EP0906696A1

EP0906696A1 - Method for scrambling and/or descrambling an image signal respecting the signal signature

Info

Publication number: EP0906696A1
Application number: EP97929368A
Authority: EP
Inventors: Rémy POULET; Louis Guillou
Original assignee: Telediffusion de France ets Public de Diffusion; France Telecom SA
Current assignee: Telediffusion de France ets Public de Diffusion; Orange SA
Priority date: 1996-06-17
Filing date: 1997-06-17
Publication date: 1999-04-07
Also published as: FR2749999A1; WO1997049246A1; FR2749999B1

Abstract

The invention discloses a method for scrambling and/or descrambling an image signal respecting the signal signature by forcing at least one address bit of each line according to the signature of this line. Each frame is thus scrambled into at least two distinct sub-frames. In PAL, one bit of the address is forced according to its signature, and in SECAM, two bits are forced. The invention is useful for pay television.

Description

METHOD OF • SCRAMPING AND / OR DE-SCALDING OF AN IMAGE SIGNAL RESPECTING THE SIGNATURE OF THE SIGNAL

DESCRIPTION

Technical area

The present invention relates to a method for scrambling and / or descrambling an image signal respecting the signature of the signal. It finds application in pay TV.

State of the art

Methods are known for scrambling and descrambling an image signal operating by line mixing. In a very general manner, in such methods, on transmission, the lines taken in their natural order are assigned different addresses formed of bits taken in a pseudo-random sequence, the lines are transmitted in a mixed order linked to their respective addresses , and, on reception, the pseudo-random sequence and the addresses of the lines are restored, and the received lines are reordered according to their addresses to restore their natural order.

Among these methods, there is one which simplifies the decoder used for reception in the sense that it makes it possible to use a low capacity memory in the decoders. This process is described in international application WO 91/13517. According to this process, the lines are mixed in the following manner: - on transmission, the useful lines of each frame

(from line 23 to line 309) are distributed in a memory comprising 32 stacks, starting at the bottom of the frame; a pseudo-random generator assigns to each line an address from 1 to 32; for each frame, a variable number of lines are therefore stacked in each of the 32 stacks; the lines are then transmitted to the antenna; the line at the bottom of the pile being emitted in place of the line immediately above it in the pile and the line at the top of the pile being emitted in place of the line at the bottom of the pile for the preceding frame ;

- on reception, each decoder has 32 line registers; the 32 lines received at the end of a scrambled frame are used to combine the 32 line registers to prepare for descrambling of the next frame; the pseudo-random generator assigns an address from 1 to 32 to each line received; when a line enters one of the 32 registers, then, at the same time, the line contained in this register leaves it to be displayed on the screen.

Although satisfactory in certain respects, this method has a drawback. Indeed, the quality of a scrambling and descrambling process is assessed above all by the resistance it offers to hacking attempts. If it is possible for a specialist to find, from the scrambled signal, the pseudo-random sequence having served for the transmission to scramble the signal, it is clear that the process will be vulnerable. However, the method described in international application WO 91/13517 reveals information which can be very useful to whom wants to attack the system. This revelation of information is carried out through the signature of the signal, a concept that should be clarified.

The first color television system used NTSC encoding. In this system, all the lines of the image signal comprise a reference burst characterized by the same phase and the same frequency (3.579545 MHz). There is only one type of lines. The NTSC has some weaknesses in color rendering, however.

To remedy this, other systems have been proposed such as PAL ("Phase Alternative Line") or SECAM

("Sequential to Memory"), with more elaborate reference bursts aimed at improving color reproduction. In PAL, the frequency of the reference bursts is fixed at 4.44 MHz, but each reference burst is characterized by a phase equal to either + 135 ° or -135 °, alternating from one line to another. There are therefore two types of lines. In SECAM, each reference burst is characterized not only by a phase which can take two values but also by a frequency which can take two values. There are therefore four types of lines. The frequency is 4.40 MHz for red and 4.25 MHz for blue, alternating from one line to another. The phase is 0 or π, with the sequence 0-0-τr for the odd frames and 7C - 71 -0 for the even frames.

Table 1 summarizes the signature of the image signal in SECAM. Table 1

To return to the scrambling and descrambling process described in WO 91/13517, it does not concern itself with the signature of the stirred lines. However, the reference burst of each line received can be analyzed and the frequency and / or phase can be obtained. For lack of precaution to respect the signal signature, the analysis of each useful line received therefore discloses to the attacker up to 1 bit of information in PAL (the parity bit) and up to 5/3 bit in SECAM (1 bit of frame parity and 2/3 of a bit for the phase sequence). This revealed information weakens the system and makes hacking more

The object of the present invention is precisely to remedy this drawback.

