The Germans did have several major code-breaking successes, mainly concerning the codes used by the Royal Navy and Merchant Navy. At the start of the war, these services were using book codes. These were books that gave code groups for enciphering messages, as well as abbreviations for common words (an example can be seen at http://www.cix.co.uk/~klockstone/codebook.htm). The German Navy's code-breaking service, the B-Dienst, had cracked those codes that were in use in the prewar period. They would then use this experience to help break codes that were introduced during the war. The RN introduced Naval Cipher No 3 in October 1941; the B-Dienst broke it by September 1942. Breaking these codes gave the German Navy a wealth of information on the locations of RN ships, and the courses of Allied convoys.However, when the Royal Navy switched to the more secure Typex machine in 1943, the B-Dienst was unable to continue reading its messages.

German successes with codebreaking were limited for two key reasons. Firstly, they were careless at disguising the sources of their information. When the Allies disseminated Ultra information to lower levels of command, they were careful to disguise the fact that it was coming from deciphered German messages. For example, coordinates were given to lower levels of precision, or the information was referred to as coming from 'special sources', with the implication being that it was obtained from spies within the German command. The Germans took no such precautions. Coordinates and names of ships were sent, giving details that could only have come from deciphered messages. This allowed the Allies to recognise that their codes had been broken, and to move to more secure methods of communication.

The other main reason was that Allied methods of encryption were typically more secure than those used by the Germans. The Western Allies used two main encryption machines, equivalent to the German Enigma. The British used Typex, while the Americans used SIGABA, both using electro-mechanical rotors in a similar fashion to Enigma. Typex was originally derived from the Enigma machine. However, it used five rotors, compared to the three or four of Enigma. This increased the complexity of encryption significantly. It also benefited from its similarity to Enigma - with the Germans being sure that Enigma was unbreakable, they put little effort into attempting to break Typex, as it would be similarly unbreakable. SIGABA was more complex than either Typex or Enigma. Both of these advanced their rotors in a sequential fashion, representing a possible weakness. SIGABA advanced its rotors pseudorandomly, making it much harder to crack. By using these more secure machines, the Western Allies ensured that their codes could not be broken by the Germans.