WATERMARK EMBEDDING AND DETECTION

Abstract: A method and arrangement for embedding and detecting a watermark in an information signal is disclosed. The embedded watermark (Wi) is selected (13) from a plurality of watermarks (W1..WN) in dependence upon a property P of the signal. An example of such a property is the distribution of luminance values of the current video image as calculated by an analysis circuit (12). The corresponding watermark detector performs the same operation: the watermark being looked for depends on the same signal property. It is achieved with the invention that the embedded watermark changes from time to time as a function of the information signal content, so that it cannot easily be hacked.

Description: Watermark embedding and detection. FIELD OF THE INVENTION The invention relates to a method and arrangement for watermarking an information signal, for example, an audio or video signal. The invention also relates to a method and arrangement for detecting a watermark in such an information signal. BACKGROUND OF THE INVENTION A known method of watermarking a video signal is disclosed in International Patent Application WO-A-99/45705. In this method, a watermark pattern is added to the video signal. A watermark detector correlates the same pattern with the suspect signal. If the correlation exceeds a given threshold, the pattern is said to be present. The presence or absence of the pattern represents a single bit of information. The embedded watermark may also carry a multi-bit payload. In the system disclosed in WO-A-99/45705, the payload is represented by a combination of one or more basic patterns and spatially shifted versions thereof. The payload is encoded into the respective shift vectors. The watermark detector correlates each basic pattern with the suspect signal, and determines the spatial positions of the basic patterns with respect to each other. The detector further checks whether said positions constitute a valid payload. The process of correlating watermark patterns with the suspect signal requires the watermark detector to have locally stored versions of said patterns. In view thereof, it is desired that the watermarking system employs only a few different patterns. The patterns being used are kept secret to the outside world. However, even without knowledge of the patterns, a hacker can compromise the system if he has the relevant embedder at his disposal. He may feed an arbitrary input signal to said embedder and subtract the signal from its watermarked version. The difference signal thus obtained resembles the watermark of any other watermarked signal, depending on the perception model used in the watermark embedder at hand. If the difference signal is combined with (e.g. added to or subtracted from) a watermarked signal, the embedded watermark will substantially be cancelled or at least no longer represent a valid payload. In either case, the embedded watermark has been made ineffective. OBJECT AND SUMMARY OF THE INVENTION It is an object of the invention to provide a more secure method and arrangement for embedding and detecting a watermark in an information signal, even if a hacker has a watermark embedder at his disposal. To this end, the method in accordance with the invention comprises the steps of analyzing a given property of the information signal and determining an actual value of said property, associating different watermarks with distinct values of said property, and embedding the watermark associated with said actual value. The corresponding watermark detection method comprises the steps of analyzing a given property of the information signal and determining an actual value of said property, associating different watermarks with distinct values of said property, and detecting the watermark associated with said actual value. It is achieved with the invention that the embedded watermark pattern changes from time to time, as a function of the information signal content. Feeding an arbitrary signal to an embedder so as to produce a signal that resembles the watermark, as described above, does not work anymore because the arbitrary signal has different properties. A significant advantage of the invention is that the number of different watermark patterns which the detector must store can be kept much lower. Said number is a result of balancing detector complexity versus security. There are numerous examples of properties of the information signal that can be used for selecting the watermark pattern to be embedded. The only requirement to be fulfilled is its robustness or invariance with respect to the embedded watermark. Advantageous examples of properties are distinct distributions of luminance values of a video signal, or distinct shapes of the frequency spectrum of an audio signal. Further aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. The examples relate to watermark embedding and detection of video signals, but it will be appreciated that the invention equally applies to audio signals or any other type of multimedia signal. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows schematically a diagram of a watermark embedder in accordance with the invention. Fig. 2 shows schematically a diagram of a watermark detector in accordance with the invention. Fig. 3 shows an arrangement to illustrate the operation of the watermark embedder and detector. Figs. 4 and 5 show further embodiments of the watermark embedder in accordance with the invention. Fig. 6 shows a further embodiment of the watermark detector in accordance with the invention. DESCRIPTION OF PREFERRED EMBODIMENTS Fig. 1 shows schematically a diagram of an embodiment of a watermark embedder 1 in accordance with the invention. It will here be assumed that the embedded watermark represents a 1-bit payload. For example, the absence of a watermark indicates that the video signal may freely be copied, whereas the presence of a predetermined watermark denotes that making a copy of the signal is prohibited. The embedder receives an input video signal I in the form of a sequence of images, and comprises an adder 11 which adds a watermark pattern Wj to each image. The embedder