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The Computer Journal 1994 37(5):429-436; doi:10.1093/comjnl/37.5.429
© 1994 by British Computer Society
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Automated Cryptanalysis of Transposition Ciphers

J. P. Giddy * and R. Safavi-Naini *

Department of Computer Science, University of Wallongong, Northfields Ave., Wollongong 2500, Australia

In this paper we use simulated annealing for automatic cryptanalysis of transposition ciphers. Transposition ciphers are a class of ciphers that in conjunction with substitution ciphers form the basis of all modern symmetric algorithms. In transposition ciphers, a plaintext block is encrypted into a ciphertext block using a fixed permutation. We formulate cryptanalysis of the transposition cipher as a combinatorial optimization problem, and use simulated annealing to find the global minimum of a cost function which is a distance measure between a possible decipherment of the given ciphertext and a sample of plaintext language. The success of the algorithm depends on the ratio of the length of ciphertext to the size of the block. For lower ratios there are cases that the plaintext cannot be correctly found. This is the expected behaviour of all cryptanalysis methods. However, in this case, examining the output of the algorithm provides valuable ‘clues’ for guiding the cryptanalysis. In summary, simulated annealing greatly facilities cryptanalysis of transposition ciphers and provides a potentially powerful method for analyzing more sophisticated ciphers.

Received April, 1994.

* Department of Computer Science, University of Wallongong, Northfields Ave., Wollongong 2500, Australia


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