The British and Americans largely used cipher machines that were similar in concept to Enigma. They used a series of rotors containing electrical circuits that advanced as letters were typed. As the rotors advanced, the circuits changed, and new letters were encoded. Other cryptosystems were also used, mainly 'book codes'. These books gave abbreviations, code groups and keys for enciphering messages (an example of these from the Royal Navy can be seen at http://www.cix.co.uk/~klockstone/codebook.htm).

The British Army and Air Force spent the war using a cipher machine called Typex. This was, in many ways, similar to Enigma - in fact, the very first Typex machine was a commercial Enigma machine with a teletype printer attached. However, further developments made it much more secure than Enigma. The ultimate variants of Typex had five rotors, compared to the three or four of Enigma (the number of rotors determines the number of ciphertext alphabets available, and hence how difficult the coded message is to decipher). Typex rotors also rotated at different speeds, resulting in a further increase in complexity. The Germans encountered Typex machines, but considered it just as unbreakable as Enigma, making no real effort to break it. The Royal Navy began the war using book codes. These codes were easy to use, produce and distribute, a major reason for the RN to favour them, especially for communication with merchant ships. However, these codes were easy to break. The German Navy's code breaking group, the B-Dienst, had broken the codes in use pre-war, and were able to keep up with the changes. When Naval Cipher Number 3 was introduced in October 1941, the B-Dienst had cracked it by September 1942. However, the Germans were careless with security around their penetration of British codes. By 1943, the Allies had enough evidence that these codes had been broken, and the RN switched to using Typex.

The US's equivalent to Typex and Enigma was called SIGABA. This was originally a USN project, but the US Army was introduced to it from 1940. It relied on the same concept as Typex or Enigma, using electromechanical rotors to generate the key. Unlike the two other machines, SIGABA used fifteen rotors, which were advanced in a pseudorandom fashion, rather than sequentially. This made it far more secure than either of the other two machines. As a result, no successful contemporary cryptanalysis of the system is known. A few other devices were used. In 1943, the US Army introduced a machine called SIGCUM, but this was found to be insecure. It was withdrawn until a redesign and improved transmission procedures were put into place. To allow intercommunication between the Allies, a device called the Combined Cipher Machine was produced from 1943. This was an attachment to Typex and Sigaba machines that let messages encrypted on one machine be decrypted on the other. At the tactical level, code talkers were used, mainly by the USMC in the Pacific, but also by the US Army's 4th Infantry Division in Europe. The USMC used Navajo, while the 4th ID used Comanche. Navajo had been only rarely studied by European anthropologists and linguists, and so few outside the tribe were familiar with it. As such, it was a good choice for a code, but it was one that could be broken with a native speaker. The code used a phonetic alphabet, combined with euphemistic terms for common words - 'egg' for bomb, and the like. At an informal level, some British Army units used a similar scheme with the Welsh language. The US State Department used a book code called the Black code for most of the 1930s and 40s. However, it had been both broken by the Germans and stolen by the Italian intelligence service. Despite this, the American military attaché in Cairo, a Colonel Bonner Fellers, sent all his messages to the US in the code. His messages included a large amount of sensitive information about the British 8th Army's situation in logistics, training and position. As a result, German and Italian forces in North Africa had knowledge of 8th Army's disposition and it's fighting ability, as well as of attempts to resupply Malta.