The Tunny Gallery

From intercept to decrypt during World War II

The Tunny Gallery, along with the Colossus Gallery, shows the entire World War II code-breaking process of the Lorenz-encrypted messages (known as Tunny in the UK) from signal intercept at the Knockholt receiving station in Kent to the production of the final decrypts on Tunny machines in Bletchley Park.

The original Tunny machine, a re-engineering of the then unseen German Lorenz SZ42 cipher machine, was designed by the Post Office Research Station in 1942. It produced the final decrypts of enciphered teleprinter communications of the German High Command during World War II.

Tunny’s design was based upon the ingenious work of a team led by Bill Tutte who worked in the Research Section at Bletchley Park. No-one at Bletchley Park saw the Lorenz until one was captured after the war, so Tutte had to work out the logical structure of the highly sophisticated 12-rotor machine using samples of its encrypted output and the manual decrypts laboriously and ingeniously produced by Colonel John Tiltman. The 12 rotors of the Lorenz machine gave it 1.6 million billion possible start positions, so the decryption of messages by the Testery was an astonishing achievement.

By the end of the war, the Tunny machines are thought to have numbered between 12 and 15, operated continuously and were probably located in Block F at Bletchley Park, a short distance from Block H where the Colossus computers were housed. The Tunny machines were dismantled and recycled after the war.

The Tunny Rebuild

After hundreds of man-hours work, a team of TNMOC volunteers led by John Pether and John Whetter has rebuilt a fully operational Tunny machine based on fragmentary evidence consisting of a few photographs, partial circuit diagrams and the fading memories of a few original Tunny operators.

The Tiltman Break

With millions of possible wheel combinations, the Lorenz-encrypted communications were thought to be unbreakable.

But a human error led to the breakthrough that cracked the code. On 30 August 1941, against all the rules, a Lorenz message was re-sent between Berlin and Athens because the first was not received properly. They used the same wheel settings – and crucially the second message was shortened by the use of abbreviations. This gave Colonel John Tiltman the insight he needed to break the code by hand in ten days.

The three phases of finding the wheel settings of the Lorenz SZ42:
Jul 1942 - Jul 1943 by hand with great patience in the Testery.
Jun 1943 - Jan 1944 with the temperamental Heath Robinson machine.
Feb 1944 - May 1945 with the fast and reliable Colossus computers.

The Testery

Set up in 1942 and led by Major Ralph Tester, the Testery was composed mainly of codebreakers and linguists. From July 1942 until the end of the war, they were breaking Tunny and the staff grew from six to 118.

The Newmanry

Set up in mid 1943 and led by Max Newman, the Newmanry aimed to build machines to help in the codebreaking of Tunny. This led to the development of Colossus computers.

The Testery and the Newmanry worked closely together especially in the last 18 months of the war to break the highly important Tunny messages. “The country was lucky to have these brilliant men in the right place at the right time.” Capt. Jerry Roberts at the opening of the Tunny Gallery in 2011.

Opening day re-union

On 26 May 2011, the Tunny Gallery was opened in the presence of four wartime veterans: Capt. Jerry Roberts, a shift leader in the Testery, Helen Currie, an ATS who operated Tunny, John Croft CBE, an early Testery codebreaker, and Gil Hayward, a Post Office engineer who helped build the first Tunny machines and later managed a Tunny maintenance team at Bletchley Park.

Here is BBC's video coverage of the Tunny Re-Opening Day.