Moscow communicated with a ring of alleged spies in America by encoding instructions in otherwise innocent-looking images on public websites. It’s a process called steganography. And it’s one of a slew of high-tech and time-tested methods that the deep-cover agents and their Russian handlers used to pass information — from private Wi-Fi networks to buried paper bags.

Steganography is simultaneously one of the oldest methods for secret communications, and one of the more advanced. The process dates back to the fifth century B.C., when the Greek tyrant Histiaeus shaved the head of one of his servants, tattooed a message on his head, and waited for his hair to grow back before sending the messenger out. When the courier arrived, his head was shaved and the missive was read, giving information about upcoming Persian attacks. Later on, secret inks were used on couriers’ backs. Morse code messages were woven into a sweater that was worn by a courier.

As information went digital, steganography changed. Messages could be hidden in the 1s and 0s of electronic files — pictures, audio, video, executables, whatever. The hidden communications could even be slowly dribbled into the torrent of IP traffic. Compression schemes — like JPEG for images or MP3 for audio — introduce errors into the files, making a message even easier to hide. New colors or tones can be subtly added or removed, to cover up for the changes. According to the FBI, the image above contains a hidden map of the Burlington, Vermont, airport.

Both before and after Sept. 11, there were rumors in the media that al-Qaida had begun hiding messages in digital porn. That speculation was never confirmed, as far as I can tell.

That's exactly what I thought, Prof!

"crikey we're going to get attacked - I had best tattoo this guy then wait a few weeks for his hair to grow back"

Sounds like suicide to me.

then you have the time to wait for hair to grow back and for your messenger to travel.

But as armies moved on foot, there were no air craft or missiles, you could do things a lot more slowly. If it takes an army 8 weeks to get to you, because they are walking, then you have the time to wait for hair to grow back and for your messenger to travel.

But you will still receive the message a few weeks later than you otherwise would, regardless of how long it takes an army to approach.

What, a little more limited?

You know the saying

"The more you learn about something, the more you realise how little you know"

Enough metaphysical tomfoolery, the important fact is the spy is actually quite hot.

Enough metaphysical tomfoolery, the important fact is the spy is actually quite hot.