Breaking substitution ciphers using a relaxation algorithm
Communications of the ACM
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This paper describes a systematic procedure for decrypting simple substitution ciphers with word divisions. The algorithm employs an exhaustive search in a large on-line dictionary for words that satisfy constraints on word length, letter position and letter multiplicity. The method does not rely on statistical or semantical properties of English, nor does it use any language-specific heuristics. The system is, in fact, language independent in the sense that it would work equally well over any language for which a sufficiently large dictionary exists on-line. To reduce the potentially high cost of locating all words that contain specified patterns, the dictionary is compiled into a database from which groups of words that satisfy simple constraints may be accessed simultaneously. The algorithm (using a relatively small dictionary of 19,000 entries) has been implemented in Franz Lisp on a Vax 11/780 computer running 4.3 BSD Unix. The system is frequently successful in a completely automated mode -- preliminary testing indicates about a 60% success rate, usually in less than three minutes of CPU time. If it fails, there exist interactive facilities, permitting the user to guide the search manually, that perform very well with minor human intervention.