How Alan Turing and His Fellow Codebreakers Broke the Nazis’ “Unbreakable” Enigma Code

There was a time when you could just go out and buy an Enig­ma machine. In the ear­ly nine­teen-twen­ties, any orga­ni­za­tion with a need to keep its com­mu­ni­ca­tions secret could order a few of those brand-new devices from Ger­many and rest assured that any mes­sages encod­ed using them would be safe from pry­ing eyes. It did­n’t take long for the mil­i­tary to catch on to the Enig­ma’s use­ful­ness. Just a few years after the machine came on the mar­ket, it had been adopt­ed by the Ger­man Navy, Army, and Air Force, and made pos­si­ble the rapid, secret coor­di­na­tion cen­tral to the tech­nique we now call blitzkrieg. When World War II began, Axis forces were con­fi­dent, as any­one in pos­ses­sion of an unbreak­able code might well be.

As his­to­ry played out, of course, that code turned out to be break­able. That this came as a sur­prise to the Nazis makes sense if you watch the new video above from Ver­i­ta­si­um, which opens up an actu­al Enig­ma machine and explains its inner work­ings. Each let­ter typed into its key­board pass­es through a series of inter­change­able rotors wired togeth­er, then comes back through in the oppo­site direc­tion, result­ing in a par­tic­u­lar sub­sti­tute let­ter.

Each char­ac­ter entered advances at least one of the rotors, each of which turns at a dif­fer­ent speed, mean­ing that the already com­plex sub­sti­tu­tion cipher con­stant­ly changes. To make it even trick­i­er, the Nazis made mod­i­fi­ca­tions to their Enig­ma machines, includ­ing a tele­phone-oper­a­tor-style plug­board that expand­ed the pos­si­ble encryp­tion keys to 7 x 10¹⁸: an infi­nite-seem­ing num­ber that fair­ly cried out for the math­e­mat­i­cal ser­vices of Alan Tur­ing.

Already famous for hav­ing come up with the “Tur­ing machine,” which could the­o­ret­i­cal­ly manip­u­late strings of sym­bols in what we’d now call an algo­rith­mic fash­ion, he was recruit­ed by Britain’s wartime code­break­ing head­quar­ters at Bletch­ley Park, where experts were already hard at work reverse-engi­neer­ing the Enig­ma machine. There he con­sid­ered flaws in the design of the device, such as its inabil­i­ty to encrypt any giv­en let­ter as itself, and the ways in which its human oper­a­tors used it, such as mak­ing pre­dictable weath­er reports. Exploit­ing these weak­ness­es required build­ing a non-the­o­ret­i­cal com­pu­ta­tion engine, the Bombe, to grind through all pos­si­bil­i­ties at what would then have seemed an unfath­omable speed. The intel­li­gence the Bombe pro­duced con­tributed to the suc­cess of the sur­prise D‑Day inva­sion and may have tak­en years off the poten­tial rav­ages of World War II. It would­n’t have hap­pened with­out Tur­ing’s for­mi­da­ble intel­li­gence — or his under­stand­ing that you need a machine to out­smart a machine.

Relat­ed Con­tent:

The Enig­ma Machine: How Alan Tur­ing Helped Break the Unbreak­able Nazi Code

How British Code­break­ers Built the First Elec­tron­ic Com­put­er

The LEGO Tur­ing Machine Gives a Quick Primer on How Your Com­put­er Works

A Tur­ing Machine Hand­made Out of Wood

Can You Crack the Uncrack­able Code in Kryp­tos, the CIA’s Work of Pub­lic Art?

The Sto­ry of Elize­beth Fried­man, the Pio­neer­ing Cryp­tol­o­gist Who Thwart­ed the Nazis & Got Burned by J. Edgar Hoover

Based in Seoul, Col­in Marshall writes and broad­casts on cities, lan­guage, and cul­ture. He’s the author of the newslet­ter Books on Cities as well as the books 한국 요약 금지 (No Sum­ma­riz­ing Korea) and Kore­an Newtro. Fol­low him on the social net­work for­mer­ly known as Twit­ter at @colinmarshall.