On this day in 1936 Alan Turing published his famous paper On Computable Numbers, with an Application to the Entscheidungsproblem. Today Turing is widely considered the father of modern computers and the field of cryptography during WWII. He worked at the Bletchley Park research complex in England, which while renowned today was kept a closely guarded secret during the war. Code breakers at Bletchley Park worked furiously to crack Nazi codes and help the Allies intercept enemy communications. Turing was the most influential of the Bletchley park codebreakers, revolutionizing modern cryptography in the process.
In order to make code breaking on a massive scale more feasible Turing worked to develop automated mathematics, what would have at the time been called computing. For part of its history NASA employed women with the job title of “computer” to solve orbital mechanics calculations. In Turing’s age, the idea that he could invent a machine to break codes was revolutionary. The German codes at this time were not very complicated, relying on a simple cypher design based on the “Caesar Wheel”. Each letter was equated to another letter in the alphabet, usually encoded by two wheels one within the other, each printed with the alphabet. Spin the wheel to any random position, matching each letter to another letter in the alphabet, and you would have a code. German decoder devices, known as enigma machines, required users on both ends to input the same three letter code, syncing the two code wheels. Three letters is a small key but with three 26-value variables there are 17576 possibilities. German forces would change the codes every 24 hours, making it a nearly Sisyphean task for the codebreakers to find the right combination before midnight.
One of Turing’s first great innovations was the Bombe, a device that could automate the process of breaking the enigma codes. The Bombe was not a full computer, but automating millions of repetitive tasks very quickly is exactly what computers are good at. The Bombe made code cracking infinitely easier and turned Bletchley park into England’s secret weapon, allowing the Allies to consistently predict Nazi military strategies. You can check out simulations of both the enigma machines and the Bombe at https://www.101computing.net/turing-welchman-bombe-simulator/. However, that still was not the heart of Turing’s 1936 paper. The Bombe was a machine that could compute one task, in mathematical terms one function or equation. A universal Turing machine, what Turing wrote about, was a device that could calculate any function, taking any input and making arbitrary modifications to it. That idea, and the theories, that surrounded it, formed the basis of modern computers, smart phones, and the internet. In a distant way, Turing’s work made it possible for us to write this article and for you to read it.