Steganography is the hiding of information in innocent
looking objects and is a part of cryptography. Steganos means
hidden and graffein write. Since the arrival of digital files
for image and sound, steganography has known an enormous
revival.
Hiding messages is something that already occurs since ancient
times, e.g. Herodotus tells how in the Fifth Century before Christ a
sovereign shaves the head of a slave and then had it tattooed. The
message contained a warning for Greece about the planned invasion by
Persia. When the hair grew back the information was hidden for the
outside world and could become visible again by shaving the
hair.
An other technique in the old Greece, also described by
Herodotus, was writing messages on wood, and then cover them with a
coating of wax, so that it looked like an unused bar.
From times immemorial people know ways to write messages on paper
with invisible ink.
During the Second World War the Germans and other spies used
microdots to send information. These microdots were about as big as
the dot written with a typewriter, and they contained nformation
made smaller in a photographical way.
Today several people
use the technique to protect their property or to send secret
information via the internet. E.g. by applying a watermark the
author can put his unique stamp or mark on an image or sound file.
It is also possible to send information openly via images on web
pages, and only insiders can retrieve the information.
Source & more about this subject;
wikipedia.org/Steganografie
