Cryptography – By modifying optical image
Reexamination Certificate
1999-01-28
2001-03-27
Gregory, Bernarr E. (Department: 3662)
Cryptography
By modifying optical image
C283S113000, C283S017000, C283S073000, C713S176000, C380S055000, C380S200000, C380S202000
Reexamination Certificate
active
06208735
ABSTRACT:
FIELD OF THE INVENTION
The present invention concerns a method of digital watermarking for use in audio, image, video and multimedia data for the purpose of authenticating copyright ownership, identifying copyright infringers or transmitting a hidden message. Specifically, a watermark is inserted into the perceptually most significant components of a decomposition of the data in a manner so as to be virtually imperceptible. More specifically, a narrow band signal representing the watermark is placed in a wideband channel that is the data.
BACKGROUND OF THE INVENTION
The proliferation of digitized media such as audio, image and video is creating a need for a security system which facilitates the identification of the source of the material. The need manifests itself in terms of copyright enforcement and identification of the source of the material.
Using conventional cryptographic systems permits only valid keyholder access to encrypted data, but once the data is encrypted, it is not possible to maintain records of its subsequent representation or transmission. Conventional cryptography therefore provides minimal protection against data piracy of the type a publisher or owner of data or material is confronted with by unauthorized reproduction or distribution of such data or material.
A digital watermark is intended to complement cryptographic processes. The watermark is a visible or preferably an invisible identification code that is permanently embedded in the data. That is, the watermark remains with the data after any decryption process. As used herein the terms data and material will be understood to refer to audio (speech and music), images (photographs and graphics), video (movies or sequences of images) and multimedia data (combinations of the above categories of materials) or processed or compressed versions thereof. These terms are not intended to refer to ASCII representations of text, but do refer to text represented as an image. A simple example of a watermark is a visible “seal” placed over an image to identify the copyright owner. However, the watermark might also contain additional information, including the identity of the purchaser of the particular copy of the image. An effective watermark should possess the following properties:
1. The watermark should be perceptually invisible or its presence should not interfere with the material being protected.
2. The watermark must be difficult (preferably virtually impossible) to remove from the material without rendering the material useless for its intended purpose. However, if only partial knowledge is known, e.g. the exact location of the watermark within an image is unknown, then attempts to remove or destroy the watermark, for instance by adding noise, should result in severe degradation in data fidelity, rendering the data useless, before the watermark is removed or lost.
3. The watermark should be robust against collusion by multiple individuals who each possess a watermarked copy of the data. That is, the watermark should be robust to the combining of copies of the same data set to destroy the watermarks. Also, it must not be possible for colluders to combine each of their images to generate a different valid watermark.
4. The watermark should still be retrievable if common signal processing operations are applied to the data. These operations include, but are not limited to digital-to-analog and analog-to-digital conversion, resampling, requantization (including dithering and recompression) and common signal enhancements to image contrast and color, or audio bass and treble for example. The watermarks in image and video data should be immune from geometric image operations such as rotation, translation, cropping and scaling.
5. The same digital watermark method or algorithm should be applicable to each of the different media under consideration. This is particularly useful in watermarking of multimedia material. Moreover, this feature is conducive to the implementation of video and image/video watermarking using common hardware.
6. Retrieval of the watermark should unambiguously identify the owner. Moreover, the accuracy of the owner identification should degrade gracefully during attack.
Several previous digital watermarking methods have been proposed. L. F. Turner in W.I.P.U. patent publication number WO89/08915 entitled “Digital Data Security System” proposed a method for inserting an identification string into a digital audio signal by substituting the “insignificant” bits of randomly selected audio samples with the bits of an identification code. Bits are deemed “insignificant” if their alteration is inaudible. Such a system is also appropriate for two dimensional data such as images, as discussed in an article by R. G. Van Schyndel et al entitled “A digital watermark” in Intl. Conf. on Image Processing, vol 2, Pages 86-90, 1994. The Turner method may easily be circumvented. For example, if it is known that the algorithm only affects the least significant two bits of a word, then it is possible to randomly flip all such bits, thereby destroying any existing identification code.
An article entitled “Assuring Ownership Rights for Digital Images” by G. Caronni, in Proc. Reliable IT Systems, VIS '95, 1995 suggests adding tags—small geometric patterns-to-digitized images at brightness levels that are imperceptible. While the idea of hiding a spatial watermark in an image is fundamentally sound, this scheme is susceptible to attack by filtering and redigitization. The fainter such watermarks are, the more susceptible they are to such attacks and geometric shapes provide only a limited alphabet with which to encode information. Moreover, the scheme is not applicable to audio data and may not be robust to common geometric distortions, especially cropping.
J. Brassil et al in an article entitled “Electronic Marking and Identification Techniques to Discourage Document Copying” in Proc. of Infocom 94, pp 1278-1287, 1994 propose three methods appropriate for document images in which text is common. Digital watermarks are coded by: (1) vertically shifting text lines, (2) horizontally shifting words, or (3) altering text features such as the vertical endlines of individual characters. Unfortunately, all three proposals are easily defeated, as discussed by the authors. Moreover, these techniques are restricted exclusively to images containing text.
An article by K. Tanaka et al entitled “Embedding Secret Information into a Dithered Multi-level Image” in IEEE Military Comm. Conf., pp216-220, 1990 and K. Mitsui et al in an article entitled “Video-Steganography” in IMA Intellectual Property Proc., vI, pp187-206, 1994, describe several watermarking schemes that rely on embedding watermarks that resemble quantization noise. Their ideas hinge on the notion that quantization noise is typically imperceptible to viewers. Their first scheme injects a watermark into an image by using a predetermined data stream to guide level selection in a predictive quantizer. The data stream is chosen so that the resulting watermark looks like quantization noise. A variation of this scheme is also presented, where a watermark in the form of a dithering matrix is used to dither an image in a certain way. There are several drawbacks to these schemes. The most important is that they are susceptible to signal processing, especially requantization, and geometric attacks such as cropping. Furthermore, they degrade an image in the same way that predictive coding and dithering can.
In Tanaka et al, the authors also propose a scheme for watermarking facsimile data. This scheme shortens or lengthens certain runs of data in the run length code used to generate the coded fax image. This proposal is susceptible to digital-to-analog and analog-to-digital conversions. In particular, randomizing the least significant bit (LSB) of each pixel's intensity will completely alter the resulting run length encoding. Tanaka et al also propose a watermarking method for “color-scaled picture and video sequences”. This method applies the same signal transform a
Cox Ingemar J.
Kilian Joseph J.
Shamoon Talal G.
Feig Philip J.
Gregory Bernarr E.
NEC Research Institute Inc.
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