How to catch a spy that is using a numbers station – The KGB Experience

  - How do you know the KGB is surveilling you?
  - A new wardrobe has just appeared in your apartment.

Talk about power though, imagine getting a knock on your door and being told "you're moving apartments, and don't ask why or who we are.".

https://www.google.com/maps/@55.7272732,37.5479753,3a,75y,15...

(unfortunately nowhere near close enough to see if the sign is still there)

In SpyCatcher, Peter Wright describes contemplating a similar system to catch spies removing sensitive documents from MI5. From the book:

"I was asked if there was any technical way we could prove Vassall was removing documents from the Admiralty. I had been experimenting for some time with Frank Morgan on a scheme to mark classified documents using minute quantities of radioactive material. The idea was to place a Geiger counter at the entrance of the building where the suspected spy was operating so that we could detect if any marked documents were being removed. We tried this with Vassall, but it was not a success. There were too many exits in the Admiralty for us to be sure we were covering the one which Vassall used, and the Geiger counter readings were often distorted by luminous wristwatches and the like. Eventually the scheme was scrapped when fears about the risks of exposing people to radiation were raised by the management"

Peter Wright also described a radioactive agent for discovering secret writing, which may be similar to how the secret writing that tipped them off was detected on the letters to the embassy to begin with:

"The techniques of secret writing are the same the world over. First the spy writes his cover letter. Then he writes the secret message on top, using a special sheet of carbon paper treated with a colorless chemical. Tiny particles of the chemical are transferred to the letter, which can then be developed by the recipient. Most developing agents make the chemical traces grow, so that the message becomes legible, and unless the correct agent is known, the message remains undetectable. But Morgan created a universal developing agent, using radioactivity, which transformed the possibilities of detection."

I had been joking about www.duckdascism.gov in https://news.ycombinator.com/item?id=24458630 but https://news.ycombinator.com/item?id=24526075 has people who've used that channel. (for the record, ~64kbps is more than most people had over dialup in the early days of the web)

  After release he demanded compensation from the US embassy, but was denied as a non citizen.

Wouldn't a MQTT station be lacking one of the major advantages of a radio number station - that it is very difficult to know who is listening to the broadcast.

Using weather reports (or obituaries in newspapers etc) are not number stations, though they can be used as covert channels.