Typex

History

Typex was the British services’ cipher machine from 1937 into the 1950s: the commercial Enigma with a printer, two fixed drums and more notches. The Germans captured machines without drums in 1940 and said after the war they had not broken it; Dermot Turing weighs whether captures let them read some in 1942–43. In 1945 the U.S. Navy’s OP-20-G examined a Royal Navy Mark II, No. 2026, as “R5W Enigma”, and tabled its drum sets; Frode Weierud published the reports in 2023.

How it works

Five drums sit left to right: slow, medium, fast, and two stators; a key passes the stators and moving drums to the reflector and back. An insert fits either way round, so Y-269’s fourteen give 28 drums. The drums step before each letter with the Enigma’s double step; on the Mark II the four notches are on the drum, so the tyre only relabels the window. The Mark 22 has nine on the tyre, and a stecker. On the Air Ministry’s keyboard X is the space bar, Z shifts to figures and V back; V, X and Z are typed in figures over C, G and D.

Without the key, the stators and entry are one fixed substitution. With the drums and reflector known, every choice of moving drums at every position is walked through a menu of the crib, as a Bombe walks it; the stators are then fitted to the stop.

How it was checked

Y-269’s inserts, turned over by OP-20-G’s rule, equal their separately typed reverse table in 363 of 364 cells; the odd cell gives a letter twice. Their printed double step, DYL to EAP, comes out. The keyboard’s rules type the Mk VI manual’s VIXEN and a Mark III test sheet’s pangram letter for letter; so does CyberChef’s vector on its random drums. A crib of 26 letters gives a test message’s key, with one stop.

Limits

No Typex message is published with its key, so none is checked against a machine. No wartime reflector setting is published; the one given completes a key line Hagelin’s sheet prints in part. The reflector plate’s lettering is CyberChef’s; no source states it. Three SP wheels, typed with a letter twice, are left out. The Mark 22’s notch letters are read as window letters. The figures over B and N are illegible. The search assumes the reflector known; with it replugged, Turing judged in 1943, a thousand letters might be needed.