Slippery hill-climbing technique for ciphertext-only cryptanalysis of periodic polyalphabetic substitution ciphers. Issue 3 (3rd May 2020)

• Record Type: Journal Article
• Authors: Kaeding, Thomas
• Is Part Of: Cryptologia. Volume 44:Issue 3(2020)
• Page Start: 205
• Page End: 222
• Publication Date: 2020-05-03
• Subjects: cryptanalysis -- hill-climbing -- periodic polyalphabetic substitution cipher -- Quagmire -- slippery hill-climbing -- Vigenère; Cryptography -- Periodicals; 652.8
• DOI: 10.1080/01611194.2019.1655504
• Languages: English
• ISSNs: 0161-1194

Abstract

Abstract: We present a stochastic method for breaking general periodic polyalphabetic substitution ciphers using only the ciphertext and without using any additional constraints that might come from the cipher's structure. The method employs a hill-climbing algorithm for individual key alphabets, with occasional slipping down the hill. We implement the method with a computer and achieve reliable results for a sufficiently long ciphertext (150 characters per key alphabet). Because no constraints among the key alphabets are used, this method applies to any periodic polyalphabetic substitution cipher.