Here’s what you’ll learn when you read this story:

• The mechanism known as the Bombe was England’s answer to Germany’s Enigma encryption machine.
• Bombe electrical data plus human clues allowed Alan Turing and others to crack many Enigma messages.
• In cryptography, the more length, complexity, and randomness you can add to a cypher, the harder it is to crack.

This story is a collaboration with Biography.com

As the Nazi party rose to power in Germany, the German military made significant use of the commercial Enigma cipher device, which went on sale beginning in 1923. To make it more secure, they modified the out-of-the-box Enigma machine, adding extra settings and widgets that made their version significantly more complex. In cryptography, longer and more confusing strings typically take much longer to decipher (this is also why longer passwords are considered more secure for the most part).

Fundamentally, the Enigma took each letter of the Latin alphabet and assigned it to a different letter. This is much more complex than a Caesar cipher, wherein each letter is “moved” the same number of letters away (A becomes E, B becomes F, C becomes G, and so forth). The Germans also chose a new key each day, which further scrambled the messages and made them incredibly difficult to decode. After all, even if you cracked one message, you’d only know the key for that day. The next day, you’d be back to square one.

The interceptable and guessable key is one of the biggest weaknesses of any encryption system, and is the exact point that was eventually taken advantage of by the codebreaking Bombe. A specialized electrical device with circular drums that are accessed by a searching mechanism, the Bombe sought to break Nazi encryption every day during World War II.

What’s a Bombe?

The Bombe, technically, followed directly from the Polish bomba—a machine built specifically to help decode German encryptions. Three Polish mathematicians first cracked the commercial Enigma’s code using the bomba, and mathematician Alan Turing brought this work into his project to continue breaking the supplemented Nazi Enigma machines.

The bomba and Bombe are both electrical machines that, on a mechanical level, “listen” for electrical connections that indicate the physical links between letters on a particular day. This information gave codebreakers a foothold as they scoured messages for patterns, context clues, and signs of that day’s scrambling key.

The Human Element

The greatest risk to any cryptographic system (besides the necessary presence of a key) is the people who must use it, and that included the Nazis. Their use of a daily key, combined with a repeated sequence that Turing and his colleagues could often pick out of individual messages, meant that the codebreakers at Bletchley Park were often able to decipher the messages narrating the upcoming movements of Nazi troops and machines.

Even with the use of the Bombe, there are still Enigma messages that were never decrypted—mostly from the German Navy, who took greater care to encrypt with greater complexity than other segments of the German military. For instance, while the German Army and Air Force used an Enigma with five wheels of scrambling patterns, the Navy’s had eight wheels. Those few extra steps added a ton of complexity. For perspective, a five-character password would take “seconds to minutes” to crack today, according to password professionals, Bitwarden; an eight-character password would still take “hours to weeks.”

The story of the Enigma and the Bombe is ultimately one of an arms race between complexity and ingenuity. Every extra rotor the German Navy added bought them time, and every pattern that a Bletchley Park codebreaker spotted clawed it back. Today, whether you’re setting a password or designing an encryption protocol, the same principles apply: length, randomness, and complexity are your allies, while predictability and human habit are your enemies.

Hopefully, whatever you’re hoping to keep private will do a lot less damage than the Nazi army.