Cryptanalysis of a Special Polybius-Like Cipher Using Hill-Climbing

Polybius cipher is a special substitution system widely used during history. In our research, we found several Polybius-like ciphers used in Czecho-slovakia and in the Slovak State from the first half of the 20th century. Various types of this cipher are described in the first Czechoslovak cryptanalysis manual “šifrovací systémy a návod k luštění kryptogramů” by plk. Josef Růžek. It can be also found in the official cryptology directive called G–VII–8 “šifrování” (encryption) from 1938, and another variant in the new version of the same directive from 1946. In this work, we focus on a special Polybius-like cipher inspired by three real ciphers used in Czechoslovakia and in the Slovak State. Two were used during WW2 and one right after the war. We will show how these ciphers were used and how they can be solved with a modern heuristic approach on a personal computer. We evaluate the effectiveness of the Hill-Climbing heuristic methods with restarts. We also investigated several different fitness functions and language models.