World War II Day by Day

Bletchley Park, 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 late July 1939, just over a month before the Wehr­macht (German armed forces) attacked Poland, the Poles delivered a Polish-recon­structed Enigma machine, along with details of the equip­ment, to Bletchley Park. This gift 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, May 9, 1941, the British retrieved a naval Enigma en­coding machine, operating instruc­tions, manuals, code­books, a stack of messages sent and received, and the rotor settings (encryp­tion keying tables) then in use from a German sub­marine (U‑110) captured east of Green­land. (Praying that the Germans not find out about the U‑boat’s cap­ture and change all naval codes and the cipher system, the British con­cocted a story that one of their destroyers had sunk the sub­marine (actually, the sub was lost three days later under tow to Ice­land) and dec­o­rated the offi­cers and crew for sinking the sub.) Two days earlier code­books and docu­ments on the oper­a­tion of another Enigma machine fell into British hands when an enemy weather ship was captured in the North Atlantic. Over a year later another U‑boat’s Enigma machine and the immensely valu­able code­books with all current settings for the encryption key were conveyed to Bletchley Park.

The combination of the two May 1941 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 reroute 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.

Bletchley Park and Decoding the Enigma

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’s decryption bombe, a fore­runner of modern computers, was capa­ble of trying all 17,756 the­o­ret­ical settings on a three-rotor Enigma machine in just 20 minutes. Turing is widely con­sidered to be the father of com­puter science and arti­ficial intelligence. The com­mer­cially and criti­cally success­ful 2014 film The Imi­ta­tion Game, starring Bene­dict Cumber­batch, is loosely based on the role Turing and his crypt­analysts played in solving the Enigma code. On June 23, 2021, the anni­ver­sary of his birth­day in 1912, Turing became the new face of Britain’s £50 note.

Left: A four-rotor German naval Enigma encoding machine, 24 inches square and 18 inches high, enclosed in a wooden box on display at Bletchley Park. The com­bi­na­tion of the two British-captured crypto­graphic (coding) 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. Amer­ica’s ver­sion of an elec­tro­mech­an­ical rotor-based cipher machine was the SIGABA, the U.S. Army’s ver­sion of the Navy’s CSP-888/889. The SIGABA, which had three banks of five rotors each, is less well known com­pared to the German Enigma machine because it was never cracked, was not so port­a­ble (thus, impractical to use in the field), and was not overused.

Right: A three-rotor Enigma machine in use by the Luft­waffe, Decem­ber 1943. Three major com­po­nents—26 wires that trans­mitted an elec­tri­cal signal that passed through a plug­board and then through three scrambler rotors moving inde­pen­dently while enci­phering the mes­sage—produced an Enigma mes­sage that was encrypted 26 times by 26 times by 26 times. A cryp­to­graphic ana­lyst would have to press 17,576 letter keys to find the starting point for the orig­i­nal mes­sage. 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