What happens to secret codes when a nation comes under control of another? I once mentioned Stephan (2022) about German Reunification in 1990. Now I learned about France under control of Nazi Germany in André Falut, "Encrypted official telegrams of the Vichy government" (HistoCrypt 2026).
A victory of Germany early in WWII led to an armistice in June 1940, whereby a major part of France was occupied by Germany. A French government in Vichy was allowed to govern the Zone libre south to the demarcation line.
In wireless telegraphy (télégraphie sans fil, "TSF"), the French officials were required to use encryption methods that the German authorities can decrypt. Telegrams sent via cable were not subject to this condition, unless they crossed the demarcation line (p.156).
In July, a copy of Code des Colonies (and later, keys used with the code) were handed over. When the German authorities could not decrypt a transmission, the French had to provide the decryption with an explanation on the encryption method. (p.157)
Occasionally, it was still desired to use an undisclosed code. At one time, a telegram from Rabat (Morocco) to Dakar (Senegal) was to be sent by the Casablanca-Dakar cable but an interruption made it ncessary to send it wirelessly. Since French stations were not allowed, it was sent via Berne and Ongar (England). (p.157) Some extant telegrams bear a note to make sure to send them via cable. (p.158)
The request from the French to allow undisclosed methods in diplomatic transmissions seems to have been granted by October 1940, but a similar request for Vichy officials in their trips to Paris seems to have been rejected in April 1941. (p.157)
The semblance of independence of the Vichy government came to an end in November 1942, when the Germans invaded the south following the Allie's invasion of French North Africa. After this, the number of Vichy officials receiving encrypted telegrams was significantly reduced. (p.159)
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