METHODS FOR RECEIVING AND SENDING A MULTIMEDIA CONTENT, RECEIVER, SENDER AND RECORDING CARRIER AND SECURITY PROCESSOR FOR THIS METHOD

The invention pertains to a method for receiving and a method for sending a first multimedia content whose display is conditional on the prior display of second multimedia contents. An object of the invention is also a receiver, a sender, an information recording carrier and a security processor for implementing these methods.

A multimedia content is a content containing audio and/or video. This
multimedia content is designed for example to be played on a television set. For
example, a multimedia content is a film, an audiovisual program or a piece of
music.

A receiving method of this kind is known from the US patent 2007 294 773 filed on behalf of Microsoft®. More specifically, the Microsoft application describes a method for receiving a first multimedia content , i.e. Video On Demand, and the de-scrambling of this video on demand is conditional on the preliminary display of a second multimedia content, namely in this case advertising material. Video On Demand is better known as VOD.

In the Microsoft method, a receiver delivers a license by which video on demand can be deciphered only if all the advertisements have first of all been viewed. Thus, the viewing of video on demand is conditional on the prior viewing of the advertisements.

Furthermore, this receiver is equipped with various means to prevent the user from short-circuiting or switching off advertisements. For example, the receiver is equipped with means to cancel fast forward operations.

This prior-art method therefore forces the television viewers to view all the advertisements that immediately precede the video on demand.

However, the field of application of this prior-art method is limited to video on demand. Indeed, if this method were to be transposed to broadcast television then, a viewer who inadvertently happened to miss the start of the advertisements immediately preceding the show that he wishes to watch would be deprived of the possibility of viewing this show. Such an operation is unacceptable for a broadcast television system.

The term "broadcast television" herein designates the simultaneous broadcasting of a same multimedia content to numerous receivers. Unlike video on demand, the user in broadcast television cannot decide on the time at which the broadcasting of the multimedia content will begin.

It is an object of the invention therefore to propose a method for receiving a first multimedia content the viewing of which is conditional on the prior viewing
of second multimedia contents which may work in the field of broadcast television.
An object of the invention therefore is a method of this kind in a context where the second multimedia contents are the only contents to be associated with instructions on the progression of the value of a same counter, these instructions on progression modifying all the values of this same counter in the same sense when they are executed by an electronic receiver of the first and second multimedia contents. More specifically, this method comprises:

the modification, whenever a second multimedia content is viewed by
means of the electronic receiver, of the value of the counter as a function
of the instructions on progression associated with this second multimedia
content,

the storage of the value of this counter between non-consecutive
viewings of second multimedia contents interspersed with the viewing of
another multimedia content that does not modify the value of the counter,
and

the making of the full viewing of the first multimedia content conditional on a predetermined value of the counter. It is assumed here that the above method is implemented in the field of broadcast television for a television show that is sought to be made conditional on the prior viewing of advertisements. We also assume that the instructions on progression increment the value of the counter when they are executed by the receiver. In this context, even if the user misses the beginning of the advertisements immediately preceding this television show, he can nevertheless be permitted to regard it. Indeed, in the above method, whenever the user watches advertisements, the value of the counter is incremented. It is not necessary for the advertisements watched to immediately precede the television show since the value of the counter is memorized. For example, the user can watch a first sequence of advertisements and then change channels to watch a film. The viewing of this film will not modify the value of the counter. Then, some time later, the user may look at another sequence of advertisements which continues to increment the value of the counter. The counter is therefore incremented without being reset even if the viewing of the advertisements is interspersed by the viewing of other multimedia contents.

This means that even if he starts watching the sequence of advertisements that immediately precedes the television show that he wishes to regard,' the value of the counter can have already reached a value sufficient to authorize the viewing of this television show. Thus, even if he misses the sequence of advertisements immediately preceding the television show, he is nevertheless authorized to watch it.

In the above method, it is not necessary for the second multimedia content to immediately precede the first multimedia content. This makes it possible to use this method in the field of broadcast television.

The embodiments of this method of reception may comprise one or more of the following characteristics:

■ before the viewing of each second multimedia content, the method comprises the demultiplexing of a stream of received multiplexed data containing the second multimedia content multiplexed with the instructions on progression and the modification of the value of the counter as a function of the instructions on progression multiplexed with this second multimedia content;

■ the method comprises the selection of the counter, the value of which must be modified from among several possible counters as a function -of an identifier of this counter associated with the instructions on progression;

■ the method comprises:

- the reception of a group of one or more crypto-periods of the second multimedia content and instructions on progression associated with this group, each crypto-period of the second multimedia content being scrambled with a respective control word, and

- the modification of the value of the counter whenever all the crypto-periods of this group are descrambled by the electronic receiver;

■ before a crypto-period of the second multimedia content, the method comprises the reception of a message broadcast simultaneously to all the receivers capable of receiving the first and second multimedia contents, this message containing a cryptogram CW* of the control word needed to descramble this crypto-period and the instructions on progression associated with the group to which this crypto-period belongs;

■ whenever the viewing of the first multimedia content by means of the electronic receiver is authorized, the method comprises the modification of the value of the counter in a sense opposite to that of the instructions on progression so make the viewing of a following multimedia content conditional on the prior viewing of other second supplementary multimedia contents;

• the method comprises:

- the reception of a group of one or more crypto-periods of the first
multimedia content and instructions on regression of the value of the counter associated with this group, each crypto-period being scrambled with a respective control word,

- whenever all the crypto-periods the group are descrambled, the value of the counter is modified as a function of the instructions on regression associated with this group in a sense opposite to that of the instructions on progression, and

- the prohibiting of the descrambling of a following group of crypto-periods of the first multimedia content as soon as the value of the counter reaches a predetermined threshold.

