No abstract

Automatic recognition of correct solutions as a result of a ciphertext only attack of simple ciphers is not a trivial issue and still remains a taxing problem. A new compression based method for the automatic cryptanalysis of simple substitution ciphers is introduced in this paper. In particular, this paper presents how a Prediction by Partial Matching ('PPM') text compression scheme, a method that shows a high level of performance when applied to different natural language processing tasks, can also be used for the automatic decryption of simple substitution ciphers. Experimental results showed that approximately 92% of the cryptograms were decrypted correctly without any errors and 100% with just three errors or less. Extensive investigations are described in this paper, in order to determine which is the most appropriate type of PPM scheme that can be applied to the problem of automatically breaking substitution ciphers. This paper shows how a new character-based PPM variant significantly outperforms other schemes including the standard Gzip and Bzip2 compression schemes. We also apply a word-based variant which when combined with the character-based method leads to further improved results.

This paper introduces a compression-based method adapted for the automatic cryptanalysis of Arabic transposition ciphers. More specifically, this paper presents how a Prediction by Partial Matching ('PPM') compression scheme, a method that shows a high level of performance when applied to the different natural language processing tasks, can also be used for the automatic decryption of transposition ciphers for the Arabic language. Another well known compression scheme, Gzip, is also investigated in this paper with less efficient performance demonstrated by this method. In order to achieve readability, two further compression based approaches for space insertion are evaluated as well in this paper. The results of our experiments with 125 Arabic cryptograms of different lengths show that 97% of the cryptograms are successfully decrypted without any errors using PPM compression models. As well in a post-processing step, we can effectively segment the output that is produced by the automatic insertion of spaces resulting with only a few errors overall. As far as we know, this is the first work to demonstrate an effective automatic cryptanalysis for transposition ciphers in Arabic.