The German Navy started buying Enigma machines in 1925 and they started to modify it. The German Army soon followed suit and they also modified the machine. Some of the modifications included adding or deleting some keys, the addition of the plug board and using more than three rotors.
The Germans placed a lot of confidence in the security of the Enigma machine because they thought that the probability of breaking a message would be too great for their enemies. They did have certain procedures on the operation of the machine. The Germans had manuals that the operator used to set the parameters of the machine for each day.
Contrary to the beliefs of the Germans, the Enigma machine was not secure. In 1928, the Poles acquired the knowledge about the German military Enigma by intercepting one, in customs, being sent to the German Embassy in Warsaw and examined it. A whole series of Enigma machines was produced at the factory in Warsaw. A group of brilliant mathematics students at the Poznan university (Rejewski, Rozycki and Zygalski) was recruited to work in the cryptological section of the Polish General Staff. On December 31, 1932, they decrypted the German Enigma signals. To facilitate decryption Rejewski designed an electromechanical programmable machine which he called 'Bomba' (Polish for bomb) because of the bomb-like ticking noise it made. On July 25, 1939, the Poles gave the French and the British replicas of Polish made Enigmas together with the drawings and information on the Enigma, Bomba and the decryption information.
Special thanks to Edward Tarchalski and Lukasz Stepien for parts of this page.