■ the method comprises:

- the reception of the first scrambled multimedia content and of conditions of access to this first multimedia content,

- the prohibiting of the descrambling of the first multimedia content if no right of access corresponding to the conditions of access is recorded in the electronic receiver, and on the contrary

- the authorizing of the descrambling of the first multimedia content if a right of access corresponding to the conditions of access to this first multimedia content is recorded in the electronic receiver, and

- the writing of an access right in the receiver corresponding to conditions of access as soon as the value of the counter to be crossed has crossed a predetermined threshold;

■ the method comprises the limiting of the value of the counter to a predetermined threshold;

■ the method comprises the automatic creation of the counter in response to the reception of instructions on progression of the value of this counter.
The embodiments of the above method of reception furthermore have the following advantages:

- multiplexing the second multimedia contents and the instructions on progression makes it possible to associate these instructions on progression simply with the second multimedia contents,

- using a counter identifier enables the simultaneous use of several counters associated with respective multimedia contents and therefore, for example, makes the viewing of a third multimedia content conditional on the prior viewing of fourth multimedia contents, the third and fourth contents being distinct and independent of the first and second multimedia contents,

- making the modification of the value of the counter conditional on the descrambling of one or more crypto-periods increases the security of the system,

- modifying the value of the counter in a sense opposite that of the instructions on progression during each viewing of a first multimedia content again enables the viewing of a next first multimedia content conditional to be made conditional on the prior viewing of second additional multimedia contents,

- modifying the value of the counter as a function of the instructions on regression as and when the first multimedia content is viewed authorizes a duration of viewing of the first multimedia content that depends on the quantity of second multimedia contents already viewed,

- a recording of a right of access authorizing the full viewing of the first multimedia content when a predetermined threshold is attained by the value of the counter authorizes the viewing of the first multimedia content for a predetermined duration,

- imposing a maximum limit on the value of the counter imposes an alternation between the viewing of the second multimedia contents and the first multimedia contents;

- automatically creating the counter in response to the reception of instructions on progression of this counter prevents the need to initialize this counter when the receiver is put into operation or by means of specific commands sent out by the sender.

An object of the invention is also an information-recording carrier
comprising instructions for executing the above method when these instructions
are executed by an electronic computer.

An object of the invention is also a method for sending a first multimedia
content the viewing of which is conditional on the prior viewing of second
multimedia contents, wherein the method comprises:

the sending of instructions on progression of the value of the counter
associated with each second multimedia content sent,

the sending, between two second multimedia contents, of other
multimedia contents not associated with instructions for modifying the
value of this counter, and

the sending of conditions of access associated with the first multimedia
contents sent, these conditions of access making the full viewing of the
first multimedia content conditional on a sufficient value of the counter.

The embodiments of the sending method may comprise the following
characteristic:

■ the method comprises the sending of instructions on regression of the value of the counter associated with the first multimedia content sent, these instructions on regression modifying the value of the same counter in a sense" opposite that of the instructions on progression when the viewing of the first multimedia content is authorized.

An object of the invention is also a receiver of a first multimedia content the viewing of which is conditional on the prior viewing of second multimedia contents, wherein the receiver comprises:

a memory capable of storing the value of the counter between non-consecutive viewings of second multimedia contents interspersed with the viewing of another multimedia content that does not modify the value of the counter, and an electronic computer capable of:

• modifying the value of the counter as a function of the instructions on progression associated with the second multimedia contents received whenever a second multimedia content is viewed by means of the receiver, and

• making the full viewing of the first multimedia content conditional on a sufficient value of the counter.

An object of the invention is also a security processor capable of being implemented in the above receiver.

Finally, an object of the invention is also a sender of a first multimedia content for which the viewing is conditional on the prior viewing of second multimedia contents, this sender being capable of:

sending instructions on progression of the value of the counter associated
with each second multimedia content sent,

sending, between two second multimedia contents, multimedia contents
not associated with instructions on progression of the value of this counter, and
sending conditions of access associated with the first multimedia content
sent, these conditions of access making the full viewing of the first multimedia content conditional on a sufficient value of the counter.

The invention will be understood more clearly from the following description given purely by way of a non-restrictive example and made with reference to the appended drawings of which:

Figure 1 is a schematic illustration of a the architecture of a system for broadcasting scrambled multimedia contents,

Figure 2 is a schematic illustration of a grid of the programs used in the
system of figure 1,

Figure 3 is a schematic illustration of the architecture of a counter used in
the system of figure 1,

Figure 4 is a flowchart of a method for sending and receiving first multimedia contents for which the viewing is conditional on the prior viewing of second multimedia contents,

Figure 5 is a schematic illustration of the architecture of an ECMcp
message used in the system of figure 1,

Figure 6 is a schematic illustration of an ECMCNP message used in the system of figure 1,

Figure 7 is a schematic illustration of a timing diagram showing the progression of the value of a counter when the method of Figure 4 is applied,

Figure 8 is a schematic illustration of the architecture of another embodiment of an ECMCP message,

Figure 9 is a flowchart of another embodiment of a method for sending and receiving a first multimedia content, the viewing of which is conditional on the prior viewing of other multimedia contents, and

figure 10 is a schematic illustration of the architecture of an ECMCNP
message used in the method of figure 9.

