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Cryptanalytic Methods for Modern Ciphers

Block ciphers like DES are intended to be very hard to break, and they are largely successful in achieving this. Having even copious quantities of corresponding plaintext and ciphertext, it is intended that the fastest way to discover the key, so as to be able to decrypt other messages, would be a brute-force search, that is, trying every possible key until the right one is found.

Many block ciphers appear to meet this condition. Two cryptanalytic methods that can do slightly better with some of the earlier block ciphers, such as DES and LUCIFER, are differential cryptanalysis and linear cryptanalysis.

Other techniques, which are of interest against weaker ciphers, and which partially account for the fact that DES has sixteen rounds, instead of eight, such as hill-climbing techniques and genetic algorithms, are discussed in the next section.

In the book The Hut Six Story, Gordon Welchman first revealed one of the innovations used with the Bombe in connection with the cryptanalysis of the German Enigma. He also noted that it embodied a general principle which made present-day ciphers weaker than they might be expected to be.

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