Published: 2019-06-12
ISBN: 978-91-7685-087-9
ISSN: 1650-3686 (print), 1650-3740 (online)
The present paper uses information theoretic entropy as a means to evaluate the authenticity of homophonic substitution ciphers. We motivate the use of entropy on n-grams and then validate its applicability, by using it on various true ciphers and pseudo-ciphers. Differences in entropy allow us to apply further formal analyses, e.g. support-vector machines, in order to make predictions about a potential cipher’s status. We train several support-vector machines and validate them. We then apply the models to two classic ciphers, the Zodiac Killer’s first major cipher, z408, which has been solved, and his second cipher, z340, which remains unsolved. The models correctly identify z408 as a substitution cipher. z340 is classified as an advanced cipher or pseudo-cipher.
cryptography
cryptology
substitution ciphers
entropy
n-gram-based analysis
support-vector machines
z340
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