Typex

In the history of cryptography, Typex (alternatively, Type X or TypeX) was a British rotor machine used from 1937; it was an adaptation of the commercial Enigma machine with a number of enhancements that greatly increased its security. The machine was initially termed the "RAF Enigma with Type X attachments".

History and development

By the 1920s, the British Government were seeking a replacement for their book code systems, which had been shown to be insecure, and which, with superencipherment, proved to be slow and awkward to use in practice. In 1926, an inter-departmental committee was formed to consider whether they could be replaced with cipher machines. Over a period of several years, the committee investigated a number of options, including a suggestion by Wing Commander O. G. W. G. Lywood to adapt the commercial Enigma, but no proposal was decided upon. In August 1934, Lywood began work on a machine regardless, authorised by the RAF. Lywood worked with J. C. Coulson, A. P. Lemmon, and W. E. Smith at Kidbrooke in Oxfordshire, with the print unit provided by Creed & Company. The first prototype was delivered to the Air Ministry on 30 April 1935. In early 1937, around 30 Typex Mk I machines were supplied to the RAF.

The design of its successor had begun by February 1937. In June 1938, Typex Mk II was demonstrated to the cipher-machine committee, who approved an order of 350 machines. After some initial trials, the machine was soon adopted by the Army and other governmental departments.

Description of Typex

Typex came in a number of variations, and was a five-rotor machine (as opposed to three or four in the Enigma) with a non-rotating reflector. In most versions, the first two rotors were stationary during encipherment, although they could be set by hand. These additional stationary rotors provided a similar sort of protection to that of the Enigma's plugboard, which the Typex lacked.

Another improvement the Typex had over the commercial Enigma machine was that each rotor in the machine contained several notches that would turn its neighbouring rotor. Whereas the Enigma would turn after every 26th key press, the Typex might change after 5, 11, 13, and 21 key-presses.

The Typex rotors came in two parts — a slug containing the wiring was inserted into a metal casing. Different casings contained different numbers of notches around the rim, such as 5, 7 or 9 notches. Each slug could be inserted into a casing in two different ways by turning it over. In use, all the rotors of the machine would use casings with the same number of notches. Normally five slugs were chosen from a set of ten.

In operation, operators could achieve 20 words a minute, and the output ciphertext or plaintext was printed on paper tape. For some portable versions, such as the Mark III, a message was typed with the left hand while the right hand turned a handle (Devours and Kruh, 1985).

Security and usage

Typex was used by the British Army and Royal Air Force. Machines were also used in Commonwealth countries such as Canada and New Zealand. By far the greatest difference from Enigma was to simply use Typex as little as possible. Whereas the Germans routinely encrypted almost all of their messages in their various networks using Enigma, only the British Army high command and the RAF, used the Typex regularly. Other branches still performed all of their encryption by hand using older book-based methods. The supply of the Typex machines was kept severely limited, and no field units were ever allowed to have machines.

From 1943 the Americans and the British signed the Holden Agreement and BRUSA to develop a Combined Cipher Machine (CCM). The American SIGABA (M-134-C) was another rotor machine, although the Americans never permitted the British to see it. Attachments were built for both that allowed them to read messages created on the other.

It is believed that none of Typex, SIGABA or the CCM were broken by the Axis. Although a British test cryptanalytic attack made considerable progress, the results were not as significant than against the Enigma, due to the increased complexity of the system and the low levels of traffic. A Typex machine was captured by German forces, but it was without rotors. Their inability to use the machine in order to crack Typex messages may have convinced some of them even more of the security of Enigma.

Typex machines continued in use after the war up until the 1970s (the New Zealand government disposed of its last machine in 1973 [1] (http://www.gcsb.govt.nz/infhist.htm)). This was one of the reasons the British kept the Ultra secret for so long; another was that they continued to read the traffic from other nations using Enigma and Typex, while their users continued to consider them secure.

External links

• A series of photograhs of a Typex Mk III (http://www.scienceandsociety.co.uk/results.asp?txtkeys1=Creed)
• Jerry Proc's page on Typex (http://www.jproc.ca/crypto/typex.html)
• Photographs of a Typex at Bletchley Park museum: [2] (http://www.maa.org/england/5_28_Bletchley_Park/image015.htm).

References

• Cipher A. Deavours and Louis Kruh, "Machine Cryptography and Modern Cryptanalysis", Artech House, 1985, pp144–145; 148–150.
• Ralph Erskine, "The Admiralty and Cipher Machines During the Second World War: Not So Stupid after All." Journal of Intelligence History 2(2) (Winter 2002).
• Ralph Erskine, "The Development of Typex", The Enigma Bulletin 2 (1997): pp69–86
• Kruh and Deavours, "The Typex Cryptograph" Cryptologia 7(2), pp145–167, 1983

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