SIGCUM

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.

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.

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).

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.

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.

Rowlett's suggestion was implemented at the same time the first M-228 machines were installed at the Pentagon in January 1943, used for the Washington-Algiers link.

The machines worked as planned, and, initially, Rowlett's study of its security, joined by cryptanalyst Robert Ferner, uncovered no signs of cryptographic weakness.

SIGCUM was immediately withdrawn from service, and work on a replacement system, SIGTOT — a one-time tape machine designed by Leo Rosen — was given top priority.

However, the machine was judged to be only secure enough to handle traffic up to SECRET by landline, and CONFIDENTIAL by radio.