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This article presents a new application for Information Retrieval techniques. We introduce the use of clustering and categorization in the attack of cryptosystems. In order to clearly present the fundamentals and understand the workings and the implications of this new technique, we developed a procedure for keylength determination in the process of cryptanalysis of polyalphabetic ciphers, the core of any attack of this type of ciphers. The basic premises are: first, a cryptogram is a normal document written in an unknown language; secondly, Information Retrieval Techniques are extremely useful in detecting string patterns in ordinary texts and might be helpful with cryptograms as well.

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This paper presents the use of context sensitive grammars in order to outwit cryptanalysis based on language character frequency. The main goal is to take a plain text written in an idiom and transform it into a new text whose character frequency mimics a different idiom. Once created this new text, it can be submitted to any cryptographic algorithm. The proposed approach uses theory of computation techniques combined with linguistics appended to traditional encode and decode algorithms. Therefore, it was possible to obtain cryptograms more resistant to this kind of attack.

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