1.1 Background: Lorenz cipher and "Tunny" traffic
The Lorenz SZ40 and its successor SZ42 were German teleprinter cipher machines used for high‑level communications between the German High Command and army group headquarters. Unlike the Enigma machine, which enciphered letters, the Lorenz machine operated on the Baudot–Murray code used by teleprinters, generating a five‑bit key stream that was added to the plaintext. The Allies assigned the codename “Tunny” to the intercepted Lorenz traffic. Early attempts at cryptanalysis by the British revealed that the cipher could be broken by statistical methods, but the volume and complexity of the traffic demanded a dedicated manual‑analysis team.
1.2 Formation under Major Ralph Tester
In July 1942, the Government Code and Cypher School (GC&CS) established the Testery at Bletchley Park. Named after its head, Major Ralph Tester, the section was created to focus specifically on the manual cryptanalysis of Tunny messages. Tester, a former army officer with a background in languages and mathematics, assembled a team of cryptanalysts, linguists, and typists. The Testery was housed in a single room (later expanded) and operated around the clock, processing intercepted Lorenz messages that had been recorded on punched paper tape.
1.3 Relationship with the Newmanry
The Testery worked alongside the Newmanry, a separate section formed in 1943 under Max Newman. The Newmanry developed and operated the Colossus computers, which performed high‑speed statistical attacks to find wheel patterns and settings. The relationship was complementary: the Testery handled the initial hand‑methods and later the detailed decryption of individual messages, while the Newmanry provided machine‑generated probabilities and candidate settings. Close communication between the two sections was essential, with results passed between them on paper slips.
2.1 Manual cryptanalysis techniques
2.1.1 "Testery method" of wheel setting
The Testery developed a distinctive hand‑method for determining the starting positions (settings) of the Lorenz wheels. Known as the “Testery method,” it involved comparing two intercepted messages that shared the same key (a “depth”) or using a known‑plaintext attack (a “crib”). Cryptanalysts would slide sheets of paper with the ciphertext and guessed plaintext over each other, looking for patterns that revealed the cam‑settings of the wheels. This process was painstaking, often taking hours or days for a single message.
2.1.2 Key breaking and message recovery
Once the wheel settings were found, the Testery could reconstruct the key stream for that message. This allowed the ciphertext to be decrypted into the original German teleprinter code. The decrypted text was then typed onto paper and forwarded to the translators. The Testery also developed techniques to break the key stream for entire periods, enabling the reading of multiple messages transmitted on the same link.
2.2 Personnel and roles
2.2.1 Cryptanalysts
The Testery employed a core of skilled cryptanalysts, many of whom were recruited from academia, including mathematicians and classicists. Notable members included W. T. Tutte, who later made fundamental contributions to graph theory, and Jerry Roberts, a linguist. The cryptanalysts worked in small teams, each focusing on a different traffic link or stage of the process.
2.2.2 Linguists and translators
After decryption, the German plaintext messages were handed to linguists and translators within the Testery. They rendered the recovered traffic into English, often adding contextual notes. The translators needed familiarity with German military terminology and slang, and they produced intelligence reports that were sent to senior Allied commanders.
2.3 Co‑operation with Colossus computers
2.3.1 Hand‑to‑machine handover
The Testery frequently passed partial results to the Newmanry to speed up the analysis. For instance, if the Testery identified a likely crib or a pattern in the key stream, that information would be used to set up the Colossus machines more efficiently. Conversely, the Newmanry would relay its computed probabilities to the Testery for final verification and decryption.
2.3.2 Post‑processing of machine output
Once Colossus had produced a candidate setting or a decrypted stream of bits, the Testery would manually check the output for plausibility. This involved comparing the bit‑stream against known German message formats, correcting minor errors from the teleprinter transmission, and then converting the bits into letters. The Testery thus acted as the quality‑control and final‑output stage for the combined manual‑machine operation.
3.1 Decrypts of German high‑command traffic
The Testery successfully decrypted a large proportion of the Tunny traffic between the German High Command and its field armies. This included strategic orders, situation reports, and intelligence assessments. The intelligence derived from these decrypts was classified under the codename “Fish” and was considered at least as valuable as Ultra intelligence from Enigma. Over the course of the war, the Testery processed tens of thousands of messages, many of them crucial to Allied planning.
3.2 Contributions to specific campaigns
3.2.1 Normandy landings (Operation Overlord)
In the months leading up to the D‑Day landings in June 1944, the Testery decrypted German reports on troop movements, fortifications, and command‑and‑control signals. These decrypts confirmed that the German deception plan (Operation Fortitude) was working, and they provided detailed information about German defensive positions on the Normandy coast. The timely intelligence from Tunny traffic helped Allied commanders adjust their plans.
3.2.2 Other major operations
Beyond Overlord, Testery decrypts supported the Italian campaign, the Battle of the Atlantic, and the Allied advance into Germany. For example, in 1943, decrypted messages revealed German order‑of‑battle changes on the Eastern Front and in the Balkans. Later, in 1945, the Testery’s work gave advance warning of German counterattacks during the Ardennes offensive.
4.1 Secrecy and declassification
For decades after the war, the existence and work of the Testery remained a closely guarded secret. The Colossus computers were dismantled, and most documents were destroyed or classified. It was not until the 1970s that the British government began to acknowledge the role of the Testery, and full declassification of its operations occurred gradually in the 1990s and early 2000s. This secrecy meant that many of the cryptanalysts’ contributions were unknown to the public during their lifetimes.
4.2 Historical research and commemoration
4.2.1 Reconstructions and museum displays
Since declassification, historians have reconstructed the Testery’s methods and personnel. The Bletchley Park museum now includes a dedicated exhibit on the Tunny‑breaking effort, featuring original equipment, photographs, and personal accounts. A working replica of a Lorenz machine and displays explaining the Testery’s manual techniques are part of the museum’s permanent collection.
4.2.2 Influence on modern cryptology
The techniques pioneered by the Testery, particularly the combination of manual cryptanalysis with early electronic computers, influenced later developments in codebreaking and information theory. The work of Testery cryptanalysts such as W. T. Tutte led to important advances in combinatorics and algebraic coding. The Testery is now recognised as a vital component of Bletchley Park’s success, and its legacy is studied in military history and computer science curricula.