Nothing seemed to work. The Allied codebreakers tried every possible trick and combination, but these new ciphers defied all attempts at decryption. On February 1, 1942, British analysts discovered they could no longer read intercepted radio communications between German U-boat captains and their commanding officer, Vice Admiral Karl Dönitz. Overnight, a vital part of the United Kingdom’s super-secret cryptanalytic program—Ultra—had been plunged into darkness.

This intelligence blackout could not have come at a worse time for the Allies. Already, a handful of aggressive Nazi submarines were wreaking havoc all along North American shipping lanes. Dönitz’s Operation Drumbeat would result in 216 merchantmen sunk off the U.S. East Coast during the first three months of 1942 alone. American countermeasures all proved hopelessly inadequate.

Something needed to be done about Admiral Dönitz’s deadly underwater predators before they completely choked off the supplies of food, munitions, and fuel needed to keep Britain in the war. It was a daunting task; defeating the U-boats meant conducting a maximum coordinated effort among the air, sea, and intelligence services of two distrustful allies. Could the United Kingdom and the United States put aside their suspicions and join in a common endeavor to penetrate the enemy’s new communications procedures?

During the 1920s and 1930s, Germany developed a portable, easy-to-operate cipher machine known by its brand name Enigma. All three branches of Hitler’s armed services utilized it to encrypt tactical communications throughout World War II. Many German civil agencies also adopted this mechanism; as a result, hundreds of thousands were built.

Weighing 23 pounds, the device resembled an oversized typewriter with a miniature telephone switchboard on its surface. Electrically powered, it also featured a 26-character light panel. Three (later four) cylindrical metal rotors fit inside a basket behind the light panel. Each rotor contained a distinct web of wiring contacts and had to be loaded into the basket in a specific order. The Enigma came with five differently wired rotors, although by 1940 those on Dönitz’s U-boats started carrying eight for additional security.

An instruction sheet called the key list told code clerks which rotors to use on any given day (operators routinely changed their settings at midnight). This list also prescribed how to arrange the rotors, to include a moveable placement notch on the outer ring of every cylinder. Lastly, the key list provided daily configurations for Enigma’s plugboard, which the Germans termed a “stecker.”

To encrypt text, Enigma employed an innovative electromechanical character substitution process. Once the operator had his apparatus set up properly, he began by pressing a key. This sent an electrical impulse through the stecker, changing its path from one letter value to another. From there, the current passed across three or four rotors. Each rotor advanced once in turn, rerouting the electric signal several times before that impulse reached a reflecting plate. This plate diverted the current once again before returning it through the rotors and stecker. Finally, a letter would flash on the light panel, which the operator would then write down.

By entering content in this fashion, German code clerks could rapidly transform important military communications into a series of seemingly meaningless letters. Deciphering an encrypted message was as simple as typing those letters and noting what lit up on the lampboard providing the recipient had a correctly configured Enigma.

Its cryptographic power was staggering. According to mathematician Dan Fleisch, one three-rotor Enigma could generate a number of possible letter combinations equaling 10 to the 114th power—or more than the number of atoms in the known universe. A four-rotor version, like the type found on German U-boats starting in 1942, produced combinations exceeding 10 to the 145th power.

British intelligence expert Stuart Milner-Barry described the Nazis’ confidence in their capable new enciphering device: “The Germans regarded the Enigma as a perfectly secure machine, proof against cryptanalysts however talented and ingenious they may be.” Yet already a group of brilliant codebreakers was learning how to crack this supposedly invincible system.

Beginning in 1932, three theoreticians with the Polish Cipher Bureau developed a mathematical equation that approximated the wiring connections of an Enigma rotor. Meanwhile, French spies had obtained other cryptographic information from a German turncoat, which they passed on to the Poles. By 1938, Poland had succeeded in constructing a mechanism that could test 17,000 possible rotor settings to help decipher intercepted radio messages. Its designers called their creation a Bomba.

No one is sure how this name came about. One story says the Poles came up with the term “Bomba” in honor of an ice cream treat they all enjoyed.

