1. Field of the Invention
The present invention generally relates to imperceptible watermarking of human-perceptible data sets such as sound tracks, images, or videos, where such data sets have been digitized.
2. Background Description
An imperceptible watermark (hereafter watermark for short), is an alteration of the data set which is mostly not perceptible to a human, but can be recognized by a machine such as a computer. For instance, if the data set represents an image, the watermark should be (mostly) invisible; if the data set represents a sound track, the watermark should be (mostly) inaudible; and so on. The general principle of such watermarking has been disclosed in prior art.
Some watermarking schemes have been proposed to protect ownership, i.e., establish who is the rightful owner in situations when the ownership is contested. To the contrary we are interested here in watermarking techniques which are used to check the authenticity of a document by identifying the identity of the owner and/or the date of creation of a document. Alterations of the image should be detectable by an authentication algorithm, preferably in such a way that the location of the alterations can be located on the image. Authentication should still be possible on a portion of the image. Such watermarks are called fragile watermarks; they are modified (and the modification is detectable) by any modification of the image
See, for example, "An Invisible Watermarking Technique for Image Verification", M. M. Yeung and F. C. Mintzer, Proceedings, International Conference on Image Processing 1997, vol. II pp. 680-683. This paper describes watermarking schemes where the owner of a data set incorporates an imperceptible watermark into the data set. As shown in prior art FIG. 1, the OWNER applies a watermarking scheme 102 to a source data set 101 to obtain the watermarked data set 103. The watermarked data set is distributed to the CUSTOMER 104. Both the OWNER and the CUSTOMER can authenticate 105 the data set by means of the
A related invention is "The Trustworthy Digital Camera: Restoring Credibility to the Photographic Image", G. L. Friedman, IEEE Trans. on Consumer Elec., vol. 39, no. 4, 1993, pp. 905-910, and U.S. Pat. No. 5,499,294 by G. L. Friedman, which describes a digital camera which uses cryptography to create a signature for authenticating the images generated. A signature is created for the entire image and appended to the image.
This invention of G. L. Friedman is not a watermark since the signature is appended to the image instead of being embedded in it. This has several drawbacks:
1) To authenticate an image, one needs more than just the image; both the image and the signature are needed.
2) The locations where changes to the image are made cannot be determined.
3) A cropped version of the image cannot be authenticated as the signature depends on the full image.
4) The authentication algorithm needs access to the human-readable part of the image. Therefore the authentication agent, if different from the CUSTOMER, will also see the human-readable content of the image, a situation which can be undesirable. Drawbacks 1 through 3 are easily corrected by using a fragile watermark instead of appending the signature. However, in the present state of the art, this is at the cost of:
5) Altering the image more severely than just modifying the least significant bits.
6) A signature which does not offer the widely recognized advantages of Secret Key/Public Key (in short SK/PK) encryption. One of the advantages is the inability of the authenticating agent to watermark an image in the case when the authenticating agent is not the owner.
The present invention will correct all of these drawbacks.