Finding the key length of a Vigenere cipher: How to improve the twist algorithm

Abstract

A Vigenere cipher applies a single short key repeatedly to encrypt a plaintext. If a cryptanalyst correctly finds out the key length, the ciphertext can be divided into multiple instances of shift cipher and be broken by frequency analysis. To determine the key length, the twist algorithm, an alternate method to the standard Kasiski and Friedman tests, was recently proposed. In this article, we propose the twist(+) algorithm, an improved twist algorithm, which can estimate the key length more accurately than the original twist algorithm.