On How Turing and Singleton Arc Consistency Broke the Enigma Code

In this paper, we highlight an intriguing connection between the cryptographic attacks on Enigma’s code and local consistency reasoning in constraint programming. The coding challenge proposed to the students during the 2020 ACP summer school, to be solved by constraint programming, was to decipher a message encoded using the well known Enigma machine, with as only clue a tiny portion of the original message. A number of students quickly crafted a model, thus nicely showcasing CP technology - as well as their own brightness. The detail that is slightly less favorable to CP technology is that solving this model on modern hardware is challenging, whereas the "Bombe", an antique computing device, could solve it eighty years ago. We argue that from a constraint programming point of vue, the key aspects of the techniques designed by Polish and British cryptanalysts can be seen as, respectively, path consistency and singleton arc consistency on some constraint satisfaction problems.