Brigadier John Hessell Tiltman (25 May 1894–10 August 1982) was a British Army officer who worked in intelligence, often at or with the Government Code and Cypher School (GC&CS) starting in the 1920s. His intelligence work was largely connected with cryptography, and he showed considerable skill at cryptanalysis. His work in association with Bill Tutte on the German teleprinter ciphers, called at Bletchley Park the FISH ciphers, led to sometimes successful attack methods. It was to exploit those methods that Colossus, the first digital programmable electronic computer, was designed and built.
Tiltman's parents were from Scotland, though he was born in London. He joined the British Army in 1914, and saw service at the front during the Great War with the King's Own Scottish Borderers. He was wounded in France, and won the Military Cross for bravery. He was seconded to MI1 shortly before it merged with Room 40.
From 1921–1929, he was a cryptanalyst with the Indian Army at Army Headquarters, Simla. They were reading Russian diplomatic cypher traffic from Moscow to Kabul, Afghanistan and Tashkent, Turkestan. In the small section of five or less he was involved in all aspects, directing interception and traffic analysis as well as working on cyphers; he said he was exceptionally lucky to have this experience in other branches of Signals Intelligence.
After a decade as a War Office civilian at GC&CS, the interwar cryptographic organization, John was recalled to active service. His experience enabled him to assist in many areas of endeavour at GC&CS. He was considered one of Bletchley Park’s finest cryptanalysts on non-machine systems.
John Tiltman was an early and persistent advocate of British cooperation with the United States in cryptology. His advocacy helped achieve smooth relations during World War II.
In 1944, he was promoted to Brigadier and appointed Deputy Director of GC&CS. He continued in 1946, as Assistant Director of the Government Communications Headquarters (GCHQ), successor to GC&CS. John Tiltman became Senior GCHQ Liaison Officer at the Army Security Agency in 1949. He retired as a Brigadier.
John Tiltman made the transition from the manual ciphers of the early 20th century to the sophisticated machine systems of the latter half of the century; he was one of a very few who were able to do so. "The Brig" as he was affectionately known in both countries, compiled a lengthy record of high achievement.
On 1 September 2004, Tiltman was inducted into the NSA's "hall of honor", the first non-US citizen to be recognised in that way. The NSA commented, "His efforts at training and his attention to all the many facets that make up cryptology inspired the best in all who encountered him."