2 cissp ® Official Study Guide Eighth Edition

 Steganography and Watermarking 
Steganography
is the art of using cryptographic techniques to embed secret messages 
within another message. Steganographic algorithms work by making alterations to the least 

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Chapter 7 
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PKI and Cryptographic Applications
significant bits of the many bits that make up image files. The changes are so minor that 
there is no appreciable effect on the viewed image. This technique allows communicating 
parties to hide messages in plain sight—for example, they might embed a secret message 
within an illustration on an otherwise innocent web page.
Steganographers often embed their secret messages within images or WAV files because 
these files are often so large that the secret message would easily be missed by even the 
most observant inspector. Steganography techniques are often used for illegal or question-
able activities, such as espionage and child pornography.
Steganography can also be used for legitimate purposes, however. Adding digital water-
marks to documents to protect intellectual property is accomplished by means of steganog-
raphy. The hidden information is known only to the file’s creator. If someone later creates 
an unauthorized copy of the content, the watermark can be used to detect the copy and (if 
uniquely watermarked files are provided to each original recipient) trace the offending copy 
back to the source.
Steganography is an extremely simple technology to use, with free tools openly avail-
able on the internet. Figure 7.2 shows the entire interface of one such tool, iSteg. It simply 
requires that you specify a text file containing your secret message and an image file that 
you wish to use to hide the message. Figure 7.3 shows an example of a picture with an 
embedded secret message; the message is impossible to detect with the human eye.
F I g u r e 7. 2
Steganography tool

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Applied Cryptography