The Yellow network

Keys:

• April 11, 1940 traffic

Introduction

The Germans decided to pre-empt a Franco-British operation in Scandinavia of which they had heard and whose objective was to cut off the iron supply of Germany. On April 9, 1940, they invaded Denmark and then landed in Norway. A Franco-British expeditionary force landed in Norway on April 14 and 17. After short successes, the allies were repulsed by the Germans and had to re-embark in early May. The government and the king of Norway took refuge in England.

For their Nordic campaign, the Germans created a new Enigma network associated with all of their armed forces. The English baptized it with a new color: yellow. Unlike the first deciphered messages, the yellow traffic was bulky, making it easy to use the Netz (Zygalski’s sheets). Furthermore, its content was militarily interesting: it talked about the operations in progress.

At first, all messages decrypted at BP were only sent to the War Office in London. Then, they were retransmitted to the theater of operation with significant delay while trying to hide their origin. The allies then invented a fictitious secret agent whose code name was Boniface and who was supposedly the origin of the information given. Many officers in the field did not credit this source. Indeed, all these decipherments were a complete surprise for the GC&CS and Whitehall, who did not know how to manage and disseminate them. Hut 3, at the moment, only translated the decipherments coming from Hut 6, and many abbreviations were incomprehensible. Likewise, the For Peer Review Only Winterbotham SLUs were not yet in place; they would solve the problems of security and direct delivery of messages to those concerned under the term "Ultra".