SIGCUM, also known as Converter M-228, was a rotor cipher machine used to encrypt teleprinter traffic by the United States Army. Hastily designed by William Friedman and Frank Rowlett, the system was put into service in January 1943 before any rigorous analysis of its security had taken place. SIGCUM was subsequently discovered to be insecure by Rowlett, and was immediately withdrawn from service. The machine was redesigned to improve its security, reintroduced into service by April 1943, and remained in use until the 1960s. Development In 1939, Friedman and Rowlett worked on the problem of creating a secure teleprinter encryption system. They decided against using a tape-based system, such as those proposed by Gilbert Vernam, and instead conceived of the idea of generating a stream of five-bit pulses by use of wired rotors. Because of lack of funds and interest, however, the proposal was not pursued any further at that time. This changed with the United States' entry into World War II in December 1941. Rowlett was assigned to develop a teleprinter encryption system for use between Army command centers in United Kingdom and Australia (and later in North Africa). Friedman described to Rowlett a concrete design for a teleprinter cipher machine that he had invented. However, Rowlett discovered some flaws in Friedman's proposed circuitry that showed the design to be flawed. Under pressure to report to a superior about the progress of the machine, Friedman responded angrily, accusing Rowlett of trying to destroy his reputation as a cryptanalyst. After Friedman calmed down, Rowlett proposed some designs for a replacement machine based on rotors. They settled on one, and agreed to write up a complete design and have it reviewed by another cryptanalyst by the following day. The design agreed upon was a special attachment for a standard teleprinter. The attachment used a stack of five 26-contact rotors, the same as those used in the SIGABA, the highly secure US off-line cipher machine. Each time a key character was needed, thirteen inputs to the rotor stack were energized at the input endplate. Passing through the rotor stack, these thirteen inputs were to be scrambled at the output endplate. However, only five live contacts would be used. These five outputs would form five binary impulses, which would form the keystream for the cipher, to be combined with the message itself, encoded in the 5-bit Baudot code. The rotors advanced odometrically; that