Statement of the invention

To this end, the invention recommends respecting the signature of the image signal both on transmission and on reception. In cryptology, it is good practice not to modify the systematic redundancy of the signals in order to avoid giving, thereby, an advantage to the attacker who would seek to break the system. For example, if a clear byte is systematically even (or odd), it must remain even (or odd) after encryption by OU-exclusive with an even encrypting byte so that the analysis of the parity of the encrypted signal reveals nothing of the encrypting byte. According to the invention, this principle is also applied to the signature of the image signal in the context of scrambling and descrambling systems by line mixing.

The method of the invention applies not only to the method described in WO 91/13517, but to any method based on the principle of line mixing.

Specifically, the subject of the present invention is a method of scrambling a television image signal, in which the lines taken in their natural order are assigned different addresses formed by bits taken from a pseudo-random sequence, transmits the lines in a shuffled order linked to their respective addresses, this process being characterized in that, each line signal having a particular signature defined by a reference burst having a frequency and / or a phase depending on the line, imposes at least one of the bits of the address of each line according to the signature of this line, each frame thus being scrambled into at least two distinct sub-frames.

The subject of the invention is also a method of descrambling a scrambled television image signal according to the method which has just been defined and in which the scrambled signal is received, the pseudo-random sequence and the addresses of lines are restored and the received lines are reordered according to their addresses to restore their natural order, this process being characterized in that at least one of the bits the address of each line corresponds to the signature of this line, the lines being thus reordered taking into account their signature.

Brief description of the drawings

- Figure 1 schematically illustrates the principle of descrambling;

- Figure 2 shows an address with a forced bit; - Figure 3 shows an address with two forced bits.

Detailed description of embodiments

Figure 1 schematically illustrates the principle of descrambling implemented at reception. We see, in this figure, a received frame 10, a memory 20 comprising, for example, 32 registers capable of each receiving a line signal, and a displayed frame 30. To display any line L in its correct place in the image, you have to go read in memory 20 the appropriate register at address x. To read this register, the line L ′ of the frame received must be written there, writing to this register having, in effect, the effect of driving out the line previously written in this register. The line L 'received must have the address x.

If one wishes to respect the signature of the lines, for example their parity (in the PAL system), a line having a given parity can only be addressed to a register already containing a line having this parity. In other words, we do not really have a choice of an address among 32, but only one address among 16, namely the 16 addresses assigned to this line parity.

This is the reason why the memory 20 of FIG. 1 is symbolically divided into two parts, a part 20ι containing registers receiving only odd lines and a part 20 ₂ containing registers receiving only even lines.

If a stack memory system is used on transmission, as described in the document WO 91/13517 already cited, to prepare the transmission of each frame, a subset of stacks must be assigned to each type of line. In each stack, the stacked lines will all have the same parity. The frame will thus be scrambled into two separate subframes. The battery addresses will be divided into two categories.

On reception, all lines passing through the same line register will have the same parity. Then, the phase indication in PAL (or the only frequency indication in SECAM) will not disclose any useful information to the attacker since the parity bit will only conform the information that can be obtained by analyzing the burst of reference.

In an exemplary embodiment, in each address used for transmission and for reception, one of the bits of the address, always the same for a given frame, is forced to 0 or to 1 depending on the parity of the number of line. This is illustrated in Figure 2 where

the address is assumed to include five bits (2 = 32) and where the first bit BO is forced to 1 or 0 depending on the parity of the line. The other four bits B1, B2, B3 and B4 are subsequently determined pseudo-random. The first BO bit indicates which of the two battery groups (on issue) or registers (on receipt) should be addressed.

To guard against the use of the signature in the case of SECAM, according to the invention, four types of lines are considered, each type being characterized by RO, R7T, BO or B π. The frame is thus scrambled into four distinct subframes.

When implementing the principle of stacks to prepare the transmission of each frame, we must assign a subset of stacks to each type of line. In each stack, the stacked lines are all of the same type, the frame being thus scrambled into four separate subframes. Battery addresses are divided into four categories. On reception, all the lines which pass through the same line register are of the same type. Then, the signature of SECAM does not disclose any useful information to the attacker.