In these figures, the same references are used to designate the same elements.

Here below in this description, the characteristics and functions well known to those skilled in the art are not described in detail. Furthermore, the terminology used is that of systems of conditional access to multimedia contents. For more information on this terminology, the reader will refer to the following document:

"Functional Model of Conditional Access System" EBU Review -Technical European Broadcasting Union, Brussels, BE, No. 266, 21 December 1995.

Figure 1 shows a pay television system 2 for the broadcasting of multimedia contents. For example, the system 2 is a system used to broadcast several scrambled television channels. The descrambling of each of these television channels or groups of television channels is generally conditional on the payment of a subscription by subscribers.

The system 2 has at least one sender 4 of scrambled multimedia contents and a multitude of receivers capable of descrambling the multimedia contents broadcast by the sender 4. To simplify figure 1, only the receiver 6 has been represented. For example, the other receivers are identical to the receiver 6.

The sender 4 has a scrambler 10 which is able, by means of a control word CW, to scramble multimedia contents received in cleartext through a port 12 as and when they are received. To this end, the scrambler 10 uses the control word contained in a memory 14. The scrambled multimedia content is transmitted to the input of a multiplexer 16.

The scrambling of the multimedia content is an encipherment operation and the control word is the encipherment key used to scramble the multimedia content.

The sender 4 also has a generator 18 of control words. The generator 18 regularly modifies the control word contained in the memory 14. The time span between two modifications of the control, word used to scramble a multimedia content is called a crypto-period.

A generator 20 generates a new ECM (entitlement control message) . message whenever the generator 18 generates a new control word. Each ECM message furthermore has:

a CW* cryptogram of the control word, and conditions of access to the scrambled multimedia content.

The cryptogram CW* is a piece of information that is necessary but insufficient by itself for scrambling the multimedia content. For example, the cryptogram CW* is obtained by enciphering the control word with a service key known to the receiver or a reference to a control word or to a cryptogram to the control word contained in the receiver.

The conditions of access are designed to be compared with rights of access pre-recorded in the receiver. The descrambling of the multimedia content is authorized only if the conditions of access with which it is associated correspond to pre-recorded rights of access. For example, a condition of access may be a moral level associated with the multimedia content.

The ECM generated by the generator 20 is transmitted to the input of the multiplexer 16.

Here, the generator 20 is connected to a memory 22 containing a grid 24 of programs.
The multiplexer 16 enables the multiplexing of the scrambled multimedia content with the ECM messages containing the CW* cryptograms needed to descramble this multimedia content. The multiplexer 16 thus creates a stream of multiplexed data broadcast to all the receivers through an information transmission network 30. This multiplexing enables the temporal synchronizing of the sending of the ECM messages when the scrambled multimedia content is sent.

For example, the network is a packet-switched network such as the Internet or a satellite television broadcasting network.

The receiver 6 has a decoder 40 provided with an antenna 42 to receive the multimedia contents broadcast by the sender 4. The decoder 40 also has a demultiplexer 44 which demultiplexes the' multiplexed data streams received in order to transmit the scrambled multimedia content to a descrambler 40 and the ECM type messages to a security processor 48.

The processor 48 is connected detachably to the decoder 40. For example, the processor 48 is a chip card.

This processor 48 has an electronic computer 50 capable of processing each ECM message to decipher the cryptogram CW* and obtain the control word CW enabling the multimedia content to be descrambled. The processor 48 is capable of transmitting the control word CW to the descrambler 48.

The computer 50 is connected to a memory 52 containing instructions for the execution of the method of figure 4 when it executes instructions. Furthermore, here, this memory 52 has pre-recorded rights of access 54 and several counters. In figure 1, only one counter 56 has been shown. The structure and use of the other counters is deduced from the following description pertaining to the particular case of the counter 56.

The receiver 6 is connected to a television set 60 capable of displaying the descrambled multimedia content by means of the decoder 40.

Figure 2 represents an example of a. structure of the grid 24. This grid here is a table containing the following columns:

a column CM-ID design to contain an identifier of a multimedia content;

a column W-ID designed to contain an identifier of a counter of the
receiver 6, a column Nb-TK designed to contain a number of steps for incrementing
the value of the counter, a column TK-C designed to contain a number of steps for decrementing the current value of the counter, a column l-TK-A designed to contain the identifier of the counter to be created, and

a column A-Max designed to contain a maximum threshold for the value
of the counter.

The increment or decrement steps can also be called "points" or "tokens".
This grid 24 therefore enables the association with a multimedia content designated by the identifier of the column CM-ID of certain number of information elements concerning a counter to be incremented, decremented or created in the receivers.
Figure 3 represents an example of a structure for the counter 56. Here, this counter 56 comprises:

a field W-ID designed to contain the identifier counter 56,

a field TK-A designed to store current value of the counter 56,

a field A-Max designed to contain a maximum threshold for the value of
the counter 56, and

a field LUD designed to contain the date of the last update of the value of
the counter 56.

For example, the date is expressed in terms of the day, the hour, the minute and the second.

