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OutlineIntroduction, ApplicationsCharacteristics and classificationPopular techniques for watermarking

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Definition: A digital watermark is a digital signal or pattern inserted into a digital document such as text, graphics or multimedia, and carries information unique to the copyright owner, the creator of the document or the authorized consumer.

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Example

Watermarking Vs EncryptionEncryption involves document transformation

so that the contents of the document are not visible without a decryption key

Watermarking leaves the original file/image intact and recognizable

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Digital Watermarking ApplicationsOwnership Assertion

‘A’ uses a private key to generate a watermark and embeds it in the document

‘A’ makes the watermarked image publicly available ‘B’ claims that he owns the image derived from the public

image ‘A’ produces the unmarked original and establishes the

presence of ‘A’s watermark

Fingerprinting Used to avoid unauthorized duplication and distribution. A distinct watermark (a fingerprint) is embedded in each copy

of the data. If unauthorized copies are found, the origin of the copy can be

determined by retrieving the fingerprint.

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Digital Watermarking Applications (2)Authentication & integrity verification

Watermarks should be able to detect even the slightest change in the document.

A unique key associated with the source is used the create the watermark and then embed in the document.

This key is then used to extract the watermark and the integrity of the document verified on the basis of the integrity of the watermark.

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Digital Watermarking Applications (3)Content labeling

Bits embedded in the data, comprise an annotation, giving some more information about the data.

Digital cameras annotate images with the time and date, when the photograph was taken.

Medical imaging machines annotate images (X-Rays) with patients name, ID.

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Digital Watermarking Applications (4)Usage control & Copy protection

Digital watermark inserted to indicate the number of copies permitted.

Every time a copy is made the hardware modifies the watermark and at the same time it would not create any more copies of the data.

Commonly used in DVD technology.Content Protection

Content owner might want to publicly and freely provide a preview of multimedia content being sold.

To make the preview commercially useless, content is stamped with visible watermarks.

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Characteristics of Digital Watermarks Readily Detectable: the data owner or an independent control

authority should easily detect it. Unambiguous: retrieval of it should unambiguously identify

the data owner.Robust: difficult to remove for an attacker, who would

like to destroy it in order to counterfeit the copyright of the data. Moreover, removal of it should cause a considerable degradation in the quality of the data.

Visible watermarks should be visible enough to discourage theft.

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Digital Watermark ClassificationBased on visibility of watermarks

- Visible Watermarks

- Invisible Watermarks

Based on the content to be watermarked

- Text Watermarking

- Image, Audio, Video Watermarking

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Techniques for Texts

Text Line Coding: Change the spacing between lines.

Word-shift Coding: Change the spacing between words.

Character Encoding: Alter the shapes of characters.

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Word-shift coding example

Easily defeated…Retyping the text destroys the watermarkWord processors change the spacing

between words and linesCharacter encoding can be defeated by

changing the font

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Techniques for ImagesSpatial Watermarking: Just change some of the values

of the pixels in the lower bit plane; e.g., Change some of the bits from 1 to 0 or 0 to 1.

Frequency Domain Watermarking: First convert the image to the frequency domain and then apply the watermark in the low frequency regions.

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Checksum Technique for imagesWatermark is formed from the 7 most significant bits of

each pixel.Eight 7-bit segments (from eight different pixels) are

concatenated and the final checksum is thus 56-bit.Locations of the pixels that are to contain one bit each of

the checksum are randomly chosen.These pixel locations along with the checksum form the

watermark, W.Last bit of each pixel is then changed to the corresponding

checksum bit.

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Bavarian couple

Original Watermarked Version

Advantages/DisadvantagesEmbedding the checksum only changes (on average) half

the number of pixel. So less visual distortion.Can hold multiple watermarks as long as they don’t

overlap.Extremely simple and fast.Extremely fragile. Any change to the checksum causes the

failure of the verification procedure.Forger could replace a section with another one of equal

size and checksum.Entire watermark can be removed by removing the LSB

plane. Can’t survive lossy compression.

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ConclusionFirst generation of copyright marking

schemes is not strong enoughExisting schemes provide only limited

measures of markingCan only meet few requirements at a time

Tradeoff - Bandwidth vs. robustnessNo single problem but a constellation!

Real problem: watermark restoration

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Q&A

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