3175	 Breaking Substitution Ciphers Using a Relaxation Algorithm	 Substitution ciphers are codes in which each letter of the alphabet has one fixed substitute and the word divisions do not change. In this paper the problem of breaking substitution ciphers is represented as a probabilistic labeling problem. Every code letter is assigned probabilities of representing plain text letters. These probabilities are updated in parallel for all code letters using joint letter probabilities. Iterating the updating scheme results in improved estimates that finally lead to breaking the cipher. The method is applies successfully to two examples. Cryptography substitution ciphers probabilistic classification relaxation