The working of the system 2 shall now be described with reference to the method of figure 4 in the particular case where it is sought to make the viewing of the non-promotional contents conditional on the prior viewing of the promotional contents. For example:

the non-promotional content is a film or a television show and
the promotional content is a sequence of advertisements.

The system 2 starts with a phase 68 for sending scrambled multimedia contents.
Initially, before the broadcasting of the multimedia contents, during a step 70, for each promotional or non-promotional multimedia content to be broadcast, the grid 24 is filled in with information provided by an operator. More specifically, for each promotional content the columns CM-ID, W-ID, Nb-TK, l-TK-A and A-Max are filled in with information and the column TK-C is empty. For example here, for each promotional content, the columns CM-ID, W-ID, Nb-TK, l-TK-A and A-Max have respectively the following values: CP1, C1, "10", "10" and "30". The identifier CP1 is the one associated with each promotional content and the identifier C1 is that of the counter 56.

For each non-promotional content, the columns CM-ID, W-ID and TK-C are filled and the columns Nb-TK, l-TK-A and A-Max are not filled. Here, the groups Nb-TK, l-TK-A and A-Max respectively have the following values: CNP1, C1 and "1". The identifier CNP1 is the one associated with each non-promotional content.

Then, during the broadcasting of each crypto-period of a multimedia content, at a step 74, the generator 20 creates an ECM message containing:

a cryptogram CW* of the control word CW used by the scrambler 14 to scramble the multimedia content during the next crypto-period, as the case may be, the condition or conditions of access needed to authorize the descrambling of this multimedia content, and as the case may be, instructions on the progression or regression of the
counter 56.

Here, an ECM message containing the instructions on progression is denoted as ECMcp. An ECM message containing regression instructions is denoted ECMCNP. An ECM message which has neither any instruction on regression nor any instruction on progression of the counter 56 is denoted ECMN More specifically, the generator 20 creates a ECMcp message when the multimedia content being broadcast is a promotional multimedia content.

One example of a possible structure for this ECMCP message is shown in figure 5. This ECMCp message contains the following fields:

-a field HD which is a header field for the message ECMCP containing all the data enabling the use of this message such as for example an index of a service key to decipher the cryptogram CW* and/or an identifier of a concerned service zone in the security processor, a field CW* containing the cryptogram CW* of the control word CW used to scramble the promotional content, a field D containing the current date,
a zone AC containing, as the case maybe, one or more conditions of access to the promotional content, a zone CU containing instructions on progression, a zone WCR containing instructions on creation of the counter, and a field S containing a signature of the ECMCP message. Typically, in a message ECMcp, there are no conditions of access pertaining to the value of the counter 56. The zone CU comprises:

- a W-ID field intended for containing the identifier of the counter to be
incremented, and

- a field Nb-TK containing the number of steps by which the value of the
counter must be incremented.

For example, the increment step is equal to "1". The zone WCR comprises:
- a field l-TK-A containing the initial value of the counter to be created, and
- a field A-Max containing a maximum threshold for the counter to be created.

The signature contained in the field S is used to verify the integrity of the ECMCP message during its reception. For example this signature is obtained by means of a hash or a MAC (Message Authentication Code) applied to the content of the ECMcp. message.

To complement the value of the fields of the CU and WCR zones, the generator 20 uses the identifier of the broadcast multimedia content and the content of the grid 24. For example, the fields W-ID, Nb-TK, l-TK-A and A-Max are respectively the following values: C1, "10", "10" and "30".

Figure 6 shows an example of a structure for an ECMCNP message associated with each broadcast non-promotional content. The structure of the ECMCNP is identical to that of the ECMCP message except that:

- it does not comprise zones CU and WCR, and that
- the zone AC contains a condition of access which depends on the value of the counter 56.

Here, this access condition is encoded, by means of the following fields:
- a field A-CND1 intended for containing an identifier of the access condition,

- a field W-ID intended for containing an identifier of the counter whose value must be verified, and

- a field TK-C intended for containing the instructions on regression of the value of the counter.

Here, the regression instruction corresponds to a number of decrement steps that must be diminished by the current value of the counter. The decrement step is chosen to be equal to "1".

As in the case of the ECMCNP, the values of the fields W-ID and TK-C are obtained by means of the identifier of the non-promotional content and the content of the grid 24. Here, the fields W-ID and TK-C respectively contain the values C1 and "1".

The zone AC may contain other conditions of access. For example, in figure 6, the first field A-CND2 of a second condition of access has been shown.

The structure of an ECMN message may be either devoid of fields containing instructions on progression or regression of the value of a counter or comprise instructions on progression and regression of the value of a counter different from the counter 56. The structure of the message is therefore deduced from the structures of the ECM messages described here above.

In parallel with the step 74, during a step 82, each crypto-period of the multimedia data content broadcast is scrambled by means of the current control word CW.

Then, in a step 84, the scrambled multimedia content and the ECM messages associated with it are multiplexed to create the stream of multiplexed data.

In a step 86, this stream of multiplexed data is broadcast by the sender 4 through a network 30 to receivers.

The step 74, 82, 84 and 86 are reiterated in a loop for each crypto-period in order to broadcast the successive crypto-periods of the multimedia contents.

At the same time, each receiver performs a phase 90 of reception of these scrambled multimedia contents.

