Mercury was a British
cipher machine used by the
Air Ministry
The Air Ministry was a department of the Government of the United Kingdom with the responsibility of managing the affairs of the Royal Air Force and civil aviation that existed from 1918 to 1964. It was under the political authority of the ...
from 1950 until at least the early 1960s. Mercury was an online
rotor machine
In cryptography, a rotor machine is an electro-mechanical stream cipher device used for encrypting and decrypting messages. Rotor machines were the cryptographic state-of-the-art for much of the 20th century; they were in widespread use from ...
descended from
Typex, but modified to achieve a longer cycle length using a so-called ''double-drum basket'' system.
History
Mercury was designed by Wing Commander E. W. Smith and F. Rudd, who were awarded £2,250 and £750 respectively in 1960 for their work in the design of the machine. E. W. Smith, one of the developers of
TypeX, had designed the double-drum basket system in 1943, on his own initiative, to fulfil the need for an on-line system.
Mercury prototypes were operational by 1948, and the machine was in use by 1950. Over 200 Mercury machines had been made by 1959 with over £250,000 spent on its production. Mercury links were installed between the UK and various Overseas stations, including in
Canada
Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...
,
Australia
Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...
,
Singapore
Singapore, officially the Republic of Singapore, is an island country and city-state in Southeast Asia. The country's territory comprises one main island, 63 satellite islands and islets, and one outlying islet. It is about one degree ...
,
Cyprus
Cyprus (), officially the Republic of Cyprus, is an island country in the eastern Mediterranean Sea. Situated in West Asia, its cultural identity and geopolitical orientation are overwhelmingly Southeast European. Cyprus is the List of isl ...
,
Germany
Germany, officially the Federal Republic of Germany, is a country in Central Europe. It lies between the Baltic Sea and the North Sea to the north and the Alps to the south. Its sixteen States of Germany, constituent states have a total popu ...
,
France
France, officially the French Republic, is a country located primarily in Western Europe. Overseas France, Its overseas regions and territories include French Guiana in South America, Saint Pierre and Miquelon in the Atlantic Ocean#North Atlan ...
,
Middle East
The Middle East (term originally coined in English language) is a geopolitical region encompassing the Arabian Peninsula, the Levant, Turkey, Egypt, Iran, and Iraq.
The term came into widespread usage by the United Kingdom and western Eur ...
,
Washington,
Nairobi
Nairobi is the Capital city, capital and largest city of Kenya. The city lies in the south-central part of Kenya, at an elevation of . The name is derived from the Maasai language, Maasai phrase , which translates to 'place of cool waters', a ...
and
Colombo
Colombo, ( ; , ; , ), is the executive and judicial capital and largest city of Sri Lanka by population. The Colombo metropolitan area is estimated to have a population of 5.6 million, and 752,993 within the municipal limits. It is the ...
. The machine was used for UK diplomatic messaging for more or less a decade, but saw almost no military use.
In 1960, it was anticipated that the machine would remain in use until 1963, when it would be made obsolete by the arrival of
BID 610 (Alvis) equipment.
A miniaturised version of Mercury was designed, named Ariel, but this machine appears not to have been adopted for operational use.
Design
In the Mercury system, two series of rotors were used. The first series, dubbed the ''control maze'', had four rotors, and stepped cyclometrically as in Typex. Five outputs from the control maze were used to determine the stepping of five rotors in the second series of rotors, the ''message maze'', the latter used to encrypt and decrypt the plaintext and ciphertext. A sixth rotor in the message maze was controlled independently and stepped in the opposite direction to the others. All ten rotors were interchangeable in any part of either maze. Using rotors to control the stepping of other rotors was a feature of an earlier cipher machine, the US
ECM Mark II.
Mercury also used double-wired rotors, consisting of "Inside and Outside Scrambled Wheels", the Outer wheels being settable in a number of positions with respect to the Inner wheels.
Security
It had been mathematically determined that Typex had a sufficiently large cycle to permit only 750 characters to be sent using a single arrangement of its rotors without fear of compromising security. A counter recorded the number of keystrokes and when these reached 750 a predetermined rotor was manually advanced one step, thus permitting TypeX to safely encrypt messages with more than 750 key strokes.
Mercury, with its longer cycle length, was judged to be safe even after 56700 characters had passed on one setting of the rotors. This cycle length was sufficient for the machine to be used as an on-line cipher machine with
traffic flow security — the machine would transmit continuously, even if not sending a message.
See also
*
5-UCO The 5-UCO (5-Unit Controlled)Ralph Erskine, "The 1944 Naval BRUSA Agreement and its Aftermath", ''Cryptologia'' 30(1), January 2006 pp14–15 was an on-line one-time tape Vernam cipher encryption system developed by the UK during World War II ...
References
* "Awards to civilian personnel in respect of cypher machine development", 1955–1960, PRO AVIA 65/977.
{{Cryptography navbox , machines
Rotor machines
Cryptographic hardware