[imglorp]

> Looking back at the incident now, one can see that the French semaphore system was hopelessly insecure,...

Actually, this would have been an ideal application of one-time-pads for end-to-end encryption of the sensitive information, needing no modern tech beyond pencil and paper. The pads could have been passed by hand between participants and would have been highly secure.

[Udik]

It seems that messages were actually "encrypted" and "compressed" with a shared code book. But encryption wouldn't have changed anything in this particular case: nobody was altering or decoding the authentic messages, they were just piggybacking on the error correction codes to insert more information. Very clever.