Initially, in a step 92, the decoder 40 receives and demultiplexes the received stream of multiplexed data. The scrambled multimedia content is then sent to the descrambler 46 while the ECM messages are sent to the processor 48.

In a step 94, the computer 50 verifies the signature S of the ECM message received. Should the ECM message be corrupt, the computer 50 proceeds to a step 96 for inhibiting the descrambling of the multimedia content. For example, during the step 96, the processor 48 inhibits the deciphering of the cryptogram CW* so that the descrambling of the multimedia content is made impossible. At the step 96, the viewing of the multimedia content in cleartext form is therefore not authorized.

If not, at a step 98, the computer 50 compares the conditions of access contained in the ECM message received with the rights of access 54. At the step 98, the conditions of access defined by means of the field TK-C are not assessed.

If the memory 52 contains no right of access corresponding to conditions of access, then the computer 50 proceeds to the step 96.

If not, it proceeds to a step 100 for verifying the existence of a counter identifier in the ECM message received. This identifier is sought either in the field W-ID of the zone CU or in the field W-ID of the zone AC.

Should the ECM message contain no counter identifier, then the viewing of this multimedia content is not conditional on the prior viewing of promotional contents. For example, it may be a message ECMN without any zone CU and conditions of access bringing the value of a counter into effect. In this case, the operation precedes directly to a step 102 in which the computer 50 deciphers the cryptogram CW* to obtain the control word CW and sends it to the descrambler 46. Then, the descrambler 46 descrambles the following crypto-period of the multimedia content received by means of this control word and transmits the descrambled multimedia content to the television set 60 which displays it in cleartext. Thus, the viewing of this multimedia content is authorized since the multimedia content is displayed in a form that is directly perceptible and comprehensible to a user.

If not, during a step 104, the computer 50 makes a check to see if the counter identifier found at the step 100 corresponds to a counter already existing 20 in the memory 52. If the answer is negative then,, at a step 106, the computer checks to see if a'zone WCR exists in the ECM message received. If the answer is negative, then the processor 48 goes directly to the step 96. If the answer is affirmative then, at a step 108, the computer 50 creates a counter in the memory2 with the structure described with reference to figure 3. The values of the fields W-ID, TK-A, A-Max and LUD of the counter created are initialized from values, respectively, of the fields W-ID, l-TK-A, A-Max and D of the ECM message received. Thus, if the counter 56 were not to exist as yet at the first reception of a non-promotional content, this counter is automatically created at the step 108.

If the counter corresponding to the identifier contained in the W-ID field
already exists in the memory 52 or at the end of the step 108, the computer 50 proceeds to a step 110 for checking the existence of the field Nb-TK in the ECM message received.

If the answer is yes, the ECM message received is an ECMCP message. Then, at the step 112, the computer 50 makes a check to see whether the date contained in the field LUD of this counter is prior to the date contained in the field D of the ECM message received.

If the answer is yes then, at a step 114, the computer 50 increments the value contained in the field TK-A of this counter 56 by the number of steps indicated in the field Nb_TK of the ECM message received. If necessary, at the step 114, the computer limits the incrementing of the value of the counter so as not to go beyond the threshold indicated in the field A-Max. The new value of the counter is then stored up to the next iteration of the step 114.

At the step 114, the field LUD of the counter 56 is also updated on the basis of the date contained in the field D of the ECM message received.

At the end of the step 114 or if the last date for updating the counter 56 is after the date contained in the ECM message received, the processor goes directly to the step 102.

Should the field Nb-TK not exist in the ECM message received, it means that the message is an ECMCNP message. The computer 50 then goes to a step 116 for checking on the existence of the field TK-C in the zone AC of the ECM message received. If the ECM message received effectively contains this field, then it means that the multimedia content currently being broadcast is a multimedia content whose viewing is conditional on the prior viewing of other multimedia contents.

In this case, at a step 118, the computer 50 makes a check to see whether the current value of the counter identified at the step 100 is higher than the value contained in the field TK-C. If the answer is negative, the operation goes directly to the step 96, thus prohibiting the viewing of this multimedia content.

If not, at a step 120, the computer 50 decrements the current value of the identified counter of a number of steps defined by the value contained in the field TK-C of the ECM message received. At the step 120, the date contained in the field LUD of the counter is also replaced by the date contained in the ECM message received.

At the end of the step 120, or if at the step 116 the ECM message does not contain the field TK-C, the operation goes directly to the step 102.

Figure 7 gives an example of the progress of the value of the counter 56.

In figure 7, the topmost band represents the succession in time of a non-promotional content 130, a promotional content 131 and a non-promotional content 132.

A band 134 represents eight successive crypto-periods CP1 to CP8 which cover the time span during which the contents 130 to 132 are received.

A band 136 represents the instants of sending of the ECM messages. Here, each ECM message is transmitted to the receiver during the crypto-period that precedes the crypto-period of the multimedia content with which it is associated. Thus, the messages ECM2 to ECM9 are respectively associated with the crypto-period CP2 to CP9. The value between square brackets represents the value contained in the field TK-C of the ECM messages when this field is negative or the value contained in the field Nb-TK when this field is positive. It can therefore be noted that the ECM5 and ECM6 messages are ECMCP messages associated with the contents 131. The other messages represented are ECMCNP messages.

