Time-memory tradeoff útoky

Seidlová, Monika

Martin Hellman proposed the first time-memory tradeoff attack on block ciphers. It is a chosen plaintext attack, in which the attacker precomputes a large amount of data for some block cipher and can then use it repeatedly in attacks on that block cipher. An improvement suggested by Ron Rivest speeds up the attack by reducing the number of memory accesses. Another modification of the original attack called rainbow tables speeds up the attack even more and brings other advantages. Time-memory tradeoff attacks can also be applied to stream ciphers as known plaintext attacks. This bachelor thesis describes in detail the original attack, its improvements and a modification to stream ciphers. As an example, we summarize an attack on A5/1, a stream cipher used in mobile phones. We also propose a new time-memory tradeoff attack on block ciphers called r-coloured rainbows. The new attack is a modification of Hellman's attack and shares similarities with the rainbow table attack. We give a comparison of the properties of the three attacks and conclude that, for certain block ciphers, our attack may be the most effective of the three.

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