 The machine's three rotors were chosen from a set of eight (rather than the other services' five).The rotors were changed every other day using a system of key sheets and the message settings were different for every message and determined from "bigram tables" that were issued to operators.In 1939, it was generally believed at the British Government Code and Cypher School (GC&CS) at Bletchley Park that naval Enigma could not be broken.