A band 138 represents the value of the field TK-A of the counter 56. A band 140 indicates in gray the crypto-periods during which the viewing of the non-promotional contents is possible and in black the crypto-periods where such a viewing is not possible.
More specifically, during the crypto-period CP1, the value of the counter 56 is equal to "1". Consequently, the descrambling of the multimedia content during the crypto-period CP2 is authorized because the field TK-C of the ECM2 message is equal to "1". The processing of the message ECM2 during the crypto-period CP1 leads to decrementing the value of the counter 56 by "1" so that this value is "0" during the crypto-period CP2.

Then, during the crypto-period CP2, the value of the field TK-C of the ECM3 message is compared with a current value of the counter 56. This time, the value of the counter 56 is smaller than the value of the field TK-C.

Consequently, the descrambling of the non-promotional multimedia content is
prohibited during the next crypto-period CP3. In this example, the execution of
the method of figure 4 also leads to prohibiting the viewing of the content 130
during the crypto-period CP4.

During the crypto-period CP4, the computer 50 receives the message
ECM5 in which the value of the field MB-TK is equal to "10". Consequently, the value of the counter 56 is incremented by 10 and the viewing of the content 131 is authorized during the crypto-period CP5.

During the next crypto-period CP5, another ECMCP message is received so that the value of the counter 56 is again incremented by "10".

Then, to descramble the content 132, only the ECMCNP messages are received. Thus, the value of the counter 56 is decremented by "1" at the end of each of the crypto-periods CP6 and CP7. However, the viewing of the content 132 is authorized during the crypto-period CP7 and CP8 because the value of the counter 56 is greater than the value contained in the field TK-C of the messages ECM7 and ECM8.

It will therefore be understood that the method of figure 7 authorizes a duration of viewing of a non-promotional content that is a function of the duration of promotional contents viewed beforehand. For example, here the authorized duration of viewing non-promotional content is directly proportional to the duration of the promotional contents viewed. This method therefore encourages users to view the promotional contents while at the same time allowing the user great flexibility in collecting and spending the points in his counter.

Figure 8 represents another possible embodiment for the structure of an ECMCP message. In this embodiment, the ECMcp message is identical to the one described with reference to figure 5 except that it does not have any WCR zone. Thus, in this embodiment, the counters such as the counters 56 must be initialized in the memory 52 either during sending or during the manufacture of the processor 48 or by means of an EMM (Entitlement Management Message).

Another embodiment of the method of figure 4 shall now be described with reference to figures 9 and 10.

The method of figure 9 is identical to the method of figure 4 except that:

- at the step 74, the structure of the ECMCP message created is the one shown in figure 10,

- the steps 110, 116, 118 and 120 are eliminated, and

- steps 162 and 164 are introduced after the step 114.

The structure of the ECMcp message shown in figure 10 is identical to that of the ECMCP message shown in figure 5 except that the zone WCR is replaced by a zone Rl-C. The zone Rl-C contains instructions for the writing to the memory 52 of a new right of access if the value of the counter goes beyond a predetermined threshold S1. This new right of access corresponds to a condition of access to the non-promotional content. More specifically, the zone Rl-C contains:

- a field TK-S intended for containing the value of the threshold S1, and

- a value Rl containing the right of access to be recorded in the memory 52 when the threshold S1 is attained.

Preferably, in this embodiment, the ECMCNP message is devoid of regression instructions. Thus, the structure of the ECMCNP message is in no way distinguished from that of a conventional ECM message.

After having incremented the value of the counter at the step 114, at the step 162 the computer 50 makes a check to see if the new value of the counter has reached the threshold S1 contained in the field TK-S. If the answer is negative, the invention proceeds directly to the step 102. If the answer is affirmative, at the step. 164 the computer 50 writes the right of access contained in the field Rl to the memory 52. Then, again at the step 164, the value of the counter 56 is reset. For example, the value of the counter is reset at zero or decremented by the value S. Rather than resetting the value of the counter, it is also possible to increment the value of the threshold S1 by a determined step value.

In this embodiment, each non-promotional content is associated with a condition of access A-CND. So long as the memory 52 has no right of access corresponding to this condition of access, the viewing of the non-promotional content is prohibited. When the value of the counter reaches the threshold S1, then the right of access corresponding to this condition A-CND is automatically recorded to the memory 52. Consequently, the viewing of the next non-promotional content received is authorized. As in the embodiment of figure 4, this encourages viewers to view the promotional contents. Furthermore, the recording of a right of access enables a precise setting of the duration of viewable non-promotional content. For example, the new right of access recorded in the memory 52 authorizes the viewing of the channel of the current day and of the next day only.
Many other embodiments are possible. For example, the viewing of the non-promotional multimedia content may be prevented by means other than scrambling of this content. In this case, the methods described here above may be applied in a context where the first and second multimedia contents are not scrambled.

As a variant, the antenna 42 is replaced by a network adaptor when the decoder receives the multimedia contents by means of a cable. For example, the cable is an Ethernet cable.

Here, the identifier of the counter enables the use and implementation of the methods of figures 4 and 9 for several distinct non-promotional multimedia contents so as to submit the viewing of one of these non-promotional multimedia contents to the value of a particular counter. In one particular embodiment, the counter identifier may be a function of the number of the channel viewed.

However, as a variant, only one counter can be used for all the non-promotional contents. In this case, the identifier of the counter may be omitted.

