I NFORMATION H IDING :
I NTRODUCTION ( PART 2)
Dr. Shahriar Bijani
Shahed University
Sep 2014

S LIDES R EFERENCES
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Sanjay Goel, Watermarking &
Steganography, University at Albany, State
University of New York.
Steganography, University of Virginia.
Anastasios Tefas , Information Hiding
Content Verification, Dept. of Informatics,
Aristotle University of Thessaloniki.
CS 4953, The Hidden Art of Steganography,
University of Texas at St Antonio, 2005.
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I NFORMATION H IDING S UB DISCIPLINES
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Information Hiding employs technologies from numerous science disciplines:
Digital Signal Processing (Images, Audio, Video)
Cryptography
Information Theory\Coding Theory
Data Compression
Human Visual/Auditory perception
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Information Hiding can be classified into 4 primary sub-disciplines of Information Hiding
Steganography
Watermarking
Covert Channels
Anonymity

C OVERT C HANNELS
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Covert channels are communication paths that were neither designed nor intended to transfer information
E.g. the telephone was designed to allow voice communication
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 information could be conveyed by letting it ring a certain number of times
The time differences between successive phone calls could be used
You could use a mobile phone and call from different locations
– the street names convey the message
Unused bits in the TCP/IP protocol headers can be used to carry information
(Hiding data in an image, then sending that image to someone else could also be considered a covert channel)

A NONYMITY
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Anonymity is about concealing the sender and receiver of messages
This is the least studied sub-discipline of information hiding

S TEGANOGRAPHY
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The art and science of communicating in a way that hides the existence of a message in some other data
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Methods of transmitting secret messages through innocuous cover carriers in such a manner that the existence of the embedded messages is undetectable
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Carriers can be audio, video, text, or any other digitally represented code or transmission
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The hidden message may be plaintext, ciphertext, or anything that can be represented as a bit stream
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T ERMINOLOGY
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The data to be hidden:
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Plaintext (from cryptography)
Secret message
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Stego-message
Embedded data
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The data which will have a stego-message embedded in it:
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Covertext
Cover-Object
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Cover-Medium
Cover-Image\Cover-Audio\Cover-Video
Target file

T ERMINOLOGY
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The key used to make the stego-message secure
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Stego-Key
Secret Key
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Key
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The file with the steganography-message embedded
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Stegotext (ciphertext in cryptography)
Stego-Object
Stego-Medium
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Stego-Image\Stego-Audio\Stego-Video

T ERMINOLOGY
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Alice and Bob
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Classical names given to the parties wishing to communicate
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Eve, an adversary, can listen to but not modify or forge a message
 passive eavesdropping
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Wendy the Warden, another adversary, can monitor, modify, or forge a message
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A passive warden simply listens (like Eve)
An active warden may modify a message
A malicious warden may forge a fake message

I NFORMATION H IDING T AXONOMY
F. A. P. Petitcolas, R. J. Anderson, M. G. Kuhn, “Information Hiding – A Survey”, Proceedings of the IEEE, special issue on protection of multimedia content, 87(7):1062-1078, July 1999

I NFORMATION H IDING T AXONOMY
Digital
Methods

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T AXONOMY uses scientific methods to hide a message, e.g. the use of invisible ink or microdots and other size-reduction methods.
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Linguistic steganography hides the message in the carrier in some nonobvious ways
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Semagrams hide information by the use of symbols or signs.
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Visual semagram uses innocent-looking or everyday physical objects to convey a message, e.g. drawings or the positioning of items on a desk or
Website.
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Text semagram hides a message by modifying the appearance of the carrier text, e.g. delicate changes in font size or type, adding extra spaces, or different flourishes in letters or handwritten text.

T AXONOMY
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Open codes hide a message in a legitimate carrier message in ways that are not obvious to an unsuspecting observer.
The carrier message: overt communication, the hidden message: covert communication.
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Jargon code uses language that is understood by a group of people but is meaningless to others. includes warchalking (symbols used to indicate the presence and type of wireless network signal underground terminology, or an innocent conversation that conveys special meaning because of facts known only to the speakers.)
A subset of jargon codes is cue codes, where certain prearranged phrases convey meaning.

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T AXONOMY
Covered or concealment ciphers hide a message openly in the carrier medium so that it can be recovered by anyone who knows the secret for how it was concealed.
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Grille cipher : employs a template that is used to cover the carrier message.
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E.g. The words that appear in the openings of the template are the hidden message.
Null cipher: hides the message according to some prearranged set of rules, such as "read every fifth word" or
"look at the third character in every word."

A SPECTS OF INFORMATION HIDING
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Three different aspects in information-hiding systems struggle with each other:
Capacity refers to the amount of information that can be hidden in the cover medium
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Security refers to an eavesdropper’s inability to detect hidden information
Robustness refers to the amount of modification the stego medium can withstand before an adversary can destroy hidden information.
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