In a first example of implementation of this variant adapted to SECAM, the addresses comprise, on the one hand, three bits whose value comes from a pseudo-random generator, and on the other hand, two bits, always the same for a given frame, the value of which is fixed by the number of the line in the frame. The following three rules allow you to set the value of the two bits:

- to the lines numbered from 0 to 3 modulo 6, any permutation of the values 00, 01, 10 and 11 is assigned;

- to the lines numbered 4 modulo 6, the value already assigned to 2 modulo 6 is assigned;

- to the lines numbered 5 modulo 6, the value already assigned to 1 modu ¹ o 6 is assigned. / 49246 PC17FR97 / 01091

Table 2 summarizes these rules

Table 2

In a second embodiment, the batteries are distributed on transmission and the line registers on reception, taking a little better account of the distribution of the lines according to their types. We assign six addresses to the 48 lines numbered 0 modulo 6. We assign ten addresses to the 96 (or 95) lines numbered 1 or 5 modulo 6. We assign ten addresses to the 95 or 96 lines numbered 2 or 4 modulo 6. We assign six addresses to the 48 lines numbered 3 modulo 6. These rules are illustrated in Table 3.

Table 3

Then, on transmission and on reception, a pseudo-random generator fixes for each line of type 0 modulo 6 an address parrrυ 6, for each line of type 1 or 5 modulo 6 address among 10, for each line of type 2 or 4 modulo 6 one address among 10 and for each line of type 5 modulo 6 one address among 6.

More generally, a control word is used on transmission and on reception to initialize the pseudo-random generator. Transmitted by the conditional access system, this word is used to initialize and configure the pseudo-random generator of the receiver. In the first example, it can advantageously also affect the permutation of the values from 0 to 3 in table 2 as well as the manner of combining the 5 address bits. In the second example, it can advantageously affect the distribution of the values from 1 to 32 in Table 3.

Claims

1. Scrambling method of a television image signal, in which the lines taken in their natural order are assigned different addresses formed by bits taken in a pseudo-random sequence, the lines are transmitted in a shuffled order linked to their respective addresses, this process being characterized in that, each line signal having a particular signature defined by a reference burst having a frequency and / or a phase depending on the line, at least one of the bits of the address of each line according to the signature of this line, each frame thus being scrambled into at least two distinct sub-frames.

2. Method according to claim 1, characterized in that, the television system being the PAL system, where the reference burst linked to a line has a phase which can take two distinct values according to the parity of the line, it is necessary to one of the bits of the address of a line the value O or 1 according to the parity of this line, each frame being thus scrambled into two sub-frames.

3. Method according to claim 1, characterized in that, the television system being the SECAM system, where the reference burst linked to a line has a frequency and a phase which can each take two distinct values depending on the nature of the line and the parity of Ja frame, we impose on two of the bits of the address of a line any one of the four values 00, 01, 10 and 11, each frame being scrambled into four separate subframes.

4. Method according to claim 3, characterized in that the value of the two imposed bits is fixed by the number of the line in the frame according to the following three rules:

- to the lines numbered 0 to 3 modulo 6, any value 00, 01, 10 or 11 is assigned, - to the lines numbered 4 modulo 6, the value already assigned to the line numbered 2 modulo 6 is assigned,

- to the lines numbered 5 modulo 6, the value already assigned to the line numbered 1 modulo 6 is assigned.

5. Method according to claim 1, in which a memory comprising N stacks is constituted, the addresses assigned to each line ranging from 1 to N, the rows being distributed in these N stacks starting with the bottom of the frame, a number variable of lines being thus stacked in each of the N stacks for each frame, the method then being characterized in that the part of the address which is imposed by the signature of the line, addresses each line in a part of the N stacks , each of these parties always receiving lines with the same signature.

6. Method according to claim 5, characterized in that the number N of batteries being equal to 32, the addresses include 5 bits.

7. Method according to claim 6, characterized in that, in the PAL system, four bits of the address are subsequently determined pseudo-randomly and one is imposed by the line parity.

8. Method according to claim 6, characterized in that, in the SECAM system, three bits of each address are subsequently determined pseudorandom and two are imposed by the signature of the line.

9. Method for descrambling a scrambled television image signal according to claim 1, in which the scrambled signal is received, the pseudo-random sequence and the line addresses are restored and the received lines are reordered according to their addresses to restore their natural order, characterized in that at least one of the bits of the address of each line corresponds to the signature of this line, the lines being thus reordered taking into account their signature.

10. The method as claimed in claim 9, in which a single memory of N registers is used, each capable of memorizing a line, each line written in a register causing the output of the line previously written in this same register and its display on a screen, characterized by the fact that the part of the address, which is imposed by the signature of the line, addresses each line in a part of the N registers of the memory, each part of the memory always receiving lines of the same signature.