Back to Basics
by Kirk Kirksey
The Art of the Secret Message
Steganography, the ancient method of hiding messages in
ordinary places, is still being used in the computer age; by hiding information
in common files.
The ancient historian Herodotus tells the, story of
Demartus, who warned his Greek compatriots of a coming invasion by hiding a
secret message on something very common—wax tablets. Demartus avoided
detection by scraping off the wax, writing his message on the underlying wood,
and then covering the tablets with fresh wax.
The tablets appeared blank and were never questioned as they
passed through enemy lines. The story of Demartus warning may be the first
recorded example of steganography—hiding a message in something ordinary
and easily overlooked.
Steganography has tamed up in some mighty odd places.
Messages have been tattooed on shaved heads. After the hair grew back, the
message, was hidden, and could be read only after a courier had an appointment
with the barber. Invisible inks are also a form of steganography. Spies have
used milk, vinegar, fruit juices and urine to record messages to their
compatriots. These substances disappeared as they dried, but darkened again
when subjected to heat.
A 'more modem example of steganography involves hiding a
secret message in text. Here's a real example taken from a message sent by a
German spy. in World War II. The original message was:
Apparently neutral's protest is thoroughly discounted and
ignored Isman hard hit. Blockade issue affects pretext for embargo on
byproducts, ejecting suets and vegetable oils.
This was the real message:
Pershing sails from NY June 1.
The technique is called "word shifting." Take the second
letter of each word in the original text, and you can easily see how the
message was hidden.
With the speed and sophistication of today's computers,
techniques, like word shifting can be easily detected. Have modem computers
done away with steganography? Not by a long shot. Take another look at that
GIF, or listen carefully to your favorite WAV file. Everything looks fine? The
music sounds good? Perhaps. Then again, there could be a secret message, right
there under your nose. No—steganography is alive and well in the computer
age. Here's a bit about how it works, and how you can use it.
Encryption vs. steganography
Encryption and steganography are easily confused. Encryption
is the art, of scrambling a message so that it becomes unreadable. Before
computers, both the sender and receiver needed the same secret code to
unincrypted the message. Young boys in the I 930s commonly used the. Captain
Marvel decoder ring to send and decode secret messages. This is called
symmetric encryption. When computers came along, symmetric encryption was not
practical.
This problem led to the development of asymmetric encryption.
For asymmetric encryption, the sender needs one type of secret code to scramble
the message, and the reader needs another key to decode the message. The
scrambler code is called the "public key," and can be used by anyone wishing to
encrypt a digital message intended for a particular individual. The message can
be unscrambled only by using a "private key," which is known only to the
receiver.
Steganography is not message-scrambling or encryption. In the
computer age, steganography is the art of hiding a message in a common file.
This file, when accessed by an unsuspecting user, may look or sound innocent.,
The real meaning emerges only after processing the file with special software.
This message may be "human readable" or encrypted. Believe it or not, common
hiding places for steganographic messages are innocent looking GIF, JPEG, and
WAV files, spreadsheets, and even word processing files.
Steganography methods
There are several methods used by steganography software to
hide messages. Take the electronic version of this article, for example. When
my editor loaded the word processing file into her computer and accessed the
file, she was looking at the same words you are—plus all the bad grammar
she had to correct. To the human eye,' the white space between words looks
normal. However, steganography software could embed a secret message by
slightly altering the space between words. This difference would go unnoticed
by the casual reader. When processed, however, the variations in word spacing
could be translated into a secret message.
GIF picture files are one of the most common hiding places
for secret messages. This technique uses something called the LSB (Least
Significant Bit) method. Here's' how it works. All colors in a GIF image are
derived using the RGB (red, green, blue) palette. When a GIF image is loaded,
every' screen pixel is assigned three RGB values. The combination of the three
values determines the color of the particular pixel. For most GIF image's on
the Web today, the three RGB values are held in an 8-bit number (a combination
of just eight 1s and 0s). Under this scheme, 256 colors are possible because
the maximum 'value of an 8-bit binary number is 256. Here are some examples of
8-bit binary numbers and their decimal equivalents:
00000000 0
00001101 = 13
01010100 =84
11111111=255
The right bit or. number in the binary number is called the
LSB. In our example, the LSB of 00000000 (decimal 0) is 0. The LSB of 00001101
(decimal 13) is 1. Changing the LSB will have the smallest effect on the binary
number. Altering the exact value of a binary number may be critical in precise
mathematical calculations, but it is not so important when it comes to
displaying a GIF image on a Web page. The reason is simple. When viewing an
image (any image), the human eye is not capable of detecting very minute
changes in color. This is especially true considering light and intensity
variations caused by typical computer screens.
It's easy to see how steganographic software can use the LSB
to hide messages in a GIF file. First, the secret message is converted to' its
binary form. Then the GIF image is processed. For the RGB value of each pixel,
the true LSB is replaced with a binary digit from the secret message. Color
changes caused by this bit-fiddling cannot be detected 'when the image is
viewed. The message is decoded by processing the GIF file, and extracting the
LSB from each pixel. Unlike Humpty Dumpty, steganographic software reassembles
the bits, and voila—the secret message is revealed.
Want to try your hand at a little steganography? Try Stego at
www.stego.com. It's one of the best
steganography products around and—best of all—it's free! Happy
hiding.
Did you know...?
Spanish scientists in Belize have discovered a fungus capable
of eating the metal surface of compact discs.
Contact Kirk at:
[email protected]
ComputerScene March 1,
2002 |
