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Computational cryptography

7 March 2012

Theodosis Mourouzis

This research will explore some existing, and develop some new, cryptanalytic attacks in the area of hash functions and block ciphers. It includes a detailed analysis regarding cryptanalysis of hash functions, particularly the specifications of the GOST hash function (a cryptographic hash function used by the Russian National Federation). The research explores the most successful existing attack on this hash function based on some specifications of the underlying block cipher and includes a new black-box attack on the compression function f of the GOST hash function. The innovation in our attack is that it is a generic attack and works also if the underlying block cipher is replaced by any other cipher of the same input-output dimension instead of the attack presented by Mendel et al which is a specific attack.

Multiplicative Complexity (MC) was introduced for cryptanalysis purposes in 2008 by Boyar and Peralta. Techniques from this area of combinatorial optimization can be used for the security analysis of current cryptographic schemes and especially in the area of multi-dimensional Boolean functions (SBoxes).

The research applies the circuit-minimization techniques as proposed by Boyar and Peralta and combines it with our SAT solver software to obtain more compact representations of the 4-bit S-boxes of the PRESENT and GOST cipher with respect to their multiplicative complexity. These minimal representations seem to be very promising at preventing side channel attacks on cryptographic chips such as Differential Power Analysis (DPA).