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Solving a 40-Letter Playfair Challenge with CrypTool 2

George Lasry
The CrypTool Team

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Ingår i: Proceedings of the 2nd International Conference on Historical Cryptology, HistoCrypt 2019, June 23-26, 2019, Mons, Belgium

Linköping Electronic Conference Proceedings 158:10, s. 87-96

NEALT Proceedings Series 37:10, s. 87-96

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Publicerad: 2019-06-12

ISBN: 978-91-7685-087-9

ISSN: 1650-3686 (tryckt), 1650-3740 (online)

Abstract

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.

Nyckelord

Playfair challenge Klaus Schmeh simulated annealing cryptanalysis

Referenser

Noor R Al-Kazaz, Sean A Irvine, and William J Teahan. 2018. An Automatic Cryptanalysis of Playfair Ciphers Using Compression. In Proceedings of the 1 st International Conference on Historical Cryptology HistoCrypt 2018, number 149, pages 115-124. Linkoping University Electronic Press.

Michael J Cowan. 2008. Breaking short Playfair ciphers with the simulated annealing algorithm. Cryptologia, 32(1):71-83.

Michael J Cowan. 2015. Chum Algorithm. https://web.archive.org/web/20150308125149/http://www.cryptoden.com:80/index.php/algorithms/churn-algorithm, [Accessed: January, 18th, 2019].

CrypTool 2 Team. 2019. CrypTool Portal – Cryptography for Everybody. http://www.cryptoo1.org/, [Accessed: January, 18th, 2019].

C.A. Deavours. 1977. Unicity Points in Cryptanalysis. Cryptologia, 1(1):46-68.

Holger H. Hoos and Thomas Stützle. 2004. Stochastic local search: Foundations and applications. Elsevier.

George Lasry, Nils Kopal, and Amo Wacker. 2014. Solving the Double Transposition Challenge with a Divide-and-Conquer Approach. Cryptologia, 38(3):197-214.

Joseph Oswald Mauborgne. 1918. An advanced problem in cryptography and its solution. Army Service Schools Press.

Alf Monge. 1936. Solution of a Playfair cipher. US Signal Corps.

Klaus Schmeh. 2018a. Playfair cipher: Is it unbreakable, if the message has only 30 letters? http://scienceblogs.de/klausis-krypto-kolumne/2019/04/15/, [Accessed: May, 18th, 2019].

Klaus Schmeh. 2018b. Playfair cipher: Is it unbreakable, if the message has only 40 letters? http://scienceblogs.de/klausis-krypto-kolumne/2018/12/08/, [Accessed: January, 18th, 2019].

Klaus Schmeh. 2018c. Playfair cipher: Is it unbreakable, if the message has only 50 letters? http://scienceblogs.de/klausis-krypto-kolumne/2018/04/07/, [Accessed: January, 18th, 2019].

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