London, England · May 9, 1941

As war loomed in Europe, British code­breakers based at Bletchley Park out­side London worked feverishly to un­ravel the Enigma cipher machine, which the Ger­mans used to en­crypt their most sec­ret commu­ni­ca­tions. The Enigma had a num­ber of differ­ently wired scrambler rotors that oper­a­tors changed and shuffled through billions of per­mu­ta­tions, making the en­coded text infuri­a­tingly diffi­cult to decipher. In August 1939, less than a month before the Wehr­macht (German armed forces) attacked Poland, the Poles delivered a five-rotor Enigma machine to Bletchley Park, and this was the spring­board that allowed the odd­ball set of Brit­ish mathe­ma­ti­cians, lin­guists, and scien­tists to repeatedly break the far more com­pli­cated and thus secure systems intro­duced after war broke out. On this date in 1941 the British retrieved a naval Enigma en­coding machine, code­books, and the rotor settings (encryp­tion keys) then in use from a Ger­man submarine (U-110) captured off Ice­land. (Praying that the Ger­mans not find out about the U‑boat’s cap­ture and change all naval codes and the cipher system, the Brit­ish scuttled the sub­marine as it was being towed to Brit­ain.) Two days earlier code­books and docu­ments on the oper­a­tion of the Enigma machine fell into Brit­ish hands when an enemy weather ship was cap­tured in the North Atlantic. The combi­na­tion of the two cap­tured prizes allowed Brit­ish cryp­to­lo­gists to even­tu­ally read the differ­ent war­time Enigma codes used by the Ger­man army, navy, and air force. Within months intel­li­gence from the decrypts (called Ultra) allowed the Allies to re-route many mer­chant con­voys past U‑boats in the North Atlantic, saving hun­dreds of lives and hun­dreds of thou­sands of tons of vital shipping. It also began to tip the Battle of the Atlan­tic in the Allies’ favor in that the hunted now became the hunters. In May 1943 alone, 43 enemy subs were sunk, bringing the total to 100 since the start of the year. So great was the stepped-up offen­sive by Allied sub­ma­rines and escort carrier- and land-based air­craft that Adm. Karl Doenitz, commander-in-chief of the Kriegs­marine, withdrew his U‑boats from the Atlan­tic for a time. Brit­ish Prime Minister Winston Chur­chill, whose island popu­lation was close to being starved by the U‑boat menace, called cracking the German Enigma code the “secret weapon” that won the war.

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Bletchley Park and Decoding the Enigma

Bletchley Park in Buckinghamshire, England, headquarters of Britain’s Government Code and Cypher SchoolAlan Turing (standing) at Bletchley Park

Left: Bletchley Park, top-secret headquarters of Britain’s Govern­ment Code and Cypher School, where ciphers and codes of several Axis coun­tries were decrypted. This mock-Tudor mansion, with its sur­rounding buildings (“huts”), was home to as many as 10,000 men and women during the war, including Britain’s most bril­liant mathe­ma­tical brains, and was the scene of im­mense advances in com­pu­ter science and modern com­puting. Chur­chill referred to the Bletchley staff as “the geese that laid the golden eggs and never cackled.”

Right: Alan Turing (standing) was an English mathema­tician and war­time code­breaker. At Bletchley Park, Turing (1912–1954) took the lead in a team that designed an electro­mechan­ical machine known as a “bombe” that success­fully broke German ciphers. Turing is widely con­sidered to be the father of com­puter science and arti­ficial intelligence­.

Four-rotor Kriegsmarine Enigma machineThree-rotor Luftwaffe Enigma machine

Left: A four-rotor German naval Enigma on display at Bletchley Park. The com­bi­na­tion of the two British-captured crypto­graphic prizes in May 1941 was cru­cially impor­tant in breaking German U‑boat codes and ulti­mately in winning the Battle of the Atlantic. One Bletchley Park vete­ran said that Ultra decrypts shortened the war by two to four years.

Right: A three-rotor Enigma machine in use by the Luftwaffe, Decem­ber 1943. Almost to the end of the war, the Ger­mans had firm faith in the Enigma ciphering machine; indeed, Adm. Doenitz had been advised that a cryptanalytic attack on his naval Enigma machines was the least likely of all his security problems. But in fact Allied code­breakers were deciphering nearly 4,000 Ger­man trans­missions daily by 1942, reaping a wealth of information.

Bletchley Park: Breaking the Unbreakable German Enigma Coding Machine