Playfair is a manual substitution cipher invented in 1854 by Charles Wheatstone. Its name and popularity came from the endorsement of his friend Lord Playfair. The Playfair cipher encrypts bigrams (pairs of letters), and is considered more secure than monoalphabetic substitution ciphers which encrypt single letters. It was used by several countries in the 19th century and in the first half of the 20th century.
Playfair ciphers can often be solved with the help of a crib. Ciphertext-only attacks usually require hundreds of letters when carried out manually (Mauborgne, 1918). More recently, computerized attacks based on hill climbing and simulated annealing have been published, that require between 60 to 100 letters of ciphertext (Cowan, 2008; Al-Kazaz et al., 2018).
In this article, the author presents a novel ciphertext-only attack, implemented in the open-source e-learning CrypTool 2 (CT2) platform, that is effective against ciphertexts as short as 40 letters (CrypTool 2 Team, 2019). This attack is based on a specialized adaptation of simulated annealing and uses hexagrams in the scoring method. With CT2, a Playfair public challenge with only 40 letters was solved, establishing an unofficial world record for decrypting short Playfair messages, encrypted with random keys, from ciphertext only (Schmeh, 2018b). The author also offers a series of new Playfair challenges.
Keywords: Playfair challenge Klaus Schmeh simulated annealing cryptanalysis
Proceedings of the 2nd International Conference on Historical Cryptology, HistoCrypt 2019, June 23-26, 2019, Mons, Belgium
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