Enigma machine was a device for encrypting messages, used by Germany from the late 1920’s through World War II (1939-1945). Encryption is a process that changes a message to disguise it. In a process called decryption, the receiver converts the disguised message back to its original form. Encrypted messages enabled German troops and spies to work and plan in secret. The Enigma machine made use of extremely complex encryption for its time. However, Allied forces were eventually able to intercept and decrypt German messages. 
Enigma machines had their roots in rotor cipher machines invented in the early 1900’s. Such machines used spinning wheels called rotors to substitute one letter for another, a type of encryption called a cipher. Each machine had a keyboard to input information, like a typewriter. Earlier models typed out the encrypted message on a piece of paper. By World War II, a lighted panel had replaced the paper printouts.
Enigma machines required electric power to operate. Some machines operated using an internal battery, making them portable. Each Enigma machine had three to four rotor slots. The rotors had wires in the middle that would connect the keyboard to the light panel. The wires were jumbled and would rotate positions with the wheel. Machine operators could also change the course of the wire by changing the positions of plugs in a plugboard on the front of the machine. The result was an electric maze that changed as the rotors turned, constantly changing the cipher alphabet.
The rightmost rotor turned one position each time a letter was typed. If the operator typed A, for example, the wheel would turn and the electric current would flow through the wire, perhaps making the letter R light up. If the operator typed A again, the wheel would turn again and might connect the current to make the letter G light up. Such encryption, in which the cipher changes with each letter typed, can be extremely tough to decipher.
Beginning in the early 1930’s, Polish forces began working to decrypt German messages. Marian Rejewski, a Polish mathematician, figured out the internal wiring of the Enigma machine. Rejewski was later joined by the mathematicians Henryk Zygalski and Jerzy Rozcki, also of Poland. By 1933, they could mimic the machine’s operation to decode German transmissions. Germany continually improved and adapted its Enigma machines. Even so, the work of Rejewski, Zygalski, and Rozcki became key to helping the Allies defeat Nazi Germany during World War II.
The mathematicians were able to pass what they knew about the encryption to British codebreakers at Bletchley Park. Bletchley Park was a British estate that had been converted into the Government Code and Cipher School. At Bletchley Park, the British cryptanalysts (codebreakers) Alan Turing, Dilly Knox, Gordon Welchman, and Stuart Milner-Barry built a machine, called the Bombe, to solve ciphers. 
The Bombe took advantage of several weaknesses in the German Enigma codes. First, the codebreakers figured out that messages might contain common words, called cribs, that could help in cracking the encryption. They also realized that the Enigma machines would always change the input letters. For example, if the letter A was entered, the machine would never output an A. The British codebreakers also began to recognize certain patterns based on when messages were sent. For example, a weather report was sent every morning. This report was always formatted the same way. This formatting enabled easier translation because the codebreakers always knew what certain words were. From there, they could work backward to figure out the code.
Some historians have suggested that World War II would have lasted years longer if not for the code-cracking work at Bletchley Park. The Enigma machine was eventually replaced with newer encryption methods. However, the machines have become popular collectibles among World War II historians and espionage enthusiasts.
