13-07-2012, 03:34 PM
Information Hiding: Steganography and Watermarking
information_hiding.pdf (Size: 1.31 MB / Downloads: 57)
Steganographyefinition 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
Steganography: Applications
• Unobtrusive communication
– Military and intelligence agencies
– Criminals !!
– Normal people
• Plausible deniability
– Fair voting
– Personal privacy
– Limitation of liability
• Anonymous communication
– Vote privately
– Make political claims
– Access censored material
– Preserve free speech
Steganography:Examples of Techniques
1. Substitution Techniques:
• Substitute redundant parts of a cover with a secret
message
• Example: Least Significant Bit (LSB) Substitution
– Choose a subset of cover elements and substitute least
significant bit(s) of each element by message bit(s)
– Message may be encrypted or compressed before hiding
– A pseudorandom number generator may be used to
spread the secret message over the cover in a random
manner
– Easy but vulnerable to corruption due to small changes in
carrier
Watermarkingefinition
• Watermarking: is the practice of imperceptibly altering a cover to
embed a message about that cover
• Watermarking is closely related to steganography, but there are
differences between the two:
– In watermarking the message is related to the cover
– Steganography typically relates to covert point-to-point communication
between two parties
– Therefore, steganography requires only limited robustness
– Watermarking is often used whenever the cover is available to parties
who know the existence of the hidden data and may have an interest in
removing it
– Therefore, watermarking has the additional notion of resilience against
attempts to remove the hidden data
• Watermarks are inseparable from the cover in which they are
embedded. Unlike cryptography, watermarks can protect content
even after they are decoded