An Automatic Cryptanalysis of Playfair Ciphers Using Compression

Noor R. Al-Kazaz
School of Computer Science, Bangor University, Bangor, UK

Sean A. Irvine
Real Time Genomics, Hamilton, New Zealand

William J. Teahan
School of Computer Science, Bangor University, Bangor, UK

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Ingår i: Proceedings of the 1st International Conference on Historical Cryptology HistoCrypt 2018

Linköping Electronic Conference Proceedings 149:21, s. 115-124

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Publicerad: 2018-06-13

ISBN: 978-91-7685-252-1

ISSN: 1650-3686 (tryckt), 1650-3740 (online)


This paper introduces a new compression-based approach to the automatic cryptanalysis of Playfair ciphers. More specifically, it shows how the Prediction by Partial Matching (‘PPM’) data compression model, 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 very short Playfair ciphers with no probable word. Our new method is the result of an efficient combination between data compression and simulated annealing. The method has been tried on a variety of cryptograms with different lengths (starting from 60 letters) and a substantial majority of these ciphers are solved rapidly without any errors with 100% of ciphers of length over 120 being solved. In addition, as the spaces are omitted from the ciphertext traditionally, we have also tried a compression-based approach in order to achieve readability by adding spaces automatically to the decrypted texts. The PPM compression model is used again to rank the solutions and almost all the decrypted examples were effectively segmented with a low average number of errors. Furthermore, we have also been able to break a Playfair cipher for a 6×6 grid using our method.


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