America’s Unbroken World War 2 Cipher Machine

How SIGABA’s Encryption Stayed Secure As Enigma Fell

SIGABA’s keyboard and printer. Photo by the author at the National Cryptology Museum

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.

An example of Hebern’s rotor machine. Photo by the author at the National Cryptologic Museum.

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…