KW-26

The TSEC/KW-26, code named ROMULUS, (in 1966 the machine based encryption system was not code-named "Romulus," rather the code-name was "Orion," at least in the US Army's variant) was an encryption system used by the U.S. Government and, later, by NATO countries. It was developed in the 1950s by the National Security Agency (NSA) to secure fixed teleprinter circuits that operated 24 hours a day. It used vacuum tubes and magnetic core logic, replacing older systems, like SIGABA and the British 5-UCO, that used rotors and electromechanical relays.

A KW-26 system (transmitter or receiver) contained over 800 cores and approximately 50 vacuum-tube driver circuits, occupying slightly more than one half of a standard 19-inch rack. Most of the space in the rack and most of the 1 kW input power were required for the special-purpose vacuum tube circuits needed to provide compatibility with multiple input and output circuit configurations. The military services' requirements for numerous modes and speeds significantly increased costs and delayed delivery. NSA says it is doubtful that more than three or four of the possible configurations were ever used.

The KW-26 used an NSA-developed encryption algorithm based on shift registers. The algorithm produced a continuous stream of bits that were xored with the five bit Baudot teleprinter code to produce ciphertext on the transmitting end and plaintext on the receiving end. In NSA terminology, this stream of bits is called the key. The information needed to initialize the algorithm, what most cryptographers today would call the key, NSA calls a cryptovariable. Typically each KW-26 was given a new cryptovariable once a day.

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