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Re: CISCO PIX Vulnerability
From: jthain () sbi bm (Jamie Thain)
Date: Sat, 20 Jun 1998 10:24:54 -0300


David,

David

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.)

Even if the ciper were a one byte char, the resulting data set size
would be 281,474 GB big, I have not heard of a 281TB tape drive yet.

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.

This is 4GB which is doable, but the resultant set of cipertexts would
still be ~24GB big, which makes you want to have a really good reason.
Although with some dedicated Hardware 281 Trillion combinations could be
tried in a few minutes, and it would be broken.

regards:jamie



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