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Equivalence Classes: toward More Efficient Search

This paper describes a simple technique for applying Known Ciphertext Attack to a certain class of cryptosystems. The feature of the class that the technique exploits is that the ciphertext characters depend only on small portions of the key and a single plaintext character. After deriving a preferential list of locations to start the search, the keyspace is attacked piecemeal in the locations most likely to lead to confirmation or contradiction. By ordering the list of search locations, the attack is made more efficient.