Remarks on breaking the Vigenère autokey cipher. Issue 6 (2nd November 2019)

• Record Type: Journal Article
• Authors: Grošek, Otokar; Antal, Eugen; Fabšič, Tomáš
• Is Part Of: Cryptologia. Volume 43:Issue 6(2019)
• Page Start: 486
• Page End: 496
• Publication Date: 2019-11-02
• Subjects: Friedman test -- Kolchak's cipher -- Vigenère autokey cipher; Cryptography -- Periodicals; 652.8
• DOI: 10.1080/01611194.2019.1596997
• Languages: English
• ISSNs: 0161-1194

Abstract

Abstract: It has been mentioned by Shannon that a Vigenère autokey ciphertext can be transformed into a Vigenère ciphertext, provided the length of the priming key is known. We revisit this transformation and argue that combining it with the Friedman test, in fact, allows revealing of the length of the priming key. Thus, Shannon's transformation together with the Friedman test allow transforming a Vigenère autokey ciphertext into a Vigenère ciphertext without any prior information. This allows an adversary to solve a Vigenère autokey ciphertext as if it was a Vigenère ciphertext. In addition, we show that the same method can be used to solve a modification of the Vigenère autokey cipher used in the Russian Civil War in 1917–1922.