The Typex cipher machine

Typex (also written Type X) was Britain's own rotor cipher machine. It was built from the commercial Enigma and improved where the Enigma was weak. It had five rotors, rotors with several notches, and a printer that put the result on paper tape. It was the main British military cipher machine in the Second World War and into the early Cold War, and there's no evidence that German codebreakers made any real progress against it.

From Enigma to Typex

In the 1920s and 1930s, a committee examining cipher machines for the British forces looked at the commercial Enigma and didn't pursue it. Wing Commander Oswyn Lywood of the RAF proposed adapting it and adding a printing unit, and in August 1934 he began work on a machine authorized by the Air Ministry. The first prototype was delivered on 30 April 1935. About 30 Mark I machines went to the RAF in early 1937. After trials a Mark II was demonstrated in June 1938, and an order for 350 was approved. Ralph Erskine estimates that about 12,000 Typex machines had been built by the end of the war, and the Royal Navy alone eventually needed more than 600.

What was different

The Enigma's regular stepping, with a single notch per rotor, was a weakness its codebreakers could model, since how often the middle and left rotors moved was predictable. Typex's rotors had several notches in different positions, which made the movement far less regular and removed a class of attacks the Enigma allowed. Printing the output also took away a common source of errors, the copying of lamp letters by hand.

The reflector, as in the Enigma, made the machine reciprocal, so the same settings decrypted what they had encrypted, and it meant a letter could never be encrypted to itself. The designers kept the reflector for the convenience of one operation in both directions.

How secure was it?

The popular account says the Germans never broke Typex. The record is less tidy than that. German cryptanalysts could read most British military ciphers within a year of the outbreak of war, but not Typex. A rotorless Typex captured at Dunkirk in 1940 was studied by the German naval codebreaking service (the B-Dienst), which gave up after six weeks. One German cryptanalyst is quoted as believing the machine had seven rotors, so the Germans weren't even certain of its construction. Historians say conclusions about Typex security are less certain than the neat story implies, partly because the German records are incomplete. The safest statement is that no wartime break is documented.

The machine stayed in service long after the war. The Combined Cipher Machine, a joint Anglo-American system built around Typex and the American SIGABA, was used by the Royal Navy from November 1943. The RAF used Typex with a Creed teleprinter until 1960, New Zealand moved to new marks on 1 January 1950 and used Typex until about 1973, and a descendant called Mercury (Typex Mark X) carried on-line traffic.

Timeline

That's a service life of nearly forty years from the first prototype. The M-209 managed about fifteen, and forty is long even for a machine that was never broken.

What it shows

• Learning from someone else's design. Britain's cryptographers took a commercial machine they knew was flawed and fixed what they could see, above all the regular stepping.
• Ergonomics is security. A printer made the machine safer because it removed a place where operators made mistakes.
• No known break isn't proof. It's good evidence, and the wartime record is a good one, but it proves nothing, which is why modern ciphers are public and open to attack.

Common questions