They called her Elizabeth Thorpe. And Betty Pack. And Cynthia. She was an American, working for the British, and sometimes the Spanish, although the French and Polish thought she was theirs for a while. She came from an upstanding family, a proper girl who was thoroughly improper. The biggest enigma, though, was the Nazi coding machine she proved instrumental in helping the Allies crack.

The 1928 patent for Scherbius’ Enigma machine.

Developed by the Germans at the end of World War I, the Enigma machine was the most complex ciphering machine of its time, and small enough to be transported to battlefield locations. It was invented by German engineer Arthur Scherbius, and patented by his firm in 1918. Its inscrutability made it impossible for the Allies to know plans and movements of the German Army.

Enigma worked through rotors and electrical pulses to create a constantly changing code. It operated on the basic premise of cryptograms found in puzzle books, in which each letter of the alphabet corresponds to another letter. Unlike most codes, though, which are based on a rule (for example, three letters after the intended letter is substituted), Enigma changed the code with each press of a letter key, so that the substitution letter was different for each character in the message. As long as the receiving machine had its rotors set in the same pattern, it would decode the message being sent. But rotors weren’t the only methods of encryption: both machines had to have starting and ending positions identical, as well as ring positions and plugboard connections. These settings were changed each day and distributed. No longer could radio messages be intercepted by the enemy.

Betty Thorpe, codename Cynthia

There was a flaw to the Enigma machine, however: a letter would never be represented as itself. Through the process of elimination, Allies were able to break the code.

None of that would have happened, though, without Thorpe. The Poles were the first to break the code, led by mathematician Marian Rejewski. Thorpe was assigned by the British to seduce Polish Count Michal Lubienski, the chief aide to Poland’s foreign minister. From him, she learned Poland had cracked the code, delivering this information to the British, who then persuaded Poland to share the information.

The Polish code-cracking didn’t last long, however; the flaw in the system that gave the Poles entry was fixed by the Germans in May of 1940. It took the secret British code-breaking group Ultra, led by renowned scientist Alan Turing, to break through once again, unlocking the movements of the German air force and army. Still, though, the German navy remained inaccessible, and German U-boats continued to wreak havoc on Allied movements at sea – causing not only loss of life, but massive food shortages in Britain.

It wasn’t until June 1941 that Turing and Ultra broke through the naval encryptions. Once they did, they were able to decrypt thousands of German transmissions each day. Those decryptions seldom left Bletchley Park; rather, intelligence reports were prepared there, allowing the Allies to anticipate German strategy and movement, and turn the tide of the war.