Conel Hugh O'Donel Alexander, CMG, CBE (19 April 1909 – 15 February 1974), was an Irish-born British cryptanalyst, chess player, and chess writer. He worked on the German Enigma machine at Bletchley Park during World War II, and was later the head of the cryptanalysis division at GCHQ for over 20 years. In chess, he was twice British chess champion and earned the title of International Master. He was usually referred to as C.H.O'D. Alexander in print and Hugh in person.