Peter Frank George Twinn (9 January 1916 – 29 October 2004) was a British mathematician, Second World War codebreaker and entomologist. The first professional mathematician to be recruited to GC&CS. Head of ISK from 1943, the unit responsible for decrypting over 100,000 Abwehr communications. Early life and education Born in Streatham, South London, Twinn was the son of a senior General Post Office official. After attending Manchester Grammar School and Dulwich College, he graduated in mathematics at Brasenose College, Oxford. He won a scholarship to pursue postgraduate studies in physics. Cryptography Twinn was the first professional mathematician to join GC&CS. In early 1939, he applied after seeing an advertisement, working first in London before moving to Bletchley Park. He worked with Dilly Knox and Alan Turing on German Enigma ciphers. In early 1942, he became the head of the Abwehr Enigma section. Recruitment to GC&CS He was in the middle of a postgraduate scholarship studying Physics when he saw an advertisement for a job with the government. "I was a bit unsettled," he remembered. "I'd finished my university degree and I didn't quite know what to do." The advertisement indicated that they were looking for mathematicians, but was unclear about what else was involved. In that unsettled period after the Munich Agreement, international relations between the major European powers were tense and getting tenser. " They offered me this job at the princely salary of, I think, £275 a year," he said, "which sounded all right to me, and I was taken along on the first day to be introduced to Dilly Knox." He began as an assistant to Alfred Dilwyn ("Dilly") Knox, who headed a team of codebreakers at GC&CS. An eccentric but brilliant character, Dilly Knox was the first British codebreaker to work on the Enigma cipher. Like most GC&CS experts, he was a classicist. But, as war loomed, GC&CS began employing mathematicians, as well as chess players and crossword experts. Twinn was in fact the first mathematician to join the team. Knox believed in throwing his new recruits in at the deep end. He gave Twinn a mere five minutes' training before telling him to go and get on with it. On the eve of war Twinn was the first British cryptographer to read a German military Enigma message, having obtained vital information from Polish cryptanalysts in July 1939. Twinn said that "It was a trifling exercise,