09-04-2012, 04:41 PM
Information Hiding:Steganography and Watermarking
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Information Hiding
• Information Hiding is a general term encompassing
many subdisciplines
• Two important subdisciplines are: steganography and
watermarking
• Steganography:
–Hiding: keeping the existence of the information secret
• Watermarking:
–Hiding: making the information imperceptible
• Information hiding is different than cryptography
(cryptography is about protecting the content of
messages)
Steganography: Definition and History
Steganography: derived from the Greek words steganos
which means “covered” and graphia which means
“writing”, i.e. covered writing
• It is the art of concealed communication; the very
existence of a message is secret
• Examples of old steganography techniques:
–Writing on shaved heads
– Invisible ink
–Microscopic images
Transform Domain Techniques:
• Embed secret message in a transform space (e.g.
frequency domain) of cover
• Example: Steganography in the Discrete Cosine
Transform (DCT) domain
– Split the cover image into 8×8 blocks. Each block is used
to encode one message bit
– Blocks are chosen in a pseudorandom manner
– The relative size of two pre-defined DCT coefficients is
modulated using the message bit
– The two coefficients are chosen from middle frequencies
(trade off between robustness and imperceptibility)
Distortion Techniques:
• Store information by signal distortion
• The encoder applies a sequence of modifications to the
cover. This sequence corresponds to the secret message
• The decoder measures the differences between the
original cover and the distorted cover to detect the
sequence of modifications and consequently recover the
secret message
• Not useful in many applications since the decoder must
have access to the original cover
• Example: vary the distance between consecutive lines or
words to transmit secret information
Watermarking:Applications
4. Transaction Tracking
– Embed a watermark to convey information about the legal
recipient of the cover
– This is useful to monitor or trace back illegally produced
copies of the cover
– This is usually referred to as “fingerprinting”
5. Broadcast Monitoring
– Embed a watermark in the cover and use automatic
monitoring to verify whether cover was broadcasted as
agreed
Watermarking:Attacks
• Signal enhancement (sharpening, contrast enhancement, etc.)
• Additive and multiplicative noise (gaussian, uniform, etc.)
• Filtering (High pass, low pass, linear, nonlinear, etc.)
• Lossy compression (JPEG, MPEG-x, H.26x, etc.)
• Geometric transforms (translation, rotation, etc.)
• Data reduction (cropping, clipping, etc.)
• Transcoding (MPEG2 ⇒H.263, etc.)
• D/A and A/D conversion (print-scan, etc.)