I've covered this before here, here, and here. The main cipher machine used by the British armed forces was Typex. This worked in a very similar manner to the German Enigma machine; indeed, the very first Typex machines were based on the commercial variant of Enigma. By the start of the war, however, Typex was more secure than Enigma, with five rotors rather than the three of Enigma. The Germans recognised this similarity, and believing Enigma to be unbreakable, made no significant attempt at a cryptanalysis of Typex. Instead, they focused their efforts on the codes used by the Royal Navy. These codes were much simpler than Typex. Rather than using cipher machines, they used code books. These books, an example of which can be found here, gave code groups - strings of numbers or letters - that corresponded to words in the plaintext. These ciphers were much easier to produce and use than cipher machines; however, they were less secure. The German Navy's cipher bureau, the B-Dienst, had broken the codes that were in use pre-war. Those codes were not changed at the start of the war, allowing the German Navy to read Royal Navy messages for much of the early part of the war. As the RN introduced new codes, the B-Dienst was generally able to keep up - when Naval Cipher No. 3 was introduced in October 1941, it had been broken within a year. Unlike the Germans, however, the RN was willing to believe that its ciphers could be broken. The RN began to believe that Naval Cipher No. 3 had been broken in early 1943, and moved to Naval Cipher No. 5 in June, after compromise of Cipher No. 3 had been proven in May 1943. Naval Cipher No 5. was broken shortly afterwards. On realising this, the RN began to adopt Typex, which also made co-operation with the USN easier.