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 Law 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. Gen. Dwight D. Eisen­hower, Supreme Com­man­der of Allied Forces, acknow­ledged in July 1945 that the Bletchley Park work­force “saved thou­sands of British and Amer­i­can lives and, in no small way, con­trib­uted to the speed with which the enemy was routed.”

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