Stardate 20011006.0012 (On Screen): There is what is known as an "open code". During the later years of WWII, the BBC would broadcast its news and then end with a sequence of strange phrases, each of which would be repeated once. It sounded like a conversation with a schizophrenic, with phrases like "Please take me to the garden. Wound my heart with monotonous langour." That latter one (or its French equivalent) is probably the most famous of the lot; it was broadcast on June 4, 1944, and what it meant was "Invasion will begin within 48 hours." Most of the phrases read over the BBC actually meant nothing whatever, but a lot of them were messages to the French Resistance; for example, one phrase might mean "Airdrop of supplies tonight at the usual place".
Open codes are particularly difficult to prevent, because nearly anything could be an open code. Radio stations in the US were forbidden during the war from accepting requests to play specific songs, because having a specific song played on a radio station might be a signal from a German agent in the US to a waiting U-boat that meant "Convoy leaving now".
This article again speculates that Taleban operatives may be receiving activation signals by 
steganography in pictures. It may be true, but if the only thing which is being sent is "Make your attack now" then nothing that elaborate is needed. The Internet is uniquely susceptible to open codes. For example, "When my weblog has an article in it lauding the Stones, that's your signal to go." But if you're in love with the idea of using a picture, there's a much easier way to use such a thing: use its "last changed date". Every html file or picture has a date associated with it, and the browser can retrieve that; it uses that to decide whether to use its cached version of the file or to retrieve it again. If you open your browser cache, you can look and see the change dates for all the files it's retrieved. So if someone wanted to send several open-code messages, the way to do it would be to create a web page which was graphics-busy, and to assign one of those pictures to each signal. Each time the change-date on that file is refreshed, the signal is sent. There is no way to defend against this; in fact, there's no easy way to even detect it, with the fantastically large number of sites out there which are constantly changing. Why would anyone screw with steganography for such messages? (discuss)
