Cryptology ePrint Archive: Report 2020/302

Slippery hill-climbing technique for ciphertext-only cryptanalysis of periodic polyalphabetic substitution ciphers

Thomas Kaeding

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.

Category / Keywords: foundations / periodic polyalphabetic substitution cipher, hill-climbing, slippery hill-climbing, cryptanalysis, Vigenère, Quagmire

Original Publication (with minor differences): Cryptologia

Date: received 7 Mar 2020, last revised 26 Apr 2020

Contact author: hippykitty at protonmail com

Available format(s): PDF | BibTeX Citation

Version: 20200426:222422 (All versions of this report)

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