Tunny Reveals B-Dienst Successes Against the ‘Convoy Code’

Abstract

During World War II, German naval codebreakers in the B-Dienst made extensive breaks into Naval Cypher No. 3, an enciphered code used by the Allies for the vital Atlantic convoys. It is often suggested that they did not discover that Cypher No. 3 was insecure until May 1943. This article shows that the British had learned about this much earlier, in August 1942, and that they informed the US Navy then. British solutions of messages encrypted with the Wehrmacht's Tunny teleprinter cipher machine had revealed that the B-Dienst was solving Naval Cyphers Nos. 3 and 4. Surprisingly, those ciphers were not replaced until June 1943.