Algebraic Attacks on Block Ciphers

[James Muir]

This may interest some list members:

http://eprint.iacr.org/2006/168

Cryptology ePrint Archive: Report 2006/168
How Fast can be Algebraic Attacks on Block Ciphers ?
Nicolas T. Courtois

Abstract. In this paper we give a specification of a new block cipher 
that can be called the Courtois Toy Cipher (CTC). It is quite simple, 
and yet very much like any other known block cipher. If the parameters 
are large enough, it should evidently be secure against all known attack 
methods. However, we are not proposing a new method for encrypting 
sensitive data, but rather a research tool that should allow us (and 
other researchers) to experiment with algebraic attacks on block ciphers 
and obtain interesting results using a PC with reasonable quantity of 
RAM. For this reason the S-box of this cipher has only 3-bits, which is 
quite small. Ciphers with very small S-boxes are believed quite secure, 
for example the Serpent S-box has only 4 bits, and in DES all the 
S-boxes have 4 output bits. The AES S-box is not quite as small but can 
be described (in many ways) by a very small systems of equations with 
only a few monomials (and this fact can also be exploited in algebraic 
cryptanalysis). We believe that results on algebraic cryptanalysis of 
this cipher will have very deep implications for the security of ciphers 
in general.

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