Watermarking University of Palestine Eng. Wisam Zaqoot May 2010

Data Hiding Secret message Secret message Key Carrier document Embedding algorithm Transmission via network Detector Key Information Hiding is a general term encompassing many sub-disciplines Two important sub-disciplines 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)

Why do we need Data Hiding? Covert communication using images (secret message is hidden in a carrier image) Ownership of digital images, authentication, copyright Data integrity, fraud detection, self-correcting images Traitor-tracing (fingerprinting video-tapes) Adding captions to images, additional information, such as subtitles, to video, embedding subtitles or audio tracks to video (video-in-video) Copy control (secondary protection for DVD)

Issues in Data Hiding Perceptibility: does embedding information “distort” cover medium to a visually unacceptable level (subjective) Capacity: how much information can be hidden relative to its perceptibility (information theory) Robustness to attacks: can embedded data survive manipulation of the stego medium in an effort to destroy, remove, or change the embedded data Trade-offs between the previous three: More robust => lower capacity Lower perceptibility => lower capacity etc.

The “Magic” Triangle Capacity Robustness Security Additional factors: There is a trade-off between capacity, invisibility, and robustness Naïve steganography Secure steganographic techniques Digital watermarking Robustness Security Additional factors: Complexity of embedding / extraction Undetectability

Watermarking Intent: data embedding conveys some information about the cover medium such as owner, copyright, or other information Watermark can be considered to be an extended attribute of the data Robustness of watermark is a main issue In some cases, you know watermark may be there Can be visible or invisible

Watermarking:Definition 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 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.

Watermarking:History More than 700 years ago, watermarks were used in Italy indicate the paper brand and the mill that produced it By the 18th century watermarks began to be used as anti-counterfeiting measures on money and other documents The term watermark was introduced near the end of the century. It was probably given because the marks resemble the effects of water on paper The first example of a technology similar to digital watermarking is a patent filed in 1954 by Emil Hembrooke for identifying works In 1988, Komatsu and Tominaga appear to be the first to use the term "digital watermarking" About 1995, interest in digital watermarking began to mushroom

Watermarking Motivations The need to establish reliable methods for copyright protection and authentication. The need to establish secure invisible channels for covert communications. Adding caption and other additional information.

Watermarking:Applications Copyright protection Most prominent application Embed information about the owner to prevent others from claiming copyright Require very high level of robustness Copy protection Embed watermark to disallow unauthorized copying of the cover For example, a compliant DVD player will not playback data that carry a "copy never" watermark Content Authentication Embed a watermark to detect modifications to the cover The watermark in this case has low robustness, "fragile"

Watermarking:Basic principles

Watermarking: Requirements Imperceptibility The modifications caused by watermark embedding should be below the perceptible threshold Robustness The ability of the watermark to resist distortion introduced by standard or malicious data processing Security A watermark is secure if knowing the algorithms for embedding and extracting does not help unauthorized party to detect or remove the watermark

Digital Watermarking - Examples Text – varying spaces after punctuation, spaces in between lines of text, spaces at the end of sentences, etc. Audio – low bit coding, random imperceptible noise, fragile & robust, etc. Images – least-significant bit, random noise, masking and filtering, etc.

Digital Watermarking – Qualities/Types Effect on quality of original content – how does watermarking technique impact level of degradation and what is the level of acceptability with the degradation Visible vs. invisible – visible such as a company logo stamped on an image or movie or invisible and imperceptible Fragile vs. robust – fragile watermarks break down easily whereas robust survive manipulations of content (in some watermarking of audio files, both are used)

Digital Watermarking –Qualities/Types. Public vs. private – private watermarking techniques require that the original be used as a basis of encryption whereas public does not Public-key vs. secret-key – secret-key watermarking uses the same watermarking key to read the content as the key that was inserted into the image; public key uses different keys for watermarking the image and reading the image

Digital watermarks categories Robust watermark- Used for copyright protection. Requirements: the watermark should be permanently intact to the host signal, removing the watermark result in destroying the perceptual quality of the signal. Fragile watermark- Used for tamper detection or as a digital signature. Requirements: Break very easily under any modification of the host signal. Semi Fragile watermark- used for data authentication. Requirements: Robust to some benign modifications, but brake very easily to other attacks. Provide information about the location and nature of attack

Attacks Attacks are carried out with an intension to destroy watermark for the purposes of use without having to pay royalties to the originator of the content. Must withstand various signal processing attacks: Compression Cropping, editing, composing. Printing. Adding small amounts of noise.