German counterintelligence services used a machine called a subtraction device to decrypt intercepted radio messages. British agents used a 5-character code, to which the same digit was added to each group of 5 digits. The intercepted groups of 5 characters were entered in the subtraction device and the digits from 0–9 were removed from them. The result was printed out on paper. The Germans knew groups of 5 that were not possible, meaning they could quickly identify the right combinations.