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 German surface ships and U‑boats and Allied supply con­voys in the Atlan­tic theater as also German orders of battle were sent and received on Nazi Enigma machines.

On this date, January 21, 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. He ordered 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 as well as heavy losses in relief supplies being ferried by sea and air across the Medi­terranean to leaks by senior Italians on his staff.

The Ultra intelligence used by Mont­gomery was pro­duced on ma­chines that some­what resembled ordi­nary type­writers but were actually electro­me­chani­cal devices that en­coded and decoded German high-fre­quency radio Morse code and tele­type traffic using pre-set alpha­be­tical rotating wheels, or rotors. The con­tin­ual move­ment of the inter­change­a­ble rotors (three, after Feb­ru­ary 1942 four rotors used by the German Kriegs­marine) resulted in a dif­fer­ent crypto­graphic sub­sti­tu­tion after each type­writer key was pressed, scram­bling sentences into illogical sequences of letters.

Following pioneering Polish work in the 1930s on German com­mer­cial en­cryp­tion devices called “Enigmas,” British code­breakers in Bletch­ley Park 50 miles north-north­west of Lon­don deci­phered the Enig­ma code in 1941 (the Poles hadn’t been able to). The Germans believed the Enig­ma code was un­break­able, but the Brit­ish team of math­e­ma­ti­cians, lin­guists, scien­tists, clas­si­cal mus­icians, card-counting poker players, cross­word puzzle enthu­si­asts and others with an apti­tude for encoding and decoding method­i­cally honed in on seve­ral design flaws, one being that Enig­ma could not en­crypt any letter as itself—that is, “E” could never become “E.” Also, Enigma had a recip­ro­cal quality in that if “E” became “G” on a certain cipher key setting, then “G” became “E.”

The capture of German encryption materials from German naval vessels (e.g., three Enigma wheels in February 1940; a working Enigma cipher machine, cipher tables, and signal books in May 1941 and again in October and September 1942 following the intro­duc­tion of the naval four-rotor Enigma machine) gave the Bletch­ley team addi­ti­onal help. So did the advan­tage gained by exploiting cer­tain German slip­shod proce­dures; e.g., cipher clerks neglecting to select a new cipher key for each new radio mes­sage but simply using the day’s ini­tial settings to save time. Though the Bletch­ley oper­a­tion was only dis­closed to the public 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: Unraveling the Secrets of the Enigma Cipher Machine

Bletchley Park in the small Buckinghamshire market town north of LondonAlan Turing (standing) at Bletchley Park

Left: Midway between the university towns of Oxford and Cambridge, Bletchley Park was the top-secret head­quarters of Britain’s Govern­ment Code and Cypher School. It was at this Buck­ing­ham­shire country estate that ciphers and codes of several Axis coun­tries were decrypted and forwarded to various Allied mili­tary head­quarters and intel­li­gence centers (e.g., the Royal Navy’s Oper­a­tional Intel­li­gence Center) to help in pros­e­cuting the war. Bletchley Park’s mock-Tudor mansion, with its surrounding buildings (called “huts”), was home to as many as 10,000 men and women during the war years, including Britain’s most bril­liant math­e­matical brains, and was the scene of immense advances in com­puter science and modern program­ma­ble elec­tronic com­puting. Mate­rially assisting Bletchley Park’s acti­vi­ties from 1943 onward were American-designed and -built high-speed, high-powered com­puting machines located in Dayton, Ohio. British Prime Minister Winston Churchill referred to the Bletchley staff as “the geese that laid the golden eggs and never cackled.”

Right: Widely considered to be the father of computer science and arti­fi­cial intel­li­gence, Alan Turing (standing) was a nerdy, soft-spoken Cam­bridge math­e­ma­tician and war­time code­breaker. At Bletchley Park, Turing (1912–1954), nick­named “Prof” by his col­leagues, and another of Britain’s top code­breakers, Gordon Welch­man (1906–1985), teamed up to design an electro­mechan­ical machine known as a “bombe” (a Polish word for a type of ice-cream dessert) that could scan and, applying math­e­mat­i­cal logic, ana­lyze and decrypt the huge amount of Enigma signal traffic gen­er­ated by the Nazi war machine and sent over high-fre­quency cir­cuits. The British built 60 bombe machines. The original machine was 7 ft tall by 6-1/2 ft wide and could drive through every poten­tial Enigma setting in about 12 minutes by elec­tri­cally per­forming a chain of logi­cal deduc­tions that provided code­breakers with a small number of possible Enigma settings. By the end of the war, 9,000 people were opera­ting these machines to process thou­sands of intel­li­gence inter­cepts every day. 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.

Four-rotor Kriegsmarine Enigma machineThree-rotor Luftwaffe Enigma machine

Left: On display at Bletchley Park is a four-rotor German naval Enigma, which com­pared with the three-rotor Enigma machine created twenty-six more ways to encipher each letter of the German alpha­bet. To encrypt or decrypt a mes­sage, an opera­tor inserted the rotors into the Enigma machine and posi­tioned them in a certain order, as specified by a multi-letter cipher key. Each rotor could be set in twenty-six dif­fer­ent positions, and each rotor—they were many to chose from—had an arrange­ment of inter­nal wiring unique to itself. The rotors were sur­rounded by mov­a­ble outer rings. Addi­tion­ally there were elec­tri­cal plugs, called “steckers,” attached by jumper cables to a plug­board. Jumper cable settings on the plug­board (front of machine, partially hidden in this photo), in com­bi­nation with rotor and ring settings, deter­mined the code when a single letter was typed. Because all three com­po­nents worked inde­pend­ently of one another, billions of com­bi­nations were pos­sible, and these billions of com­bi­na­tions changed daily based on the setting order (cipher key) for the spinning rotors and other mov­a­ble parts. With each key­stroke creating an elec­tri­cal path through this program­ma­ble maze of hard­ware wiring, the corres­pon­ding coded (or decoded) letter lighted up on the out­put panel above the key­board, allowing the opera­tor or an assis­tant to tran­scribe the secret mes­sage. The British-captured crypto­graphic prize in late October 1942 of equip­ment along with a weather cipher book giving current key settings 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, German Army (Heer), and shore weather stations, Decem­ber 1943. An esti­mated 100,000 Enigma machines were con­structed during the war and used princi­pally by the three German armed services, the third being the Kriegs­marine. Almost to the end of the war, Germans had firm faith in the Enigma machine. Indeed, Adm. Karl Doenitz had been advised that a crypt­analytic attack on his navy’s 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 land, air, and naval forces.

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