But in recent weeks, US officials have made a startling admission: the key intelligence that prompted the security alert was seriously flawed. CIA analysts believed they had detected hidden terrorist messages in al-Jazeera television broadcasts that identified flights and buildings as targets. In fact, what they had seen were the equivalent of faces in clouds -- random patterns all too easily over-interpreted.

At the heart of the fiasco lies a technique called steganography, the art, and now hardcore science, of hiding messages. There's nothing new about steganography, in principle at least. Herodotus tells the tale of Histiaeus who, in the sixth century BC, shaved the head of his most trusted slave, tattooed a message on his scalp and let his hair regrow. The slave then travelled unchallenged to Aristagoras, who was instructed to shave the slave's head, revealing the message urging him to revolt against the Persians. In common with modern steganography, it ensured that outsiders didn't know a secret message existed.

But to experts, the idea al-Qaeda would be passing steganographic messages through TV broadcasts is ludicrous. ...