WO2000052923A1

WO2000052923A1 - Method and arrangement for scrambling and descrambling information

Info

Publication number: WO2000052923A1
Application number: PCT/EP2000/001742
Authority: WO
Inventors: Antonius A. C. M. Kalker
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 1999-03-02
Filing date: 2000-02-29
Publication date: 2000-09-08

Abstract

The invention addresses the aspect of transmitting descrambling keys to a device for descrambling scrambled information. Prior art methods use a dedicated channel for that purpose. In accordance with the invention, the descrambling key (k1, k2) is hidden in the scrambled content (X'). In an embodiment, the keys are encoded as payload of a watermark (W) which is embedded (400) in the scrambled information signal. The invention is particularly useful to secure an analog link from a high-definition television set top box to a compliant monitor or recorder.

Description

Method and arrangement for scrambling and descrambling information.

FIELD OF THE INVENTION

The invention relates to a method and arrangement for scrambling information using a scrambling key. The invention also relates to a method and arrangement for descrambling scrambled information using a descrambling key.

BACKGROUND OF THE INVENTION

The purpose of scrambling is to prevent the playback and/or recording of protected content on unauthorized playback and/or recording devices. A typical example is in the content of set top boxes. A movie title is scrambled before transmission, and a consumer is only allowed to watch the title after paying certain fees. Upon reception of the fees, the consumer is handed descrambling information and subsequently watch the video. The video coming from the set top box is "self protecting" which means that it is useless unless processed by a compliant device. Self protection can be easily achieved for digital video streams by using well-known scrambling and encryption techniques. Currently there is an increasing demand for self protection of analog high- definition (HD) video. Of particular concern is the short and reliable link between source (set top box) and display (monitor) device. The bit rates for HD video are so high that digital data links supporting the required data rates are not yet expected to be available. This implies that the above-mentioned known digital scrambling encryption can not be used for some time. However, the quality of analog HD video is so high that there is real concern for illegal (digital) copies from sampling the analog link.

Scrambling implies modifying the video signal in a given manner defined by one or more scrambling keys. Descrambling keys are to be transferred to a compliant display or recording device along with the scrambled signal. A scrambling system is known in which a video signal is scrambled by dynamically modifying the positions of the active video with respect to the horizontal sync pulses. The deviations with respect to the proper position of the horizontal sync pulse are accommodated in the vertical sync pulse in the vertical blanking interval of the video signal. There is a concern that this system does not have the required security. A pirate might easily and cheaply build a device which restores the proper horizontal sync. It is envisaged that a simple analysis of the video content will allow restoration of the original quality, even in the absence of offset information. Another scrambling system is known in which the video signal is scrambled by giving an offset to all values (Y/C or RGB) of a video line. The offsets are transmitted to the compliant destination device over a separate secure low-rate digital link.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the invention to provide an method of scrambling an information signal and transmitting descrambling information along with said signal, which overcomes disadvantages of the prior art methods.

To this end, the method of scrambling information using a scrambling key is characterized in that the method includes the step of embedding said scrambling key in the scrambled information. Herewith is achieved that a separate link is dispensed with and that the (de)scrambling key is not easily accessible. The scrambling key is preferably encoded as payload of an embedded watermark, for example, by accommodating the payload in cyclic shifts of a given sequence of watermark signal samples. This is particularly advantageous in combination with an embodiment, in which the information signal is organized as a sequence of series of signal samples (e.g. a sequence of video lines constituting a video field or frame), wherein the signal is scrambled by adding a random offset to all signal samples of a series, and the watermark is embedded by further adding a watermark sample. An advantage of this combination is that the embedded watermark enforces the scrambling.

A corresponding method of descrambling scrambled information is characterized in that the method comprises the step of retrieving the (de)scrambling key from said the information. If the descrambling key is accommodated as payload of an embedded watermark, the watermark can be regenerated and removed if desired.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a schematic diagram of an arrangement for scrambling an information signal in accordance with the invention.

Fig. 2 shows a schematic diagram of an arrangement for descrambling a scrambled information signal in accordance with the invention.

Fig. 3 shows a schematic diagram of a further embodiment of an arrangement for descrambling a scrambled information signal in accordance with the invention. DESCRIPTION OF PREFERRED EMBODIMENTS

Fig. 1 shows a schematic diagram of an arrangement for scrambling an information signal in accordance with the invention. The information signal is a high- definition interlaced analog video signal having 1080 lines per picture (540 lines per field) and 1920 signal samples (pixels) per line. The arrangement receives a video field X of video signal samples x where i=1..540 denotes line numbers of the field and j=L.1920 denotes samples of line i. The signal is scrambled by adding random offsets to the analog luminance and/or chrominance components (or the RGB components) of the video signal in this embodiment. The same offset is added to all signal samples of a line but varies from line to line. It is possible to add different offsets to sub-series of signal samples (or even to individual samples) but that renders the system more complicated. Other scrambling methods may also be used.