further comprises an image analyzer 12, a selector 13 and a read-only memory 14 in which a plurality of different watermark patterns W] ..WN are stored. The analyzer 12 receives the video signal and analyzes a given property P of the video signal as a function of time. The actual value of property P found by analyzer 12 is applied to the selector 13. In response thereto, the selector selects one of the stored watermark patterns Wι..W N to the adder 11 for embedding. The analyzer 12 may take numerous forms. A few examples will be given to provide sufficient teaching to enable a skilled person to design appropriate alternative embodiments. The property being analyzed may be the distribution of luminance values across the image (spatial distribution) or across a sequence of images (temporal distribution). In a first example, the analyzer divides each image into sub-images, and determines which of said sub-images has the highest average luminance. The relevant sub-image number is the actual value of property P. In a second example, the analyzer assigns a "0" to each sub-image having a low average luminance and a "1" to each sub-image having a high average luminance. Each video image is now characterized by an n-bit code, where n is the number of sub-images. The relevant n-bit code is the actual value of property P. The property being analyzed may also be local image activity. Such an analysis can easily be carried out in the frequency domain. Fig. 2 shows schematically a diagram of a preferred embodiment of a watermark detector 2 in accordance with the invention. The detector receives a suspect video signal J and comprises an image analyzer 22, a selector 23 and a read-only memory 24 which are identical to the corresponding counterparts of embedder 1. Accordingly, the analyzer 22 analyzes the same property P of the video signal, and the selector 23 selects the same watermark pattern W from the stored patterns W]..W N , as the embedder. The detector further comprises a correlation circuit 21 which calculates the correlation between each image of the suspect video signal and the applied watermark pattern Wj. If the correlation exceeds a predetermined threshold, the selected watermark pattern Wj is said to be present (D=l), otherwise it is said to be absent (D=0). The correlation circuit 21 is preferably of a type which performs the correlation for all possible spatial positions of the applied watermark with respect to the image. Such a correlation circuit is disclosed in WO-A-99/45705. It generates a correlation pattern which exhibits a peak for each spatial position of the watermark. WO-A-99/45705 describes that multiple peak positions may represent a payload. However, as mentioned above, the payload in this example is a 1-bit copy control signal. The detection circuit 2 will consider the presence of 2 or more peaks as an invalid payload (D=0). It is assumed that the watermark patterns W J ..W N are secret and can neither be retrieved by interrogating the embedder or detector circuits. As will now be explained with reference to Fig. 3, the invention prevents a hacker from compromising the system when he happens to have an embedder at his disposal. In Fig. 3, a potential hacker receives a video signal V being watermarked by an embedder la. The signal V may be a recorded signal, in which case the actual embedding took place a long time ago. The embedder la is of a type as described above with reference to Fig. 1. The hacker has an identical embedder lb at his disposal. An arbitrary video signal X is applied to said embedder lb so as to locally generate a watermarked video signal X'. An adder 3 subtracts the arbitrary signal X from its watermarked version X'. The difference signal (which strongly resembles the embedded watermark pattern) is then combined with (added to or subtracted from) the watermarked signal V by a further adder 4. The suspect signal V" thus processed is applied to a watermark detector 2 as described above with reference to Fig. 2. Without the provisions of the invention, both embedders la and lb embed the same watermark in the respective input signals. This results either in a cancellation of the watermark in the suspect signal V" or in an invalid payload due to multiple occurrences of the watermark pattern W at different positions. In both cases, the detector generates an output signal D=0 and the hacking attack is successful. With the provisions of the invention, the watermark W; (i=l..N) in signal V will generally differ from the watermark W j (j=l..N) in signal X', because the contents of the original video signals V and X are different. The property analysis algorithm of detector 2 responds to the contents of signal V" which is substantially equal to the contents of V. Consequently, the watermark pattern being checked by detector 2 is the watermark pattern Wj which has been embedded by embedder la. The detector ignores the additional presence of a different pattern W j , and the hacking attack thus fails. A possible work-around is feeding the watermarked signal V instead of an arbitrary signal X to embedder lb, so as to force embedder lb to select the same watermark Wj as embedder la. To avoid this, the embedders la and lb are preferably of a type that refrains from embedding a watermark in a signal that has already been watermarked. Fig. 4 shows a schematic diagram of such an embedder. It comprises the same adder 11, image analyzer 12, selector 13 and ROM 14 as the embedder which is shown in Fig. 1. It further comprises the correlation circuit 21 of the detector which is shown in Fig. 2. The correlation circuit 21 detects whether the input signal I already includes the watermark pattern Wj to be embedded. If that is the case (D=l), a switch 15 is controlled to prevent the watermark pattern Wj from being embedded multiple times. Fig. 5 shows a schematic diagram of a watermark embedder for embedding multi-bit payload in the video signal. The embedder comprises the same adder 11, image analyzer 12, selector 13 and ROM 14 as described before with reference to Fig. 1. The ROM 14 now stores a plurality of sets of watermark patterns. The embedder further includes an encoding circuit 