Information Hiding: Steganography and Watermarking


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Information Hiding: Steganography and Watermarking
Dr. Mohammed Al-Mualla and Prof. Hussain Al-Ahmad Multimedia Communication and Signal Processing (MCSP)
Research Group
Etisalat College of Engineering
P.O.Box: 980, Sharjah, UAE Tel: +971 6 5611333, Fax: +971 6 5611789, e-mail: {almualla, alahmad}@ece.ac.ae

Outline
• Introduction to Information Hiding • Steganography
– Definition and History – Applications – Basic Principles – Examples of Techniques – Demos
• Watermarking
– Definition and History – Applications – Basic Principles – Requirements – Attacks – Evaluation and Benchmarking – Examples

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

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: Basic Principles

TRANSMITTER cover c

RECEIVER

Encoder

stego-object s

(Hiding Mechanism)

stego-object

Insecure

s

Decoder

Channel

(Extraction Mechanism)

message m key k

key k message m

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

Steganography: Examples of Techniques
2. 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)

Steganography: Examples of Techniques
3. Spread Spectrum Techniques:
• Adopt ideas from spread spectrum communication where a signal is transmitted in a bandwidth in excess of the minimum necessary to send the information
• In other words, the message is spread over a wide frequency bandwidth
• The SNR in every frequency band is small (difficult to detect) • Even if parts of the message are removed from several bands,
enough information is present in other bands to recover the message • Thus, it is difficult to remove the message completely without entirely destroying the cover (robustness)

Steganography: Examples of Techniques
4. Statistical Techniques: • Encode information by changing several statistical
properties of a cover • The cover is split into blocks. Each block is used to hide
one message bit • If the message bit is “1” then the cover block is
modified, otherwise the cover block is not modified • Difficult to apply in many cases, since a good test must
be found which allows distinction between modified and unmodified cover blocks

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Information Hiding: Steganography and Watermarking