METHODS AND DEVICES FOR SENDING AN ENCIPHERED MULTIMEDIA CONTENT IN BURSTS, RECORDING CARRIER FOR THESE
METHODS

The invention relates to methods and devices for sending an enciphered multimedia content in bursts as well as an information-recording carrier to implement these methods.
There are methods known to the Applicant for sending an enciphered multimedia content in bursts, this method comprising:
a) the choosing of any unspecified duration T for successive crypto-periods,
b) the replacing, by means of a synchronizer, of an encipherment current key TEKj by a new current key TEKj+1 at each end of a crypto-period,
c) the enciphering of Pi immediately consecutive segments of the multimedia content with the current encipherment key, each segment Pi starting at a respective instant tdi,
d) the building, by means of a generator, of a burst Si for which the duration
of reception is shorter than the duration of playing of the segment Pi, the burst S,
containing the enciphered segment Pi and a cryptogram of each current key
used to encipher the segment Pi.

Bursts* are most commonly known by the English term "burst". (Translator's note: * "salves" in the French. When translating this sentence into a language other than English, please translate the first occurence of "burst" but not the second).

The multimedia content is a content containing audio and/or video material. For example, a multimedia content may be a film, an audiovisual program, a television channel, music or the like. A multimedia content is sometimes also called a "service". The multimedia content is designed to be played by a receiver as and when it is received.

The duration of play of a segment of the content is the duration needed to play the segment considered at normal speed on a receiver.

Here, the terms "encipher" and "scramble" are considered to be inter changeable.

The sending of enciphered multimedia contents in bursts is described for example in the DVB-H (Digital Video Broadcast-Handheld) standard or similar standards. The reader could refer to this standard for more details on the designing and sending of bursts.

Sending enciphered multimedia contents in bursts has been defined to enable a mobile receiver to save energy during the reception of these multimedia contents. The mobile receiver is for example a mobile telephone, a personal digital assistant, a portable television set, a portable media player (PMP) or again a laptop computer.

In these sending methods, the multimedia content is divided into a succession of immediately consecutive segments Pi where the index i indicates the order number of a particular segment in this succession. A segment corresponds to a fraction of the multimedia content incorporated and transmitted
by a single burst. Thus, a burst contains one and only one segment Pi. The burst which contains the segment Pi is denoted as Si.

Each segment is compressed and enciphered before being transmitted in a burst. This compression of the segment gives a burst whose duration of reception is shorter than the duration of playing of the segment Pi.

In practice, several multimedia contents are sent simultaneously. To this
end, bursts corresponding to different multimedia contents are temporarily multiplexed. For example, time slots at regular intervals are allocated to the bursts of a specific multimedia content. These time slots allocated to a specific multimedia content define a channel.

The receiver of these bursts processes only one channel at a time. Thus, between two consecutive bursts of this channel, the receiver can remain inactive, thus saving energy.

The management of the encipherment keys to secure the transmission of this multimedia content is done by a "key management system" which is part of the contents protection system implemented. For example, the description here is made with reference to a key management system of the contents protection system defined in the OMA-BCAST Smartcard Profile (Open Mobile Alliance-Broadcast Services Enabler Suite Smartcard Profile). Thus, in this description, the terminology used is the one defined in this standard.

In a system compliant with the OMA-BCAST Smartcard Profile standard, a TEKj (Traffic Encryption Key) key is used to encipher the segments currently sent out by the multimedia content. The TEKj is changed at regular intervals. These intervals are known as crypto-periods. For example, a crypto-period lasts less than one minute. Typically, a crypto-period lasts five to ten seconds. Here, each crypto-period is denoted as Tj, where j is an order number of the crypto-period. During the crypto-period Tj, only the key TEKj is used to encipher the multimedia content. Then, during the immediately consecutive crypto-period Tj+1, only the key TEKj+1 is used to encipher the multimedia content and so on and so forth.

At least one STKMj (Short Time Key Message) is included in each burst so that the segment Pi contained in this burst can be deciphered. Each STKMj message contains a cryptogram of the TEKj. In the OMA-BCAST Smartcard Profile standard, an STKM message can contain only one cryptogram of a TEK key. An STKM message often also contains conditions of accessing the multimedia content that have to be compared with rights of access pre-recorded in a memory of the receiver in order to authorizat and alternately prohibit the deciphering of the multimedia content.

When a mobile receiver receives a burst, it must decipher the cryptogram of the TEKj contained in the STKMj message before it can decipher the segment Pi contained in this burst. The deciphering of the cryptogram of the key TEKj takes a certain time denoted TSTKM- Thus, when a user changes a channel, i.e. when he zaps or when he tries to turn his receiver on, the deciphering of the segment Pi encapsulated in the first burst Si received can start, at the earliest, TSTKM after the start of reception of this burst.

Furthermore, another problem may arise during a change in channel. To explain this, reference is made to figures 1 and 2. Figure 1 represents a succession of immediately consecutive segments Pi of a multimedia content played at normal speed. Each segment Pi starts at an instant tdi. This instant tdi is indicated on a time axis 2.

A second time axis 4 represents the crypto-periods on the same scale. Here, two crypto-periods Ti and Tj+1 are shown. During the crypto-period Tj, the segments are enciphered by means of the TEKj key. This crypto-period Tj is completed at the instant at which the next crypto-period Tj+1 starts, i.e. at an instant tej+1. During the crypto-period Tj+1, the key used to encipher the segments of the multimedia content is the TEKj+i key.

