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Watermark

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Transcript

  • 1. 1
  • 2. Outline  Introduction, Applications  Characteristics and classification  Popular techniques for watermarking 2
  • 3. Definition: A digital watermark is adigital signal or pattern inserted into adigital document such as text, graphicsor multimedia, and carries informationunique to the copyright owner, thecreator of the document or theauthorized consumer. 3
  • 4. Example 4
  • 5. Watermarking Vs Encryption Encryption 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 5
  • 6. 6
  • 7. Digital Watermarking Applications Ownership 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. 7
  • 8. 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. 8
  • 9. 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. 9
  • 10. 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. 10
  • 11. 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. 11
  • 12. Digital WatermarkClassification Based on visibility of watermarks - Visible Watermarks - Invisible Watermarks Based on the content to be watermarked - Text Watermarking - Image, Audio, Video Watermarking 12
  • 13. Techniques for TextsText Line Coding: Change the spacingbetween lines.Word-shift Coding: Change the spacingbetween words.Character Encoding: Alter the shapes ofcharacters. 13
  • 14. 14
  • 15. Word-shift coding example 15
  • 16. Easily defeated…  Retyping the text destroys the watermark  Word processors change the spacing between words and lines  Character encoding can be defeated by changing the font 16
  • 17. Techniques for Images  Spatial 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. 17
  • 18. Checksum Technique for images Watermark 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. 18
  • 19. 19
  • 20. Bavarian coupleOriginal Watermarked Version 20
  • 21. Advantages/Disadvantages Embedding 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. 21
  • 22. Conclusion First generation of copyright marking schemes is not strong enough Existing schemes provide only limited measures of marking Can only meet few requirements at a time  Tradeoff - Bandwidth vs. robustness  No single problem but a constellation! Real problem: watermark restoration 22
  • 23. Q&A 23