SIGABA

For the secure email corporation, see Secure Data in Motion.

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-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.

Contents

1 History 2 Description 3 See also 4 References 5 External links

History

It was clear to US cryptographers well before WWII that the single-stepping mechanical motion of new rotor machines (e.g. the Hebern machine) introduced patterns into the resulting cyphertext that 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 in addition to the message settings it had in common with, among others, the Enigma machines, it added the positioning of the tape and the settings of a plugboard that said which line of holes on the tape controlled which rotors.

The M-134 had one distinct disadvantage compared to the Enigma – that the tape had to be identical for any machines hoping to decypher messages from other machines. If this tape were intercepted the number of potential settings remaining was large, but not infinite. And there were problems for any machine 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. This is not as trivial as it may seem. The Enigma rotors each take one input signal (current from a battery) and create one output signal, but in Rowlett's design each rotor must be constructed such that between one and five 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, 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 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 cyphertext. Unlike Enigma's three rotors however, the SIGABA included no less than fifteen.

Simply increasing the number of rotors does not make the machine more secure. This is because in the Enigma system, the rotors only move if the one to their right does so first, and it does that only after 26 key presses. In other words the message has to contain at least 676 (26²) characters before the third rotor comes into play, and around 17,000 for before the fourth. For most messages of a few hundred letters, more rotors added no security.

What SIGABA did with these extra rotors was increase the complexity of the movement of the main rotors in the machine. In the Enigma the rotors turned one location with every key press, which led to a number of patterns in the cyphertext. While these patterns were hard to find, the British and US applied intensive effort to the problem, and by the end of the war were able to read practically everything the Germans encrypted.

In the case of the SIGABA, a simple modification was applied that made it more secure. Instead of the rotors being turned by mechanical action of the keyboard, they were instead turned by the electrical action of a separate set of rotors. 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 cihpertext 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.

In summary, the SIGABA advanced one or more of its main rotors in a complex, pseudorandom fashion. This meant that the patterns used to break Enigma were completely hidden. In fact 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 Navaho wind talkers who provided tactical field communications in parts of the Pacific Theater beginning at Guadalcanal. In other theatres, less secure, but smaller lighter and tougher machines were used. SIGABA, impressive as it was, was overkill for tactical communications.

See also

• Typex

References

• 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) (http://www.cs.ucsb.edu/~kirbysdl/broadcast/thesis/thesis.pdf).
• John J. G. Savard and Richard S. Pekelney, "The ECM Mark II: Design, History and Cryptology", Cryptologia, Vol 23(3), July 1999, pp211–228.

External links

• Electronic Cipher Machine (ECM) Mark II (http://www.maritime.org/ecm2.htm) by Rich Pekelney
• SIGABA simulator for Windows (http://frode.home.cern.ch/frode/crypto/simula/sigaba/)
• The ECM Mark II, also known as SIGABA, M-134-C, and CSP-889 (http://www.hypermaths.org/quadibloc/crypto/ro0205.htm) — by John Savard
• A cipher which is similar to the Sigaba machine (http://fgerlach.com/Sigaba2005.java)

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