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SIGABA
In the history of cryptography, the ECM Mark II was a rotor machine used by the United Statesmarker 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.

History

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).

Oddly the Army was unaware of either the changes or the mass production of the system, but were "let in" on the secret in early 1940. In 1941 the Army and Navy joined in a joint cryptographic system, based on the machine. The Army then started using it as the SIGABA.

Description

SIGABA
SIGABA was similar to the Enigma in basic theory, in that it used a series of rotors to encipher every character of the plaintext into a different character of ciphertext. Unlike Enigma's three rotors however, the SIGABA included fifteen, and did not use a reflecting rotor.

The SIGABA had three banks of five rotors each; the action of two of the banks controlled the stepping of the third.

  • The main bank of five rotors was termed the cipher rotors, and each had 26 contacts. This acted similarly to other rotor machines, such as the Enigma; when a plaintext letter was input, a signal would enter one side of the bank and exit the other, denoting the ciphertext letter.
  • The second bank of five rotors was termed the control rotors. These were also 26 contact-rotors. The control rotors received four signals at each step. After passing through the control rotors, the outputs were divided into ten groups of various sizes, ranging from 1–6 wires. Each group corresponded to an input wire for the next bank of rotors.
  • The third bank of rotors was called the index rotors. These rotors were smaller with only ten contacts, and did not step during the encryption. After travelling though the index rotors, one to four of five output lines would have power. These then turned the cypher rotors.


The rotors (wheels) and rotor unit
The SIGABA advanced one or more of its main rotors in a complex, pseudorandom fashion. This meant that attacks which could break other rotor machines with more simple stepping (for example, Enigma) were made much more complex. Even with the plaintext in hand, there are so many potential inputs to the encryption that it is difficult to work out the settings.

On the downside, the SIGABA was also large, heavy, expensive, difficult to operate, mechanically complex and fragile. It was nowhere near as practical a device as the Enigma, which was smaller and lighter than the radios it was used with. It found widespread use in the radio rooms of the US Navy's ships, but as a result of these practical problems the SIGABA simply couldn't be used in the field, and, in most theatres other systems were used instead, especially for tactical communications. The most famous may be the Navajo code talker who provided tactical field communications in parts of the Pacific Theater beginning at Guadalcanalmarker. In other theatres, less secure, but smaller lighter and tougher machines were used, such as the M-209. SIGABA, impressive as it was, was overkill for tactical communications.


Combined cipher machine

SIGABA was also adapted for interoperation with a modified British machine, Typex. The common machine was known as the Combined Cipher Machine (CCM), and was used from November 1943.

See also

  • Mercury — British machine which also used rotors to control other rotors
  • SIGCUM — teleprinter encryption system which used SIGABA-style rotors


References

  • Mark Stamp, Wing On Chan, "SIGABA: Cryptanalysis of the Full Keyspace", Cryptologia v 31, July 2007, pp 201–2222
  • Rowlett wrote a book about SIGABA (Aegean Press, Laguna Hills, California).
  • Michael Lee, "Cryptanalysis of the Sigaba", Masters Thesis, University of California, Santa Barbara, June 2003 (PDF) (PS).
  • John J. G. Savard and Richard S. Pekelney, "The ECM Mark II: Design, History and Cryptology", Cryptologia, Vol 23(3), July 1999, pp211–228.
  • Crypto-Operating Instructions for ASAM 1, 1949, [22297].
  • CSP 1100(C), Operating Instructions for ECM Mark 2 (CSP 888/889) and CCM Mark 1 (CSP 1600), May 1944, [22298].


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