The Axis saw considerable success breaking some Allied ciphers. The book ciphers used by the Royal Navy in the early part of the war were particularly vulnerable. These ciphers were relatively simple, using books (like this one) that contained groups of numbers that corresponded to words in a plaintext message. The Italians and Germans both managed to break these ciphers, allowing them to track British fleet units and convoys. The German Navy's code-breaking service, the B-Dienst, broke the ciphers in use pre-war, and were generally able to keep up with the changes in the cipher. When Naval Cipher Number 3 was introduced in October 1941, the B-Dienst had cracked it by September 1942. The Italian successes were equally impressive; in some cases, they were able to decode messages before British commanders received them. Another notably broken cipher was the Black Code, used by the US State Department. The German intelligence service managed to break this cipher, while the Italians stole a copy. This was a significant leak, as the American military attache in Cairo, Bonner Fellers, used the cipher to communicate with the USA. Fellers' messages included considerable detail on British forces and plans in North Africa and the Mediterranean. 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.

Despite these successes, the Axis never had the same in-depth penetration of Allied ciphers as the Allies did of Axis ciphers. There were several reasons for this. Firstly, Allied ciphers were generally more secure than Axis ones. The main cipher machine used by the Germans and Italians was the Enigma machine, which used three to four rotors to scramble messages. The British equivalent to Enigma was Typex, which had originally been based on the commercial variant of the Enigma machine. Typex used five rotors, which rotated at different speeds. The American equivalent, SIGABA, used 15 rotors, which advanced in a pseudorandom fashion. While both machines could be broken using similar techniques to those used to break Enigma, the Axis had fewer chances to do so thanks to the second factor; Allied radio procedure was generally better. To carry out a cryptanalysis of Enigma, the Allies took advantage of lazy techniques by German radiomen - such as reusing settings or using stereotypical message headings. Allied radiomen were generally better about avoiding these 'cribs', making it harder for the Axis to break the Allied codes. The Germans also had poor radio procedure when it came to disseminating the fruits of code-breaking. When the Allies distributed Enigma decrypts, they took steps to conceal the source of the information, such as implying it came from agents on German-held soil, or using less-precise coordinates. The Germans did not; this meant that the Allies were able to determine that the RN's ciphers had been broken from reading German messages. Finally, the Axis put comparatively little effort into reading Allied ciphers. They were generally confident in the security of their own ciphers. As many of the Allied ciphers used similar techniques to the Axis ciphers, it was felt that trying to break them would be a waste of time and resources. When the Germans captured a Typex machine at Dunkirk in 1940, they noticed the similarity to Enigma, and made no effort to break it as a result.