Enigma Code-Breaking: Replicating a 1930s Polish Cyclometer

Encryption is incredibly important for security today, just as it has been since long before digital computers were ever even envisioned. Julius Caesar, for example, used a simple cipher that shifted each letter of a message by three spaces, now named for him, for military communications. In English, that would mean an A would become a D, a P would become an S, and so on. A Caesar cipher is laughably easy to crack, which is why Nazi Germany developed and employed Enigma machines during World War II. Allied efforts to decipher Enigma machine encryption were critical to victory, which is why Hal Evans built a functional replica of a 1930s Polish cyclometer used for that effort.

The encryption provided by German Enigma machines makes traditional ciphers look like child’s play. Ciphers rely on a key that describes the steps to encrypt the message, and that key doesn’t generally change. They’re easy to crack using frequency analysis (how often particular letters statistically appear) or by looking at multiple messages. An Enigma machine, though purely electromechanical, was far more complex. A series of rotating disks were used to pass electrical signals through the machine in order to create purposefully inconsistent relationships between input text and output. To be able to decrypt a message, the receiver would need both the same set of rotors and to know their settings.

Incredibly, thanks to brilliant mathematics and the work of a French spy, Poland was able to reverse-engineer an Enigma machine and decrypt German messages in January of 1933. They ended up building a cyclometer to take advantage of what they’d learned in order to decrypt messages — a tool that would be indispensable as Germany revised their Enigma machines throughout the war. Evans, an alumnus of the University of Cambridge’s Department of Engineering, used now-published information to build a replica of a Polish cyclometer. Like the Enigma machines themselves, it is purely electromechanical. Rotors can be assembled and placed inside of the machine, and then set to the proper key. Switches and light bulbs let the user set inputs and receive outputs. Evans' replica is currently on display in King’s College, Cambridge.