Article | Proceedings of the 2nd International Conference on Historical Cryptology, HistoCrypt 2019, June 23-26, 2019, Mons, Belgium | Solving a 40-Letter Playfair Challenge with CrypTool 2 Linköping University Electronic Press Conference Proceedings
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Title:
Solving a 40-Letter Playfair Challenge with CrypTool 2
Author:
George Lasry: The CrypTool Team
Download:
Full text (pdf)
Year:
2019
Conference:
Proceedings of the 2nd International Conference on Historical Cryptology, HistoCrypt 2019, June 23-26, 2019, Mons, Belgium
Issue:
158
Article no.:
010
Pages:
87-96
No. of pages:
10
Publication type:
Abstract and Fulltext
Published:
2019-06-12
ISBN:
978-91-7685-087-9
Series:
Linköping Electronic Conference Proceedings
ISSN (print):
1650-3686
ISSN (online):
1650-3740
Series:
NEALT Proceedings Series
Publisher:
Linköping University Electronic Press, Linköpings universitet


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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

Author:
George Lasry
Title:
Solving a 40-Letter Playfair Challenge with CrypTool 2
References:

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].

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

Author:
George Lasry
Title:
Solving a 40-Letter Playfair Challenge with CrypTool 2
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Last updated: 2019-10-02