Hidden in Plain Sight
By Al Williams
I've been accused of having strange dreams, and I suppose that's true. One night I dreamt of sending secret data by taking advantage of a subtle drift in the signals between airplane transponders. To outward appearances, the signals were ordinary, but if you knew where to look, there was a message. When I woke up, I was intrigued by the idea, so I did some research.
As it turns out, I was 7,000 years too late to file a patent application. The Greek Histiaeus wrote secret messages on the shaved head of a slave, and dispatched the slave after his hair grew back. This sort of code writing is known as steganography and it is in use to this day. (Well, maybe not the part about the slave's shaved head.)
Today, computers handle most of the work involved in steganography. Think of all the files that fly across the Internet every day. If you see an email message that starts with BEGIN PGP, you can guess it was encrypted with the Pretty Good Privacy (PGP) program. This is a good sign that the sender and the recipient are exchanging information that they wish to keep private. However, if that email message had contained a picture of a car, you probably wouldn't have suspected that it contained any private information. It's possible, though, that the sender and recipient were transmitting the same private information in the picture of the car as they were in the PGP-encrypted email.
Steganography programs can hide data inside a picture so that the picture appears the same (or nearly the same) to the naked eye, but contains data if you know where to look.
