Tunny: Hitler’s biggest fish

Abstract

After 1942, the Tunny cipher machine took over increasingly from Enigma for encrypting Berlin’s highest-level army communications. Hitler used Tunny to communicate with his generals at the front lines. Turing tackled Tunny in the summer of 1942, and altered the course of the war by inventing the first systematic method for breaking into the torrent of priceless Tunny messages. The story of Enigma’s defeat by the Bletchley Park codebreakers astonished the world. Less well known is the story—even more astounding—of the codebreakers’ success against a later, stateof- the-art German cipher machine (Fig. 14.1). This new machine began its work encrypting German Army messages in 1941, nearly two years into the war. At Bletchley Park it was codenamed simply ‘Tunny’. Broken Tunny messages contained intelligence that changed the course of the war and saved an incalculable number of lives. How Bletchley Park broke Tunny remained a closely guarded secret for more than 50 years. In June 2000 the British government finally declassified the hitherto ultra-secret 500-page official history of the Tunny operation. Titled ‘General report on Tunny’, this history was written in 1945 at the end of the war by three of the Tunny codebreakers, Donald Michie, Jack Good, and Geoffrey Timms. Finally the secrecy ended: the ‘General report’ laid bare the whole incredible story of the assault on Tunny. Far more advanced than Enigma, Tunny marked a new era in crypto-technology. The Enigma machine dated from the early 1920s—its manufacturer first placed it on the market in 1923—and even though the German Army and Navy made extensive modifications, Enigma was certainly no longer state-of-the-art equipment by the time the war broke out in 1939. From 1942, Hitler and the German Army High Command in Berlin relied increasingly on the Tunny machine to protect their ultra-secret communications with the front-line generals who commanded the war in the eastern and western theatres. Germany’s compromised Tunny radio network carried the highest grade of intelligence, giving Bletchley Park the opportunity to eavesdrop on lengthy back-and-forth communications between the grand architects of Germany’s battle plans. Tunny leaked detailed information about German strategy, tactical planning, and military strengths and weaknesses.