16 which receives a selected set i of basic watermark patterns Wj ιl ; Wj t2 , ..., and encodes a multi-bit payload d into the relative positions of said patterns. The basic patterns have a relatively small size (e.g. 128x128 pixels). The watermark pattern generated by encoder 16 is subsequently tiled over the image by a tiling circuit 17. The ROM 14 stores different sets of basic patterns for different values of signal property P. The actual set of basic patterns being applied to encoder 16 is controlled by the actual value of property P and changes as a function of time. Fig. 6 shows the corresponding watermark detector. The detector comprises a folding circuit 25 for folding and storing image segments of 128x128 pixels in a buffer prior to correlation. The detector further comprises the same correlation circuit 21, image analyzer 22, selector 23 and read only memory 24 as described before with reference to Fig. 2. The ROM 24 stores different sets of basic patterns for different values of signal property P. The actual set of basic patterns being applied to the correlation circuit 21 is controlled by the actual value of property P. It should be noted that the invention is not limited to the watermarking systems described in the embodiments. For example, a watermarking system is known that uses n different watermark patterns, each pattern corresponding to one bit of an n-bit payload. In accordance with this invention, the embedder and detector of such a system include different sets of n patterns. A particular set is then selected in response to the actual value of a signal property. In summary, a method and arrangement for embedding and detecting a watermark in an information signal is disclosed. The embedded watermark (Wj) is selected (13) from a plurality of watermarks (WI...W N ) in dependence upon a property P of the signal. An example of such a property is the distribution of luminance values of the current video image as calculated by an analysis circuit (12). The corresponding watermark detector performs the same operation: the watermark being looked for depends on the same signal property. It is achieved with the invention that the embedded watermark changes from time to time as a function of the information signal content, so that it cannot easily be hacked.

Claims: CLAIMS: 1. A method of embedding a watermark in an information signal, comprising the steps of: - analyzing a given property of the information signal and determining an actual value of said property; - associating different watermarks with distinct values of said property; and selecting the watermark associated with said actual value for embedding in the information signal. 2. A method as claimed in claim 1, in which the information signal is a sequence of video images, said analyzing step comprising analyzing a spatial or temporal distribution of luminance values, each distinct distribution of luminance values constituting a value of said property of the information signal. 3. A method as claimed in claim 1, in which the information signal is a sequence of audio signal segments, said analyzing step comprising analyzing a shape of the frequency spectrum of said audio segments, each distinct shape of the frequency spectrum constituting a value of said property of the information signal. 4. A method as claimed in claim 1, in which the embedded watermark is a combination of two or more basic watermark patterns constituting a set of basic watermark patterns being selected from different sets in dependence upon the actual value of the property of the information signal. 5. A method of detecting a watermark in an information signal, comprising the steps of: analyzing a given property of the information signal and determining an actual value of said property; associating different watermarks with distinct values of said property; and selecting and detecting the watermark associated with said actual value. 6. A method as claimed in claim 5, in which the information signal is a sequence of video images, said analyzing step comprising analyzing a spatial or temporal distribution of luminance values, each distinct distribution of luminance values constituting a value of said property of the information signal. 7. method as claimed in claim 5, in which the information signal is a sequence of audio signal segments, the method comprising the steps of calculating the frequency spectrum for each segment, each distinct shape of said frequency spectrum constituting a value of said property of the information signal. 8. method as claimed in claim 5, in which the embedded watermark is a combination of two or more basic watermark patterns constituting a set of basic watermark patterns being selected from different sets in dependence upon the actual value of the property of the information signal. 9. watermark embedder for embedding a watermark in an information signal, comprising: - means (12) for analyzing a given property (P) of the information signal and determining an actual value of said property; means (14) for associating different watermarks with distinct values of said property; and means (13) for selecting the watermark associated with said actual value for embedding (11) in the information signal. 10. A watermark detector for detecting a watermark in an information signal, comprising: - means (22) for analyzing a given property of the information signal and determining an actual value of said property; means (24) for associating different watermarks with distinct values of said property; and - means for selecting (23) and detecting (21) the watermark associated with said actual value. 11. A watermark embedder as claimed in claim 9, further including a watermark detector as claimed in claim 10, and comprising means (15) for refraining from embedding the selected watermark in response to said detector detecting said selected watermark in the information signal.

Inventors: DEPOVERE GEERT F G, EVELEENS JAN, KALKER ANTONIUS A C M, DEPOVERE, GEERT, F., G., EVELEENS, JAN, KALKER, ANTONIUS, A., C., M.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2000
Main classification: G06T1
IPC codes: G06T1, G10L11, G11B20, H04N1, H04N5, H04N7