction with the
weakening of the key down to 48 bits.  In export-weakened SSL, one
adds some public salt to the 40-bit secret key, to stop precomputation
attacks; but note that CISCO's algorithm adds no salt, so there are
all sorts of precomputation attacks possible.

The simplest attack (``the Exabyte attack'') is to encrypt some
common plaintext block (e.g. "\nlogin: ") under all 2^48 possible
keys, and store the 2^48 ciphertext results on a big Exabyte tape;
then each subsequent link-encryption key can be broken with O(1)
effort.  Thanks to the ECB mode, such a common plaintext block
should be easy to find.  (With a real chaining mode, these attacks
are not possible under a ciphertext-only assumption, because the
chaining vector serves as a kind of salt.)

A much more practical approach would use Hellman's time-space
tradeoff.  There, you'd need only about 2^32 space (e.g. $100 at
Fry's for a cheap hard disk), plus you'd need to do a 2^48 precomputation.
After the precomputation, each subsequent link-encryption key
can be broken with about 2^32 trial encryptions.

So you should think of CISCO PIX as roughly 32-bit crypto...
and that might be an overstatement.

I don't think I need to tell you that a 2^32 work factor is
*trivial*.  I could be breaking this in real-time, and I'm only
a grad student.



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