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16 August 1998
Date: Sun, 16 Aug 1998 08:53:37 -0500
To: cypherpunks@toad.com
From: Bruce Schneier <schneier@counterpane.com>
Subject: Announcement: Cryptanalysis of Frog (an AES Candidate)
Results Announcement:
D. Wagner, N. Ferguson, and B. Schneier, "Cryptanalysis of Frog,"
Counterpane Systems Report, Aug 1998.
Abstract:
We examine some attacks on the FROG cipher. First we give a
differential attack which uses about $2^{58}$ chosen plaintexts and
very little time for the analysis; it works for about $2^{-33.0}$ of
the keyspace. Then we describe a linear attack which uses $2^{56}$
known texts and works for $2^{-31.8}$ of the keyspace. The linear
attack can also be converted to a ciphertext-only attack using $2^{64}$
known ciphertexts. Also, the decryption function of FROG is a lot
weaker than the encryption function. We show a differential attack
on the decryption function that requires $2^{36}$ chosen ciphertexts
and works on $2^{-29.3}$ of the keyspace. Using our best attack an
attacker with a sufficient number of cryptanalytical targets can expect
to recover his first key after $2^{56.7}$ work. Taken together, these
observations suggest that FROG is not a very strong candidate for the
AES.
This paper is available at http://www.counterpane.com/publish.html, and
will be made available at the AES Workshop next week.
Cheers,
Bruce
**********************************************************************
Bruce Schneier, President, Counterpane Systems Phone: 612-823-1098
101 E Minnehaha Parkway, Minneapolis, MN 55419 Fax: 612-823-1590
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