> "I'm sure that even before LLMs one could find a way to create a story where certain numbers / code words would be embedded without altering the underlying story too much."

It's called steganography, and it's a centuries if not millennia old technique.

Seems to me like coordinating with an entity outside of the spooks' control, such as the BBC, would give more opportunities for leaks. It would also reveal some information about who is controlling the signal--someone with some kind of relationship with the broadcaster.

who's to say they aren't doing both? They may not even be sending anything over the number station; these stations will continue on a schedule even when there is nothing to say and nobody is listening because it makes it harder to eek out a foothold in the event of a weakness in the encryption.

Shortwave propagates better and also its just a one time pad being distributed so embedding doesn't matter as much as long as the one time pad is longer than the intended message to send. There is no way to decrypt it because once you encrypt a message using a one time pad it is impossible to decrypt without the exact one time pad that it was encrypted with.

I think you're massively overestimating the amount of control the US has over news broadcasters.

The previous time that the US and UK overthrew Iran's government (https://en.wikipedia.org/wiki/1953_Iranian_coup_d'%C3%A9tat), they used the BBC in that way.

  Roosevelt told the Shah that he was in Iran on behalf of the American and British secret services, and that this would be confirmed by a code word the Shah would be able to hear on the BBC the next night. Churchill had arranged that the BBC would end its broadcast day by saying not 'it is now midnight' as usual, but 'it is now exactly midnight'

I can't find it immediately, but I've read about something even sneakier than this. A standard broadcast station was modified such that its carrier signal was modulated by a PSK signal. The intended listener would use e.g., a PSK-31 modem to listen to the carrier signal and would be able to obtain the encoded digital data. Everyday listeners would hear the regular broadcast. The station involved _might_ have been a BBC station, but I don't recall.

I think they do this, too.

However, the numbers stations transmissions are never a big secret. They're intentionally powerful so someone can pick them up on simple equipment without raising suspicion. A person can modify an off-the-shelf AM radio to pick up shortwave, for example, even in an oppressive regime.

It's a one-time pad, so the encryption is unbreakable.

regular AM/FM stations are not broadcasting on shortwave bands

[dead]