The scrambling algorithm requires 540 offsets per field to be generated. To this end, the arrangement comprises an offset generator 200 which generates a random sequence N of 540 offset values n;, one for each line i. The offset generator includes a pseudo random sequence generator 202 which receives a seed S2. The seed S2 is derived from a scrambling key, which is this embodiment comprises two sub-keys kl and k2, via a mathematical unit 201. The offset generator further includes a scaling unit 203 which receives an appropriate scaling factor A. The larger A is, the better scrambled (more disturbed) the signal is. The offset sequence N is added to the video field X by an adder 100.

The scrambling key (kl,k2) used to generate the offsets may change from field to field. They are transmitted along with the scrambled signal so as to allow a compliant receiver to reconstructed the offsets and descramble the signal. In accordance with an aspect of the invention, the scrambling keys are embedded in the scrambled signal as payload of a watermark W which is generated by a watermark embedder 300. The watermark W is added to the scrambled signal by an adder 400.

The watermark embedder 300 comprises a pseudo random sequence generator 301 which receives a fixed seed SI being known to the compliant display or recording device. The generator 301 produces a random sequence of 540 random numbers which constitute a basic watermark noise pattern. The scrambling key (kl,k2) is encoded in cyclic shifts kl and k2 of the basic watermark pattern. This method of embedding payload data in one or more cyclic shifts of a basic watermark pattern is exhaustively described in Applicant's International Patent Application WO-A-99/45705. Any shift k in the range 0-540 can be chosen, but in order to improve the reliability of key retrieval, the shifts are chosen to be multiples of 4, which restricts the range of k to 0<k<135. A first shift unit 302 cyclically shifts the basic watermark by 4*kl. An inverter 303 multiplies the basic watermark by -1 and a second shift unit 304 cyclically shifts the inverted watermark by 4*k2, where kl≠k2. Both shifted sequences are added by an adder 305 to constitute a watermark sequence W of 540 values w,.

After adding the offsets n, and watermark values w, to the input field X, the scrambled field Y obeys the equation: y,,_J ^{= χ}.,_J ^{+ n}ι + ι Eq-¹ Fig. 2 shows a schematic diagram of an arrangement for descrambling the scrambled video signal. The arrangement receives the scrambled video field Y. It comprises a pseudo random generator 21 which is identical to, and receives the same seed SI as pseudo random generator 301 in Fig. 1. The generator 21 thus produces the same basic watermark pattern as the scrambler. The arrangement further comprises a summing circuit 22 which calculates the row-wise sum Z of Y. That is, the circuit 22 sums all the signal samples y,_j of line i. In mathematical notation:

which, after substitution of Eq.l and normalization, can be written as:

₁ 1920 z = w . + n. H x, ,

¹ ' 1920 ^ ^,,J

J=l

As this equation shows, the sequence Z of 540 numbers z, (one for each line of the field Y) includes the watermark sequence W. The sequence Z is applied to a watermark detector 22 which cyclically correlates the sequence with the basic watermark pattern. Such a watermark detector is exhaustively described in International Patent Application WO-A-99/45705 already mentioned above. If the scaling factor A is properly chosen, the correlation pattern will exhibit a positive peak at 4*kl and a negative peak at 4*k2. The scrambling key (kl,k2) to be used for descrambling is thus retrieved by the watermark detector 23.

The retrieved key (kl,k2) is applied to a an offset generator comprising a mathematical unit 24, a pseudo random generator 25 and a scaling unit 26. The offset generator is identical to, and produces the same offset sequence N as offset generator 200 in Fig. 1. The scaling factor A is known to both the scrambling and descrambling device. To this end, the scaling factor A is either fixed or also hidden in and to be retrieved from the scrambled information. Descrambling is subsequently effectuated by subtracting the reconstructed offsets from the scrambled video field Y by a subtractor 27.

It should be noted that the descrambling key (kl,k2) is not retrieved until the whole field has been received. In view hereof, descrambling requires the field Y to be delayed by a field buffer 28. The field buffer 28 can easily be dispensed with by accommodating the descrambling keys for the next field in the current field.

The arrangement which is shown in Fig. 2 generates a descrambled field X'. The descrambled field is not perfectly equal to the original field X, because it still includes the watermark W. This may be not a problem if the watermark is embedded, as usual, in an imperceptible manner. A significant advantage of the invention is that the embedded watermark may be perceptible. In technical terms, the watermark embedding depth may be large. The larger the embedding depth is, the more the perceptible watermark contributes to the desired effect of scrambling. If the embedded watermark W is perceptible, it must be removed from the scrambled signal as well. Fig. 3 shows a schematic diagram of an embodiment of a descrambling arrangement for such a situation. The same reference numerals are used in this Figure to denote same elements of the arrangement which is shown in Fig. 2. The arrangement differs from the one which is shown in Fig. 2 in that it further comprises a watermark regeneration circuit which receives the basic watermark pattern from the pseudo random generator 21. The watermark regenerator includes a first shift unit 31, an inverter 32, a second shift unit 33, and an adder 34, which are identical to, and produce the same watermark W as the first shift unit 302, inverter 303, second shift unit 304, and adder 305, respectively, of watermark embedder 300 which is shown in Fig. 1. The regenerated watermark sequence W is subsequently added to the offset sequence N by an adder 35, and the sum is subtracted from the scrambled signal Y. Note that this is equivalent to subtracting the regenerated watermark W from the descrambled signal X'.

