Image analysis – Applications
Reexamination Certificate
1999-11-30
2003-10-14
Mehta, Bhavesh M. (Department: 2625)
Image analysis
Applications
C382S232000, C380S051000, C380S054000, C713S176000
Reexamination Certificate
active
06633652
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns a method of inserting an additional information item such as a watermark in a digital signal.
It also concerns a method of authenticating a digital signal from a watermark inserted in the signal.
Correlatively, the present invention concerns a device for inserting an additional information item and a device for authenticating a digital signal adapted respectively to implement the insertion and authentication methods according to the invention.
2. Description of Related Art
The present invention concerns in general terms the authentication of digital data, and more particularly digital images. Its purpose is to allow the detection of changes occurring in original digital data.
More precisely, the authentication method according to the invention comes within the technical field of watermarking of digital data which can be interpreted as the insertion of a seal in the digital data, enabling the content of an original digital data file to be authenticated.
In a conventional fashion, and as described for example in the European patent application EP 0 766 468 filed in the name of NEC CORPORATION, watermarking techniques can be used for protecting copyright over a digital document. In such a case, the watermark inserted must, amongst other things, be robust to the different manipulations performed on the digital data, and notably the conventional techniques of digital data compression.
Unlike the digital data watermarking techniques aimed at protecting the author of a digital document, watermarking techniques for authenticating the data themselves must make it possible to detect any change to or manipulation of the digital data. The watermark inserted for authenticating original digital data must consequently not be robust to the various conventional manipulations of the image.
In addition to its fragility with regard to the changes undergone by the document, this inserted watermark must be imperceptible, for the purpose of preserving the quality of the digital document, and notably the visual quality when the document to be authenticated is a digital image, and be difficult to counterfeit in order to prevent a counterfeiter from reinserting the same watermark in digital data which have been modified.
Such a method of authenticating digital data by the insertion of a watermark in the data is described in the article entitled “Digital watermarking using multiresolution wavelet decomposition” by D KUNDUR and D HATZINAKOS, Proc. ICASSP, pages 2969-2972, May 1998.
This method thus makes it possible to insert an additional information item, which can for example contain information on the origin of the data or their date of creation, and to sign these original data.
However, in this article, the insertion and authentication method described requires the spectral wavelet decomposition of a digital image to be authenticated and then the reverse wavelet spectral recomposition of the digital image after insertion of the watermark.
In addition to the fact that this spectral decomposition is expensive in calculation time, the method described makes it possible to authenticate only an original digital image but does not make it possible to protect digital data already compressed by various coding methods.
BRIEF SUMMARY OF THE INVENTION
The aim of the present invention is notably to propose a method of inserting an additional information item with a view to the authentication of digital data which make it possible to detect digital data in various spectral or spatial representation fields.
It makes it possible notably to insert an additional information item in digital data which are compressed in order to be stored or transmitted, in a manner compatible with the compression used.
The insertion method is applied to a digital signal decomposed into a set of coefficients, each coefficient being represented in L bits, and makes it possible to insert an additional information item represented by a set of binary values with the same size as the set of coefficients representing the digital signal to be authenticated.
The method comprises calculating, for at least one coefficient, a check bit, and then substituting a coefficient watermarked as a function of the check bit and a binary value of the additional information item for the said at least one coefficient.
According to the invention, the insertion method is characterised in that it includes the following steps:
calculating, for at least one coefficient, a check bit in accordance with a predetermined operation as a function of L-M first bit planes of said coefficient;
calculating M last bit planes according to a predetermined rule which is reversible as a function of the check bit and a binary value of the additional information item; and
substituting the M last bit planes calculated for the M last bit planes of said coefficient.
Thus the method of inserting a watermark according to the invention is applied to any digital signal provided that it is represented by a set of spatial or spectral coefficients.
The insertion method can thus be applied notably directly to a compressed representation of a digital signal.
The additional information is incorporated directly in the digital signal, by modifying only the last bit planes of the coefficients, that is to say the least significant bits LSB of the coefficients so that it remains imperceptible.
In addition, the check bit calculated from the coefficient itself depends only on the first bit planes, that is to say the most significant bit planes of the coefficient. These first L-M bit planes are not modified during the substitution step, which has the advantage, during the authentication of the digital signal, of being able to recalculate, for each coefficient, a check bit identical to that calculated at the time of insertion of the additional information and to facilitate the detection of any changes made to the original digital signal.
According to a symmetrical aspect of the invention, a method of authenticating a digital signal, from an additional information item, such as a watermark, inserted in the digital signal by the insertion method as described above, is characterised in that it includes the following steps:
calculating, for at least one coefficient, a check bit according to said predetermined operation as a function of L-M first bit planes of said coefficient;
extracting the value of the additional information item inserted in accordance with said predetermined rule which is reversible as a function of the check bit and the last M bit planes;
comparing the extracted value of the additional information item inserted and the binary value of the additional information item; and
deciding whether or not to authenticate the digital signal depending on whether or not said extracted value and said binary value of the inserted additional information item are identical.
As described previously, the authentication method is all the more simple to implement since the check bit, calculated from unmodified bit planes of the coefficients, is identical to that calculated during the calculation step of the insertion method.
It is thus possible, at the extraction step, to use a deterministic rule which is simple to reverse in order to extract the binary value of the additional information item inserted.
In addition, according to the spectral or spatial representation of the digital signal by the coefficients, the authentication method makes it possible to locate, in the representation space, the changes made to the digital signal.
Moreover, as before, the authentication method makes it possible to authenticate a compressed representation of a digital signal and is therefore intrinsically robust with respect to the compression algorithm used.
According to a preferred characteristic of the invention, at the step of calculating a check bit of the insertion method and authentication method, said predetermined operation is a function of the L-M first bit planes and a confidential key represented in L bits.
Although optional, th
Canon Kabushiki Kaisha
Carter Aaron
Fitzpatrick ,Cella, Harper & Scinto
Mehta Bhavesh M.
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