Bletchley Park and "PC Bruno" worked together against the German message traffic. In the interest of security, they themselves corresponded using the supposedly "unbreakable" Enigma cipher. In early 1940, the two centres read some old Enigma messages, and in March they broke some German daily keys, and read some messages "in real time" (i.e., sometimes as soon as their intended German recipients). During the next few months, the two centres decrypted several thousand Enigma messages, about half at each centre. Some of the messages gave notice of the German invasion of Denmark and Norway and of the German invasions on Belgium, the Netherlands, and France. However, no effective Allied use was made of these warnings.