Background to Steganography
The earliest records of the use of steganography
being used were recorded by the Greek historian Herodotus.
When the Greek tyrant Histiaeus was held prisoner by King
Darius in Susa during the 5th century BCE, he had to send
a secret message to his son-in law Aristagoras in Miletue.
Histiaeus shaved the head of a slave and tattooed a message
on his scalp. When the slave's hair had grown long he was
dispatched to Miletus. As soon as this steganography technique
was discovered people were stopped at the borders, heads
shaved and the message was revealed. Thus the technique
became useless and unreliable and a new way to send messages
had to be found.
In Ancient Rome, they used invisible ink
to write between already written texts. The invisible inks
used were substances such as fruit juices, urine and milk.
These substances would darken and become legible when heated,
allowing the message to be read.
Invisible inks were used frequently during
World War II. They also used a form of steganography involving
"null ciphers", which were unencrypted messages
used to hide real messages. For example the following message
was sent by a German spy during WWII
"Apparently neutrals protest is thoroughly
discounted and ignored. Isman hard hit. Blockade issue affects
pretext for embargo on by-product, ejecting suets and vegetable
This message at face value look innocent
and would not alert suspicion but if you were to take the
second letter of ever word the secret message would be encrypted.
" Pershing sails from NY June 1"
Another well-publicized example of steganography
occurred during the height of the Vietnam War, when Commander
Jeremiah Denton, a naval aviator who had been shot down
and captured by North Vietnamese forces, was paraded in
front of the news media as part of well-staged propaganda
event. Denton knew he would be unable to say anything critical
of his captors outright, so as he spoke to the media, he
blinked his eyes in Morse code, spelling out T-O-R-T-U-R-E.
With the advent of photography, microfilm
was created as a way to store a large amount of information
in a very small space. In both world wars, the Germans used
"microdots" to hide information, a technique which
J. Edgar Hoover called "the enemy's masterpiece of
espionage." A secret message was photographed, reduced
to the size of a printed period, then pasted into an innocuous
cover message, magazine, or newspaper. The Americans caught
on only when tipped by a double agent: "Watch out for
the dots -- lots and lots of little dots."
Modern updates to these ideas use computers
to make the hidden message even less noticeable. For example,
laser printers can adjust spacing of lines and characters
by less than 1/300th of an inch. To hide a zero, leave a
standard space, and to hide a one leave 1/300th of an inch
more than usual. Varying the spacing over an entire document
can hide a short binary message that is undetectable by
the human eye. Even better, this sort of trick stands up
well to repeated photocopying.
Steganography, as the above examples demonstrate,
is not limited to one particular medium or technology --
it's simply a matter of disguising a covert message within
an overt one, whether that overt message is an ancient wax
tablet, a telegram or a person speaking through a television
broadcast. So it should come as no surprise that the technique
has also found its way onto the Internet. In fact, steganography
tools are freely available for public use. Steganography
software allows users to secretly incorporate data into
various digital media - text, jpg images, MP3 audio files,
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