The methods of figures 4 and 9 can be modified to authorize the incrementation of the counter only when a group of several consecutive crypto-periods of the promotional content has been entirely viewed. To this end, an ECM message comprises an indication of the start of a group of crypto-periods and then a subsequent ECM message broadcast at the end of this group of crypto-periods comprises an indication of an end of the group of crypto-periods. Furthermore, a crypto-period group identifier and a counter of continuity of the crypto-periods of the group are inserted into each ECM message received during this group of crypto-periods. From this information, the computer 50 ascertains that the crypto-periods of this group have all been received immediately one after the other. If the answer to this check is affirmative, the value of the counter is incremented by the number of steps indicated in the field Nb-TK. If the answer is negative, i.e. if one of the crypto-periods has not been descrambled, the value of the counter remains unchanged. By synchronizing this group of crypto-periods in time or temporally with a promotional content, it is possible to encourage the user to watch the entire promotional content without changing channels in the middle.

Another method used to subject the incrementation of the counter 56 to the viewing of an entire segment of promotional content extending over several crypto-periods consists in reporting the start and end of this group of crypto-periods and adding, to each ECM message associated with this group, a fragment of a signature of a local EMM message containing the date, the identifier of the counter and the value of the Nb-TK field for which this counter must be incremented. Before incrementing the counter corresponding to the identifier contained in this EMM message built locally by the receiver, the computer 50 checks the signature of this message by comparing it with the signature obtained in concatenating all the signature fragments contained in the ECM messages associated with this group. Should the two signatures correspond, the incrementing of the value of the counter is authorized. If not, it is prohibited.

In these ECM messages, the zone AC may be omitted if the promotional content is freely accessible.

The instructions on progression associated with the promotional contents have been described in the particular case where these instructions correspond to a increment step that is constant by crypto-periods. Thus, the incrementing of the counter is directly proportional to the number of crypto-periods of promotional contents viewed. As a variant, the increment step is not constant and may vary between two crypto-periods of promotional contents.

Here, the counter has been described in the particular case where its value is incremented when a promotional content is viewed and decremented when a non-promotional content is viewed. In another embodiment, the sense of variation of the value of this counter are opposite, i.e. the counter is decremented when the promotional contents are viewed and incremented when a non-promotional content is viewed.

It is possible to replace the operation of incrementation of the value of the counter by an operation of incrementation of the value of the predetermined threshold beyond which the viewing of the non-promotional multimedia content is authorized.

The increment step or decrement step is not necessarily an integer. It may be a rational number or a real number.

What has been described in the particular case of the management of encipherment keys using ECM and EMM messages can be applied to any method for the management of keys involving the sending of cryptograms of the control word used to send the multimedia contents.

The counter is not necessarily registered in the memory of the security processor. For example, as a variant, the value of the counter is recorded, in the decoder in enciphered form by means of a key contained in the security processor.

The processor 48 can be integrated non-detachably in the decoder or in the descrambler. Conversely, the descrambler may be detachable.

What has been described here above can also be applied to making the viewing of a non-promotional content conditional on the prior viewing of a same promotional content a certain number of times. In this case, the value of the counter is for example incremented according to the number of times that this particular promotional content has been viewed. The structure of the ECMs described and the above method remain unchanged. Only the use of the methods changes.

What has been described here above can also be applied in the context of video on demand. For example, the above methods can be implemented to make the free viewing of a video on demand conditional on the prior viewing of a certain duration of promotional content. In this case, these promotional contents are for example those played just before the video on demand.

The first and second multimedia contents may be different respectively from the non-promotional and promotional contents. For example, the second contents may be first episodes of televised series and the first multimedia contents may be the last episodes of this series. Thus, the users are incited to look at all the episodes including the first ones. In general, what has been described here above is not limited to particular types of first and second multimedia contents.

CLAIMS

1. Method for receiving a first scrambled multimedia content the viewing of
which is conditional on the prior viewing of scrambled second multimedia contents, these second multimedia contents being the only contents to be associated with access control message containing a cryptogram of a control
word to descramble them and instructions on progression of the value of a
same counter, these instructions on progression modifying all the value of
this same counter in the same sense when they are executed by an electronic receiver of the first and second multimedia contents, characterized in that this method comprises:

the modification (114), whenever a second multimedia content is viewed by means of the electronic receiver, of the value of the counter as a function of the instructions on progression associated with this second multimedia content,

the storage (114) of the value of this counter between non-consecutive viewings of second multimedia contents interspersed with the viewing of another multimedia content that does not modify the value of the counter,

the making of the viewing of the first multimedia content conditional (118; 98) on a predetermined value of the counter, and

whenever the viewing of the first multimedia content by means of the electronic receiver is authorized, the method comprises the modification (120; 164) (120; 164) of the value of the counter in a sense opposite to that of the instructions on progression.

2. Method according to claim 1 wherein, before the viewing of each second multimedia content, the method comprises the demultiplexing (92) of a received stream of multiplexed data containing the second multimedia content multiplexed with the instructions on progression and wherein the modification of the value of the counter is a function of the instructions on progression multiplexed with this second multimedia content.

3. Method according to any one of the preceding claims, wherein the method comprises the selection (112) of the counter whose value which must be modified from among several possible counters as a function of an identifier of this counter associated with the instructions on progression.

