Mercury (cipher machine)

The Mercury was a rotor-based cipher machine developed by the British during World War II. Unlike the more widely known Enigma machine used by the Germans, Mercury was specifically designed for high-level strategic communications and was intended to be more secure than existing Allied cipher systems.

Developed by the Post Office Research Section at Dollis Hill, work began in 1940 with the aim of producing a machine that could be used to encipher and decipher messages transmitted via teleprinter circuits. A key design requirement was resistance to cryptanalysis, especially given the advances being made in codebreaking techniques by the Axis powers.

The Mercury used ten rotors, significantly more than the three or four found in standard Enigma machines. This increased the key space and therefore the complexity of the cipher. Furthermore, the machine featured a plugboard, similar to the Enigma, which allowed for additional substitution of letters, further enhancing its security. The stepping mechanism of the rotors was irregular and designed to resist differential cryptanalysis, a technique that had shown promise against simpler rotor machines.

The first Mercury machine was delivered to Bletchley Park in 1943, where it was rigorously tested. Following successful trials, a small number of Mercury machines were deployed for use by high-ranking military personnel and government officials. It remained in use until the 1950s.

Because of the high level of secrecy surrounding the Mercury project and its limited deployment, relatively little information about its operational use and cryptographic vulnerabilities is publicly available. It is considered a significant, though lesser-known, contribution to Allied cryptographic capabilities during the war.