Mercury (cipher machine)
Encyclopedia
Mercury was a British cipher machine used by the Air Ministry
from 1950 until at least the early 1960s. Mercury was an online rotor machine
descended from Typex
, but modified to achieve a longer cycle length using a so-called double-drum basket system.
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.
It had been estimated 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 56,700 characters had passed on one setting of the rotors. This 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.
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.
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
, Australia
, Singapore
, Cyprus
, Germany
, France
, Middle East
, Washington
, Nairobi
and Colombo
. 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.
Air Ministry
The Air Ministry was a department of the British Government with the responsibility of managing the affairs of the Royal Air Force, that existed from 1918 to 1964...
from 1950 until at least the early 1960s. Mercury was an online rotor machine
Rotor machine
In cryptography, a rotor machine is an electro-mechanical device used for encrypting and decrypting secret messages. Rotor machines were the cryptographic state-of-the-art for a prominent period of history; they were in widespread use in the 1920s–1970s...
descended from Typex
Typex
In the history of cryptography, Typex machines were British cipher machines used from 1937. It was an adaptation of the commercial German Enigma with a number of enhancements that greatly increased its security....
, but modified to achieve a longer cycle length using a so-called double-drum basket system.
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.
It had been estimated 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 56,700 characters had passed on one setting of the rotors. This 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.
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.
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
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
, Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
, Singapore
Singapore
Singapore , officially the Republic of Singapore, is a Southeast Asian city-state off the southern tip of the Malay Peninsula, north of the equator. An island country made up of 63 islands, it is separated from Malaysia by the Straits of Johor to its north and from Indonesia's Riau Islands by the...
, Cyprus
Cyprus
Cyprus , officially the Republic of Cyprus , is a Eurasian island country, member of the European Union, in the Eastern Mediterranean, east of Greece, south of Turkey, west of Syria and north of Egypt. It is the third largest island in the Mediterranean Sea.The earliest known human activity on the...
, Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
, France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
, Middle East
Middle East
The Middle East is a region that encompasses Western Asia and Northern Africa. It is often used as a synonym for Near East, in opposition to Far East...
, Washington
Washington, D.C.
Washington, D.C., formally the District of Columbia and commonly referred to as Washington, "the District", or simply D.C., is the capital of the United States. On July 16, 1790, the United States Congress approved the creation of a permanent national capital as permitted by the U.S. Constitution....
, Nairobi
Nairobi
Nairobi is the capital and largest city of Kenya. The city and its surrounding area also forms the Nairobi County. The name "Nairobi" comes from the Maasai phrase Enkare Nyirobi, which translates to "the place of cool waters". However, it is popularly known as the "Green City in the Sun" and is...
and Colombo
Colombo
Colombo is the largest city of Sri Lanka. It is located on the west coast of the island and adjacent to Sri Jayawardenapura Kotte, the capital of Sri Lanka. Colombo is often referred to as the capital of the country, since Sri Jayawardenapura Kotte is a satellite city of Colombo...
. 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.