America’s Unbroken World War 2 Cipher Machine
How SIGABA’s Encryption Stayed Secure As Enigma Fell
The story of America’s SIGABA cipher machine, which resisted the efforts of the best Axis cryptographers and provided secure communications throughout the war, is not nearly as well known as that of the German Enigma. Though it used very similar technology to the Enigma, it employed a key difference which was primarily the invention of two men, and remained unbroken during its operational lifetime. Without a doubt, it played an important role in the Allied victory in the war, and its story is a fascinating one.
Cipher Machines Before World War 2
Edward Hebern began sketching designs for the first encryption machine using rotors in 1917, and built beautiful working machines from brass over the next several years. He was issued a patent for a rotor machine in 1924.
The simplest ciphers, known as monoalphabetic ciphers, replace all instances of a single letter with another. For example:
These codes are simple to implement, but are trivial to break. Hobbyists routinely break them for fun as “Cryptogram” puzzles. Since some letters occur more frequently than others in written language, if you have an encrypted message of any significant length, it’s easy to make good guesses as to what the cipher scheme is. This is referred to frequency analysis, and a monoalphabetic cipher will rapidly fall to it once enough encrypted text has been captured.
A much better code is one which doesn’t use the same letter to encode a given character each time. The first time you encounter it, an A might be encoded as M, but the next time it occurs, it could be encoded as J, and later a T. This is called a polyalphabetic cipher and it is much harder to break.
Hebern’s rotor machines, and the other secure machines for the next few decades, all automated the process of encoding a message with a polyalphabetic cipher. This was accomplished with one or motor rotors with electrical pathways built around the edges. When a key was pressed, the encoded letter was output, and at least one rotor would advance…
