London, England · January 21, 1943

Arguably one of Germany’s greatest assets early in World War II was the Enig­ma ma­chine. It could encrypt and decrypt sensi­tive diplo­matic and mili­tary mes­sages in billions of ways (actually 10 to the 23rd power). The loca­tion of U‑boats in the Atlan­tic, supply con­voys, and the orders of battle were sent and received on Ger­man Enig­ma machines. On this date in 1943 in Italy’s North African colony of Libya, British com­mander Bernard Mont­gomery, using Ultra inter­cepts (the code­name for the secret Enig­ma mes­sages), changed his plans to attack Ger­man Field Marshal Erwin Rommel’s Afrika Korps, ordering the drive on the Libyan capital, Tri­po­li, be directed along the coastal road rather than to the south as planned. Two days later a con­fi­dent Brit­ish Eighth Army entered Tri­po­li. Rom­mel’s last assault on the Eighth Army—indeed his last offen­sive in North Africa—ini­ti­ated on March 6, 1943, was turned back based on Ultra inter­cepts. (Looking for a scape­goat, Rom­mel attri­bu­ted his battle­field failures and losses in relief supplies being ferried across the Medi­ter­ra­nean to leaks by senior Ital­ians on his staff.) The Ultra intel­li­gence used by Mont­gomery was pro­duced on ma­chines that looked like ordi­nary type­writers but were electro­me­chani­cal devices that en­coded and decoded mes­sages using pre-set alpha­be­tical moving rotors. The con­tin­ual move­ment of the rotors (three, later four rotors) resulted in a dif­fer­ent crypto­graphic sub­sti­tu­tion after each type­writer key was pressed, scram­bling sen­tences into il­logi­cal se­quences of letters. Following pio­neering Polish work, Brit­ish code­breakers in Bletch­ley Park north of Lon­don deci­phered the Enig­ma code in 1941. The Ger­mans believed the Enig­ma code was un­break­able, but the Brit­ish team of math­e­ma­ti­cians, lin­guists, and scien­tists honed in on seve­ral design flaws, one being that Enig­ma could not en­crypt any letter as itself. The cap­ture of Ger­man code­books gave the Bletch­ley team addi­ti­onal help. Though the Bletch­ley oper­a­tion was only dis­closed in 1974, histo­rians since have gene­rally con­cluded that the intel­li­gence gained from the Ultra breakthrough shortened the war by two or more years.

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Bletchley Park and Decoding the Enigma

Bletchley Park in the small Buckinghamshire market town north of LondonAlan 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 (called “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. British Prime Minister Winston 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. To encrypt or decrypt a mes­sage, an opera­tor typed on the key­board. Settings on the plug­board (front of machine, partially hidden), in com­bi­nation with the rotors on the top, deter­mined the code. Billions of com­bi­nations were pos­sible. With each key press, the corres­pon­ding coded (or decoded) letter lit up the out­put panel above the key­board, allowing the opera­tor to copy down the mes­sage. 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.

Right: A three-rotor Enigma ciphering machine in use by the Luft­waffe, Decem­ber 1943. An esti­mated 100,000 Enigma machines were con­structed during the war and used princi­pally by the Wehr­macht (German armed services). Almost to the end of the war, Germans had firm faith in the Enigma machine. Indeed, Adm. Karl Doenitz of the Kriegs­marine had been advised that a crypt­analytic attack on his Enigma machines was the least likely of all his secu­rity prob­lems. But the truth was that by 1942 Allied code­breakers were deci­phering nearly 4,000 Ger­man trans­mis­sions daily, reaping a wealth of infor­mation used by the British and Americans against German naval, air, and land forces.

Scholarly Explanation of Encryption Technology: How Enigma Machine Worked and How the German Code Was Cracked