422 digital watermarking for information security

8/2/2019 422 Digital Watermarking for Information Security

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OutlineIntroduction, Applications

Characteristics and classification

Popular techniques for watermarking

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Example


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Watermarking Vs

EncryptionEncryption involves document transformation

so that the contents of the document are notvisible without a decryption key

Watermarking leaves the original file/imageintact and recognizable

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Digital Watermarking ApplicationsOwnership Assertion

A uses a private key to generate a watermark and embeds it inthe 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 As 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 theslightest change in the document.

A unique key associated with the source is usedthe create the watermark and then embed in thedocument.

This key is then used to extract the watermarkand the integrity of the document verified on thebasis of the integrity of the watermark.

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(3)Content labeling

Bits embedded in the data, comprise anannotation, giving some more information aboutthe data.

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

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

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(4)Usage control & Copy protectionDigital watermark inserted to indicate the

number of copies permitted.

Every time a copy is made the hardwaremodifies the watermark and at the same time itwould not create any more copies of the data.Commonly used in DVD technology.

Content ProtectionContent owner might want to publicly and freely

provide a preview of multimedia content beingsold.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 wouldlike to destroy it in order to counterfeit the copyright ofthe data. Moreover, removal of it should cause aconsiderable degradation in the quality of the data.

Visible watermarks should be visible enoughtodiscourage 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 ofcharacters.


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Word-shift coding example


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Easily defeatedRetyping the text destroys the watermark

Word processors change the spacing

between words and lines

Character encoding can be defeated bychanging 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 ofthe bits from 1 to 0 or 0 to 1.

Frequency Domain Watermarking:First convert theimage to the frequency domain and then apply thewatermark 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 thewatermark, W.

Last bit of each pixel is then changed to the correspondingchecksum bit.

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Bavarian couple

Original Watermarked Version


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Advantages/Disadvantag

esEmbedding the checksum only changes (on average) half

the number of pixel. So less visual distortion.

Can hold multiple watermarks as long as they dont overlap.

Extremely simple and fast.

Extremely fragile. Any change to the checksum causes thefailure of the verification procedure.

Forger could replace a section with another one of equalsize and checksum.

Entire watermark can be removed by removing the LSBplane. Cant survive lossy compression.

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ConclusionFirst generation of copyright marking

schemes is not strong enough

Existing schemes provide only limitedmeasures of marking

Can only meet few requirements at a time

Tradeoff - Bandwidth vs. robustness

No single problem but a constellation!Real problem: watermark restoration

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Q&A

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422 digital watermarking for information security

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