A Constraint Satisfaction Algorithm for the Automated Decryption of Simple Substitution Ciphers
CRYPTO '88 Proceedings of the 8th Annual International Cryptology Conference on Advances in Cryptology
Bayesian inference for Zodiac and other homophonic ciphers
HLT '11 Proceedings of the 49th Annual Meeting of the Association for Computational Linguistics: Human Language Technologies - Volume 1
ACL '12 Proceedings of the 50th Annual Meeting of the Association for Computational Linguistics: Short Papers - Volume 2
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A homophonic substitution cipher maps each plaintext letter of a message to one or more ciphertext symbols [4]. Monoalphabetic homophonic ciphers do not allow ciphertext symbols to map to more than one plaintext letter. Homophonic ciphers conceal language statistics in the enciphered messages, making statistical-based attacks more difficult. We present a dictionary-based attack using a genetic algorithm that encodes solutions as plaintext word placements subjected to constraints imposed by the cipher symbols. We test the technique using a famous cipher (with a known solution) created by the Zodiac serial killer. We present several successful decryption attempts using dictionary sizes of up to 1,600 words.