Intelligence agencies are adept at transmitting information in plain sight. I already mentioned numbers stations [1] in another comment, but they're a perfect example of how intelligence data is broadcast through a easily intercept-able channel but totally unusable to anyone except the recipient due to the use of a one-time pad. The only way you can stop them is to jam them, which is easier said than done.
Intelligence agencies use the simplest systems of transmitting data because they're proven to work. Dead drops are another great example of what on the surface seems horrifically insecure (you want me to leave my intelligence in a public place for anyone to find?) but that in practice work extremely well.
Intelligence agencies use the simplest systems of transmitting data because they're proven to work. Dead drops are another great example of what on the surface seems horrifically insecure (you want me to leave my intelligence in a public place for anyone to find?) but that in practice work extremely well.
[1]: http://en.wikipedia.org/wiki/Numbers_station