Thomas Ralph Erskine CB (14 October 1933 – 9 April 2021) was a senior Northern Ireland government lawyer and historian of wartime codebreaking.
His work included the drafting of legislation for the first Northern Ireland Assembly in 1973 and the 1986 Anglo-Irish Agreement, for which he was appointed Commander of the Order of the Bath.
In his book The Secret War, the military historian Sir Max Hastings said: "My obligation is immense to Ralph Erskine, a fount of information about every aspect of wartime code-breaking, which is both a maze and a minefield for the uninitiated."
After hearing of Erskine's death, Hastings added: "Every historian of the Second World War and student of cryptology owes him a debt for his extraordinary contribution to the scholarship."
Hugh Sebag-Montefiore called him "the greatest expert on the Enigma-M4", which was the Enigma cipher system used by the German Navy from February 1942 to the end of the war.