Hebern rotor machine

The Hebern Rotor Machine was an electro-mechanical encryption machine built by combining the mechanical parts of a standard typewriter with the electrical parts of an electric typewriter, connecting the two through a scrambler. It is the first example (though just barely) of a class of machines known as rotor machines that would become the primary form of encryption during World War II and for some time after, and which included such famous examples as the German Enigma.

History

Edward Hugh Hebern was a building contractor who was jailed in 1908 for stealing a horse. It is claimed that, with time on his hands, he started thinking about the problem of encryption, and eventually devised a means of mechanizing the process with a typewriter. He filed his first patent application for a cryptographic machine (not a rotor machine) in 1912. At the time he had no funds to be able to spend time working on such a device, but he continued to produce designs. He made his first drawings of a rotor-based machine in 1917, built a sample, and patented it in 1918. He continued to make improvements, adding additional rotors.Agnes Driscoll, the chief civilian employee of the US Navy's cryptography operation (later to become OP-20-G) between WWI and WWII, spent some time working with Hebern before returning to Washington and OP-20-G in the mid-'20s.

Rotor machine

In cryptography, a rotor machine is an electro-mechanical stream cipher device used for encrypting and decrypting secret messages. Rotor machines were the cryptographic state-of-the-art for a prominent period of history; they were in widespread use in the 1920s–1970s. The most famous example is the German Enigma machine, whose messages were deciphered by the Allies during World War II, producing intelligence code-named Ultra.

The primary component is a set of rotors, also termed wheels or drums, which are rotating disks with an array of electrical contacts on either side. The wiring between the contacts implements a fixed substitution of letters, replacing them in some complex fashion. On its own, this would offer little security; however, after encrypting each letter, the rotors advance positions, changing the substitution. By this means, a rotor machine produces a complex polyalphabetic substitution cipher, which changes with every keypress.

Background

In classical cryptography, one of the earliest encryption methods was the simple substitution cipher, where letters in a message were systematically replaced using some secret scheme. Monoalphabetic substitution ciphers used only a single replacement scheme — sometimes termed an "alphabet"; this could be easily broken, for example, by using frequency analysis. Somewhat more secure were schemes involving multiple alphabets, polyalphabetic ciphers. Because such schemes were implemented by hand, only a handful of different alphabets could be used; anything more complex would be impractical. However, using only a few alphabets left the ciphers vulnerable to attack. The invention of rotor machines mechanised polyalphabetic encryption, providing a practical way to use a much larger number of alphabets.