Astakhov M. A. Late Medieval Homophonic Substitution Cipher and its Decipherment (Evidence from Little Known Spanish Document from the Archive of the Saint Petersburg Institute of History of the Russian Academy of Sciences) [Digital Resource] // Vox medii aevi. 2018. Vol. 2(3). P. 105–125. URL: https://voxmediiaevi.com/2018-2-astakhov
DOI: 10.24411/2587-6619-2018-00018
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Marat Astakhov
Independent Scholar, graduate of Lomonosov Moscow State University
marat1089@mail.ru
Late Medieval Homophonic Substitution Cipher and its Decipherment (Evidence from Little Known Spanish Document from the Archive of the Saint Petersburg Institute of History of the Russian Academy of Sciences)
The article presents a brief overview of the history of cryptography before the beginning of the Early Modern Period. Particular attention is paid to the homophonic substitution cipher used in the Late Middle Ages where each letter in the plaintext was replaced by several (more than one) characters in the ciphertext in accordance with its frequency. After examination of an encrypted letter of 1460 from the royal chancery of the Crown of Aragon kept in the archive of the Saint Petersburg Institute of History of the Russian Academy of Sciences we summarize some general principles of decipherment and give some practical recommendations to the researchers engaged in such domain. As soon as the language of ciphertext is recognized, cryptography manuals usually advise the further frequency analysis and comparison of results with the already known data for the distribution of letters in a particular language. However, basing on the fact that the homophonic substitution cipher was aimed to distort frequency analysis and complicate the cracking, we developed our own approach.
After cataloguing the symbols, numbering and transferring them into a spreadsheet, the “perspective” of the text became visible, revealing stable combinations of signs and words-palindromes, which could be decrypted unambiguously. Therefore, like solving a crossword without clues, we deciphered the entire ciphertext. This experience could be useful for everyone who deals with medieval cryptographic documents.
Key words: Crown of Aragon; Cryptography; Medieval Ciphers; Medieval Diplomacy; Methodology of Decipherment; Substitution Cipher.