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In the history of cryptography, the ECM Mark II was a rotor machine used by the United States from World War II (WWII) until the 1950s. The machine was also known as the SIGABA or Converter M-134 by the Army, or CSP-888/889 by the Navy, and a modified Navy version was termed the CSP-2900.

Like many machines of the era it used an electromechanical system of rotors in order to encipher messages. No successful cryptanalysis of the machine during its service lifetime is publicly known.



It was clear to US cryptographers well before WWII that the single-stepping mechanical motion of rotor machines (e.g. the Hebern machine) could be exploited by attackers. William Friedman, director of the US Army's Signals Intelligence Service, devised a system to correct for this by randomizing the motion of the rotors. His modification consisted of a paper tape reader from a teletype machine attached to a small device with metal "feelers" positioned to pass electricity through the holes. For any given letter pressed on the keyboard, not only would the machine scramble the letters in a fashion largely identical to other rotor machines, but any holes in the tape at that location would advance the corresponding rotors, before the tape itself was advanced one location. The resulting design went into limited production as the M-134, and its message settings included the position of the tape and the settings of a plugboard that indicated which line of holes on the tape controlled which rotors. However, there were problems using fragile paper tapes under field conditions.

Friedman's associate, Frank Rowlett, then came up with a different way to advance the rotors, using another set of rotors. In Rowlett's design each rotor must be constructed such that between one and four output signals were generated, advancing one or more of the rotors.

There was little money for encryption development in the US before the war, so Friedman and Rowlett built a series of "add on" devices called the SIGGOO (or M-229) that were used with the existing M-134s in place of the paper tape reader. These were external boxes containing a three rotor setup in which five of the inputs were live, as if someone had pressed five keys at the same time on an Enigma, and the outputs were "gathered up" into five groups as well — that is all the letters from A to E would be wired together for instance. That way the five signals on the input side would be randomized through the rotors, and come out the far side with power in one of five lines. Now the movement of the rotors could be controlled with a day code, and the paper tape was eliminated. They referred to the combination of machines as the M-134-C.

In 1935 they showed their work to a US Navy cryptographer in OP-20-G, Joseph Wenger. He found little interest for it in the Navy until early 1937, when he showed it to Commander Laurance Safford, Friedman's counterpart in the Navy's Office of Naval Intelligence. He immediately saw the potential of the machine, and he and Cmdr. Seiler then added a number of features to make the machine easier to build, resulting in the Electric Code Machine Mark II (or ECM Mark II), which the Navy then produced as the CSP-889 (or 888).

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