In late July 1939, just before the Nazi invasion, these Polish cryptanalysts shared their intelligence breakthrough with French and British agencies. The Poles even sent along an operating replica of the Enigma. In the United Kingdom, a Polish-built Bomba made its way to Bletchley Park, Buckinghamshire, and into the possession of a highly classified facility there known as the Government Code and Cypher School (GC&CS).

Teams of mathematicians, engineers, linguists, and military intelligence officers at GC&CS studied with astonishment what their Polish and French colleagues had provided. The British now understood Enigma’s logical foundation; furthermore, they obtained several tools that one day might enable them to penetrate their foe’s sophisticated enciphering protocol. For the time being, however, it remained a mystery just beyond the Allies’ ability to solve.

In a collection of rude huts located behind Bletchley Park’s Victorian-era manor house, some of Britain’s brightest minds began examining the problem. Two young theoreticians named Alan Turing and Gordon Welchman formulated several techniques intended to rapidly decipher Enigma messages. Welchman focused on defeating the Germans’ stecker plugboard, his “diagonal board” test dramatically reducing the number of possible combinations any decryption system would need to examine.

Alan Turing drew up an electromechanical means of proving or disproving millions of possible rotor settings. Based on but logically distinct from the Polish Bomba, Turing’s design operated in conjunction with Welchman’s diagonal board to attack Enigma’s vulnerable daily key codes. An ingenious electrical engineer named Harold “Doc” Keen transformed Turing and Welchman’s drawings into a functioning mechanism; by August 1940, the first operational “Bombe” (somewhere along the line its name changed to the French spelling) was delivered to GC&CS.

The British Bombe was a masterwork of engineering. Standing 61/2feet tall, seven feet long, and two feet wide, each unit weighed one ton. More than 200 were constructed during the war years, put to use at Bletchley Park and in a number of outstations scattered across Great Britain.

The Bombe operated like a series of Enigma machines in reverse. A series of 96 commutator wheels spinning at various speeds tested possible letter matches; if the Bombe found a “stop,” or possible solution, it would slow to a halt. An operator (often a Women’s Royal Naval Service member, or Wren) then rewound its commutators to where the Bombe stopped and wrote down that wheel position. Cryptanalysts tested this suspected key setting on a “checking device.” Whenever plain text emerged, they knew the day’s codes had been broken.

It should be remembered that the Bombe was only one part of an extraordinarily complicated system utilized by GC&CS to read enemy communications. Keys to this process were “cribs”—words or phrases likely to form a part of the intercepted message. A constant supply of good cribs was necessary to peer inside Enigma; obtaining these clues was equal part dogged persistence, inspired guesswork, and luck.

Fortunately for the Allies, German faith in Enigma’s security often led to procedural mistakes. These blunders, usually committed by lazy or bored operators, were quickly exploited by the talented analysts at Bletchley Park. One commander, for instance, invariably opened his daily reports with the salutation “Heil Hitler.” Famously, another code clerk used his girlfriend’s nickname—Cillie—for every message indicator he sent. These and other easily detectable patterns provided British codebreakers with many valuable cribs, starting points for their attacks into the foe’s ciphers.

Yet GC&CS’s most dangerous adversary was also its most security conscious. Unlike Germany’s army and air force, which could send messages by courier, teletype, or wire, the Kriegsmarine had no choice but to rely on radio communications. Admiral Dönitz regularly communicated with his U-boats via Morse code; from the war’s outset, Dönitz rightly suspected that Allied detection stations were listening in as he daily exchanged attack orders and situation reports with submarine captains operating across wide swaths of hostile ocean.

No one under Admiral Dönitz’s command got to choose his sweetheart’s name as an Enigma message indicator. Those settings were found in special codebooks known as Bigram tables, which also employed a second set of randomly generated letter pairs to doubly encrypt the initial rotor position for each message. Moreover, after February 1940, U-boats began utilizing eight rotors instead of the normal five as another layer of security.

All these precautions notwithstanding, for 18 months Bletchley Park experienced fair success in reading German naval radio traffic. Their efforts were aided immeasurably by the occasional capture of Kriegsmarine rotors, Bigram tables, and weather keys by the Royal Navy. In late 1941, for example, GC&CS knew Dönitz was about to issue new codes for his U-boats and even suspected the existence of a four-rotor Enigma.