The thought to be 'uncrackable' Enigma code (transmitted by Enigma machines) was made by a German in World War 2 called Arthur Scherbius. It helped the Germans transmit messages without other countries (especially enemies) knowing what they were saying so that they could talk about war and who they would attack. It was important as well as that so that messages could be sent to the German air force and more importantly the U-boat fleet.
The Polish were the first to start decrypting it and find out the code and how it was used. They were so nearly there when the Germans attacked Poland. They found them and tortured most of them or killed them.
Britain persuaded a Polish worker who worked in a factory that made enigma machines to send one over via the Polish Intelligence. Once Britain got their hands on one, they got to work on decrypting the code and how they used it. They made this machine called the "Bronze Goddess" that finally worked it out!
Although everyone thinks that Britain decrypted it,there are many doubts. Some Poles believe that the Polish found out first. No one knows for sure as the Polish may have kept it from Great Britain before the Germans invaded, or maybe the Germans invaded just in time so that the Polish never found it out.