We now assume that a set top box is capable of scrambling analog high- definition video signals as described above. A compliant monitor or recorder is capable of retrieving the descrambling information and display or record the original content. If the scrambled signal is applied to a non-compliant recorder, playback is only possible (on a compliant monitor) if a perfect copy of the analog signal is made. Such a perfect copy is unlikely since the required storage (both analog and digital) is out of reach for years to come. And even if a professional pirate is capable of making such a copy, the customers will not have the capabilities of displaying directly from that content carrier. Such illegal copies will thus be distributed in compressed form, with a reduced quality. Upon playback on compliant devices this gives rise to a serious degradation of quality due to the compression algorithm and/or the incapability of the device to retrieve the descrambling keys. The invention addresses the aspect of transmitting descrambling keys to a device for descrambling scrambled information. Prior art methods use a dedicated channel for that purpose. In accordance with the invention, the descrambling key (kl,k2) is hidden in the scrambled content (X¹). In an embodiment, the keys are encoded as payload of a watermark (W) which is embedded (400) in the scrambled information signal. The invention is particularly useful to secure an analog link from a high-definition television set top box to a compliant monitor or recorder.

Claims

CLAIMS:

1. A method of scrambling information (X) using a scrambling key (kl,k2), characterized in that the method includes the step of embedding (400) said scrambling key in the scrambled information (X').

2. A method as claimed in claim 1, wherein the step of embedding said scrambling key includes encoding the scrambling key as payload of a watermark (W), and embedding said watermark in the scrambled information.

3. A method as claimed in claim 2, wherein said step of encoding the scrambling key as payload of the watermark includes generating a given sequence of watermark samples and subjecting said sequence to a cyclic shift representing said scrambling key.

4. A method as claimed in claim 2, in which the information is organized as a sequence of series of one or more information samples, the method comprising the steps of: - generating for each series of the sequence a sample value offset in response to the scrambling key (kl,k2), generating for each series of the sequence a sample of a sequence of watermark samples constituting said watermark, and modifying (100,400) the signal samples of said series in accordance with said offset and watermark sample.

5. A method as claimed in claim 4, wherein said information is a video frame or picture, each series of information samples constituting a line of said video signal, and the sequence of series constituting said video frame or picture.

6. A method as claimed in claim 4, wherein the scrambling key for the next sequence of information samples is embedded in the current sequence of information samples.

7. - A method as claimed in any one of claims 1-6, wherein the scrambled information is an analog audio or video signal.

8. A method of descrambling scrambled information (Y) using a descrambling key (kl,k2), characterized in that the method comprises the step of retrieving said descrambling key from said scrambled information.

9. A method as claimed in claim 8, wherein the step of retrieving said scrambling keys includes detecting a watermark (W) embedded in the scrambled information (Y) and decoding payload of said watermark as said descrambling key.

10. A method as claimed in claim 9, further comprising the step of regenerating the embedded watermark and removing (27,35) said watermark from the descrambled information.

11. A method as claimed in claim 9, in which the scrambled information is organized as a sequence of series of one or more information samples, the method comprising the steps of: generating for each series of the sequence an offset in response to the descrambling key (kl,k2),

- restoring (27) the original signal samples of said series in accordance with said offset.

12. A method as claimed in claim 11, wherein said scrambled information is a scrambled video frame or picture, each series of information samples constituting a line of said scrambled video signal, and the sequence of series constituting said video frame or picture.

13. A method as claimed in claim 11, wherein the descrambling key retrieved from the current sequence of information samples is used to descramble the next sequence of information samples.

14. A method as claimed in any one of claims 8-13, wherein the scrambled information is an analog video signal.

15. An arrangement for scrambling information (X) using a scrambling key

(kl,k2), characterized in that the arrangement comprises means (300, 400) for embedding said scrambling key in the scrambled information (X').

16. An arrangement as claimed in claim 15, wherein said embedding means comprises a watermark embedder (300) having said scrambling key as payload of the embedded watermark (W).

17. An arrangement as claimed in claim 16, in which the information is organized as a sequence of series of one or more information samples, comprising: an offset generator (200) generating for each series of the sequence an offset in response to the scrambling key (kl,k2), - the watermark embedder (300) being arranged to generate for each series of the sequence a sample of a sequence of watermark samples constituting said watermark, and - means (100,400) for modifying the signal samples of said series in accordance with said offset and watermark sample.

18. An arrangement for descrambling scrambled information (Y) using a descrambling key (kl,k2), characterized in that the arrangement comprises means for retrieving said descrambling key from said scrambled information.

19. An arrangement as claimed in claim 18, wherein said retrieving means comprise a watermark detector (23) including a payload decoder for decoding payload of said watermark as said descrambling key.

20. An arrangement as claimed in claim 19, further comprising means (31-34) for regenerating the embedded watermark and means (27,35) for removing said watermark from the descrambled information.

21. A scrambled information signal (Y), characterized in that the scrambled information represented by said signal has embedded therein a descrambling key (kl,k2) for descrambling said scrambled information.