4. Method according to any one of the preceding claims, wherein the method
comprises:

- the reception (92) of a group of one or more crypto-periods of the second multimedia content and instructions on progression associated with this group, each crypto-period of the second multimedia content being scrambled with a respective control word, and

- the modification (114) of the value of the counter whenever all the crypto-periods of this group are descrambled by the electronic receiver;

5. Method according to claim 4 wherein, before a crypto-period of the second multimedia content, the method comprises the reception of a message broadcast simultaneously to all the receivers capable of receiving the first and second multimedia contents, this message containing a cryptogram CW* of the control word needed to descramble this crypto-period and the instructions on progression associated with the group to which this crypto-period belongs.

6. Method according to any one of the preceding claims, wherein the method comprises:

- the reception (92) of a group of one or more crypto-periods of the first multimedia content and of instructions on regression of the value of the counter associated with this group, each crypto-period being scrambled with a respective control word,

- whenever all the crypto-periods the group are descrambled, the modification (120) of the value of the counter as a function of the instructions on regression associated with this group in a sense opposite to that of the instructions on progression, and

- the prohibiting (96) of the descrambling of a following group of crypto-periods of the first multimedia content as soon as the value of the counter reaches a predetermined threshold.

7. Method according to any one of the preceding claims, wherein the method
comprises:

- the reception (92) of the first scrambled multimedia content and of
conditions of access to this first multimedia content,

- the prohibiting (98, 96) of the descrambling of the first multimedia content if no right of access corresponding to the conditions of access is recorded in the electronic receiver, and on the contrary,

- the authorizing (98) of the descrambling of the first multimedia content if a right of access corresponding to the conditions of access to this first multimedia content is recorded in the electronic receiver, and

- the writing (164) to the receiver of an access right corresponding to conditions of access as soon as the value of. the counter has crossed a predetermined threshold.

8. Method according to any one of the preceding claims, wherein the method comprises the limiting of the value of the counter to a predetermined threshold;

9. Method according to any one of the preceding claims, wherein the method comprises the automatic creation (108) of the counter in response to the reception of instructions, on progression of the value of this counter.

10. Information-recording carrier characterized in that it comprises instructions for executing a method of receiving according to any of the above claims when these instructions are executed by an electronic computer.

11. Method for sending a first scrambled multimedia content, the viewing of which is conditional on the prior viewing of scrambled second multimedia contents, these second multimedia contents being the only contents to be associated with access control messages containing a cryptogram of a control word to decramble them and instructions on progression of the value of a same counter of an electronic receiver of the first and second multimedia contents, these instructions on progression modifying all the value of this same counter in the same sense when they are executed by the electronic receiver, characterized in that this method comprises:

- the sending (86) of the access control messages associated with each second multimedia content sent,

- the sending (86), between two second multimedia contents, of other multimedia contents not associated with access control messages containing instructions for modifying the value of this counter, and

- the sending (86) of conditions of access associated with the first multimedia contents sent, these conditions of access making the viewing of the first multimedia content conditional on a predetermined value of the counter.

12. Method according to claim 11, wherein the method comprises the sending (86) of instructions on regression of the value of the counter associated with the first multimedia content sent, these instructions on regression being capable of increasing the difference between the value of a same counter and said predetermined value when the viewing of the first multimedia content is authorized.

13. Receiver of a first scrambled multimedia content, the viewing of which is conditional on the prior viewing of scrambled second multimedia contents, these second multimedia contents being the only contents to be associated with access control messages containing a cryptogram of a control word to descramble them and instructions on the progression of the value of a same counter, these instructions on progression modifying all the value of this same counter in the same sense when they are executed by the electronic receiver of the first and second multimedia contents, characterized in that the receiver comprises:

a memory capable of storing the value of the counter between non-consecutive viewings of second multimedia contents interspersed with the viewing of another multimedia content that does not modify the value of the counter, and

an electronic computer (52) capable of:

modifying the value of the counter as a function of the instructions on progression associated with the second multimedia contents received whenever a second multimedia content is viewed by means of the receiver, and

making the viewing of the first multimedia content conditional on a predetermined value of the counter.

whenever the viewing of the first multimedia content by means of the electronic receiver is authorized, modifying the value of the counter in a sense opposite to that of the instructions on progression.

14. Security processor capable of being implemented in a receiver compliant
with claim 13, characterized in that this security processor comprises an electronic computer (50) capable of:

modifying the value of the counter as a function of the instructions on progression associated with the second multimedia contents received whenever a second multimedia content is viewed by means of the receiver, and

making the viewing of the first multimedia content conditional on a predetermined value of the counter.

whenever the viewing of the first multimedia content by means of the electronic receiver is authorized, modifying the value of the counter in a sense opposite to that of the instructions on progression.

15. Sender of a first scrambled multimedia content, the viewing of which is conditional on the prior viewing of scrambled second multimedia contents, these second multimedia contents being the only contents to be associated with access control messages containing a cryptogram of a control word to descramble them and instructions on progression of the value of a same counter of an electronic receiver of the first and second multimedia contents, these instructions on progression modifying all the value of this same counter in the same sense when they are executed by the electronic receiver of the first and second multimedia contents, characterized in that the sender (4) is capable of:

- sending access control messages associated with each multimedia content sent,

- sending, between two second multimedia contents, other multimedia contents not associated with control access message containing instructions on progression of the value of this counter, and

- sending conditions of access associated with the first multimedia content sent, these conditions of access making the viewing of the first multimedia content conditional on a predetermined value of the counter