page_title: SIGABA
section_title: Description
hierarchical_section_title: SIGABA / Description
context_page_description: In the history of cryptography, the ECM Mark II was a cipher machine used by the United States for message encryption from World War II 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, but with a number of security improvements over previous designs. No successful cryptanalysis of the machine during its service lifetime is publicly known.
context_section_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 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 (Army) or alphabet maze (Navy) and each rotor had 26 contacts. This assembly acted similarly to other rotor machines, such as the Enigma; when a plaintext letter was entered,  a signal would enter one side of the bank and exit the other, denoting the ciphertext letter. Unlike the Enigma, there was no reflector.
  The second bank of five rotors was termed the control rotors or stepping maze. 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 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 were so many potential inputs to the encryption that it was 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 with which it was used. It found widespread use in the radio rooms of US Navy ships, but as a result of these practical problems the SIGABA simply couldn't be used in the field. In most theatres other systems were used instead, especially for tactical communications. One of the most famous was the use of Navajo code talkers for tactical field communications in the Pacific Theater. In other theatres, less secure, but smaller, lighter, and sturdier machines were used, such as the M-209. SIGABA, impressive as it was, was overkill for tactical communications. This said, new speculative evidence emerged more recently that the M-209 code was broken by German cryptanalysts during World War II.
caption: The rotors (wheels) and rotor unit
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