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 of a class of machines known as the rotor machines that would became the primary form of encryption during World War II and for some time after, and included such famous examples as the German Enigma.

History

Prior to the introduction of the Hebern machine, encryption almost always consisted of matching the plaintext to be encyphered against a set of substitution alphabets[?], randomized series of letters. Early codes used a single such substitution, but by the 1500s these were routinely being "broken" by looking for common letters, a process known as frequency analysis. A new system then started being used that used a selection of such substitution alphabets, with various methods for deciding which alphabet to use to encode any particular letter. This was a very time consuming process, with each letter of the plaintext taking a few seconds to be encoded into the cyphertext.

Edward Hugh Hebern (April 23, 1869- 1952) was a building contractor who was jailed in 1908 for stealing a horse. With time on his hands, he started thinking about the problem of encryption, eventually devising a means of mechanising the process with a typerwriter. At the time he had no funds to be able to spend time working on such a device, but he dusted off his thinking in 1917, built a sample, and patented it in 1918. Agnes Driscoll, civilian mainstay of the US Navy's 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.

He was so convinced of the future success of the system that he formed the Hebern Electric Code company with money from several investors. Over the next few years he repeatedly tried to sell the machines both to the US Navy and Army, as well as commercial interests like banks. Neither were terribly interested, and cryptology at the time was considered generally unimportant by everyone. Perhaps the best indication of this was the statement by Stimson in his memoirs that, gentlemen should not read each other's mail (but note the Friedman connection below). It was Stimson, as Secretary of State under Hoover, who withdrew State Department support for Herbert Yardley's American Black Chamber[?], leading to its closing shortly thereafter.

Eventually the investors got fed up, and sued Herbern for stock manipulation[?]. He spend another brief period in jail, but never gave up on the idea of his machine. In 1931 the Navy finally purchased several systems, but this was to be his only real sale.

Although Hebern's machine was the first rotor machine, several other machines were designed independantly at about the same time.

Description

Hebern single-rotor machine Courtesy Jerry Proc (http://webhome.idirect.com/~jproc/crypto/menu)

The key to the Hebern design was a disk with electrical contacts on either side, known today as a rotor. Linking the contacts on either side of the rotor were wires, with each letter on one side being wired to another on the far side in a random fashion. The wiring encoded a single substitution alphabet.

When the user pressed a key on the typerwriter keyboard, a small amount of current from a battery flowed through the key into one of the contacts on the input side of the disk, through the wiring, and back out a different contact. The power then operated the mechanicals of an electric typewriter to type the encrypted letter, or alternately simply lit a bulb or paper tape punch from a teletype machine.

Normally such a system would be no better than the single-alphabet systems of the 1500s. However the rotor in the Hebern machine was geared to the keyboard on the typewriter, so that after every keypress, the rotor turned and the substitution alphabet thus changed slightly. At this point the system was just as effective as the polyalphabetic cyphers, but was trivially easy to use. Operators simply rotated the rotor to a pre-chosen starting position and started typing. To decrypt the message, they simply turned the rotor around in its socket so it was "backwards", thus reversing all the substitutions. They then typed in the cyphertext and out came the plaintext.

Hebern five-rotor machine Note the lamps for reading the cyphertext

Better yet, several rotors can be placed such that the output of the first is connected to the input of the next. In this case the first rotor operates as before, turning once with each keypress. Additional rotors are then spun with a cam on the one beside it, each one being turned one position after the one beside it rotates a full turn. In this way the number of such alphabets increases dramatically. For a rotor with 26 letters in its alphabet, five such rotors "stacked" in this fashion allows for 26⁵ = 11,881,376 different possible substitutions.

William F. Friedman attacked the Hebern machine soon after it came on the market in the 1920s. He quickly "solved" any machine that was built similar to the Hebern, in which the rotors were stacked with the rotor at one end or the other turning with each keypress, the so-called fast rotor. In these cases the resulting cyphertext consisted of a series of single-substitution cyphers, each one 26 letters long. He showed that fairly standard techniques could be used against such systems, gievn enough effort.

Of course, this fact was itself a great secret. This may explain why the Army and Navy were unwilling to use Hebern's design, much to his surprise.