Ciphertext-only cryptanalysis of Hagelin M-209 pins and lugs

The Hagelin M-209 was a mechanical encryption device used by the US Army during World War 2 and the Korean War, as well as by other armies and in embassy settings. In this paper, we present a 4-stage hillclimbing algorithm for recovering the full key settings from ciphertext only. The two first stages are based on a divide-and-conquer approach developed by Geoff Sullivan. In our performance evaluation, we show that our method is able to recover key settings from messages as short as 750 letters, compared to 2 000− 2 500 letters with prior methods. With this method, we solved several M-209 ciphertext-only challenges, including the final exercises in W. Barker’s 1977 book, Cryptanalysis of the Hagelin Cryptograph.