In the particular case shown in figure 1, the change in crypto-period takes place between the instants tdi and tdi+TsTKM. Thus, the start of the segment Pf is first of all enciphered with the key TEKj up to the instant tej+1. Then, the end of this segment is enciphered with the key TEKj+1. The burst Si which contains the segment Pi must therefore contain the two keys TEKj and TEKj+i to enable the deciphering of the segment Pi. To this end, to be compliant with the OMA-BCAST Smartcard Profile standard, this burst Si contains two STKMj messages and STKMj+1 respectively containing the cryptograms of the keys TEKj and TEKj+1.

Let us now assume that a user has just changed channels and has zapped to the channel corresponding to the burst Si so that the receiver does not yet have any information on this channel. In these conditions, the receiver awaits reception of the first complete burst on this channel. It is assumed that this first burst is the burst Si containing the segment Pi. The burst Si is received at the instant tSi shown in the time axis 6 of figure 2.

Once the burst Si has been received, the receiver deciphers the cryptogram of the key TEKj which takes a period of time TSTKM- Hence, before the instant tsi TSTKM, the receiver cannot play the multimedia content received in cleartext. This corresponds to a period of time 8 in figure 2. For example, during this period 8, the receiver displays only a black screen or allows no sound to be heard.

The term playing as "cleartext" designates the playing of a multimedia content after it has been deciphered. Thus, a multimedia content in cleartext corresponds to images or sounds that are directly perceptible and comprehensible to the user of the receiver.

Starting with the instant tSi + TSTKM, the receiver starts displaying the start of the segment Pi very clearly.

From the instant tsi + TSTKM to the instant tsi + TSTKM + tej+1 - tdi, the start of the segment Pi is displayed in cleartext. This period bears the reference 10 in figure 2.
Furthermore, in parallel and starting from the instant tsi + TSTKM, the receiver deciphers the cryptogram of the TEKj+1. Consequently, the display in cleartext mode of the end of the segment Pi can at the earliest start only from the instant tSi + 2TSTKM-
Now, here the instant tSi + TSTKM + tej+1 - tdi is prior to the instant tsi + 2TSTKM- Therefore, the TEKi+1 key is not yet available at the end of the display of the start of the segment Pj. Thus, the receiver again displays a black screen until the instant tsi + 2TSTKM- This second period of display of a black screen has the reference 12 in figure 2.

Starting from the instant tsi+2TSTKM, the receiver displays the end of the segment Pi (period 14) in cleartext mode.

The appearance of a black screen during the period 12 after a period of cleartext display of the multimedia content is unpleasant to the user.

Naturally, one solution to prevent this phenomenon would be to await tSi+2TsTKM before starting to display the segment Pi in cleartext. However, this solution unacceptably increases the time needed to display the multimedia content in cleartext after a change of channels.

The invention seeks to overcome this drawback by proposing a method for sending a multimedia content in bursts, wherein the appearance of a black screen during the period 12 is eliminated.

An object of the invention therefore is a method for sending an enciphered multimedia content in bursts, wherein the replacement of a current key TEKj by the current key TEKj+i to encipher the segment Pi is delayed up to a point in time tdi + TSTKM or brought forward to the instant tdi or before this instant tdi in response to a synchronization signal exchanged between the generator and the synchronizer, the duration TSTKM being greater than or equal to the time needed for a receiver to decipher the cryptogram of a current key contained in the burst Si and strictly smaller than the chosen duration T.

In the above method, the replacement of the key TEKj by the key TEKj+1 is delayed or brought forward so as to prevent the end of the crypto-period from falling between the instants tdi and tdi+TSTKM. In other words, the duration T of the crypto-period Ti is lengthened or shortened dynamically during the enciphering of the multimedia content so that the instant tej+1 does not occur during the interval ]tdi ; tdi+TSTKM[ of a segment Pi,. This means that the situation described with reference to figures 1 and 2 can no longer occur, thus preventing the appearance of a black screen during the period 12. Besides, this method can be used to resolve the problem whatever the duration chosen. In particular, it is not necessary to choose the duration T as being an integer multiple of the minimum duration of play TA of the segments considered.

The embodiments of this method can show one or more of the following characteristics:
■ the method comprises:
- before the enciphering of a new segment Pi, the comparing of a current instant tc established on the basis of the synchronization signal with a theoretical instant ttej+1 of an end of the current crypto-period calculable from the instant tej and of the chosen duration T, the instant tej being the instant of a start of the current crypto-period, and
- if the instant tc is prior to the instant ttej+1, the enciphering of the totality of the segment Pi with the current key TEKj even if the end of the crypto-period Tj occurs during the segment Pi, and
- if the instant tc is later than the instant ttej+1, the enciphering of the totality of the segment Pj with the new current key TEKj+1;
■ the method comprises:
- before the enciphering of a new segment Pi during which there occurs an instant ttej+1 of an end of the current crypto-period calculable from the instant tej of a start of the current crypto-period and of the chosen duration T, comparing the instant ttej+1 with an interval ]tdi; tdi + TSTKM[ established from the synchronization signal, and
- only if the instant ttej+1 is included in the interval ]tdi; tdi + TSTKM[, the replacing (140, 142) of the current key TEKj by the current key TEKj+1 to encipher the segment Pi is delayed until after an instant tdi + TSTKM or brought forward to the instant tdi or before the instant tdi.

These embodiments furthermore have the following advantages:
- the comparing of the instants tc and ttej+1 facilitates the implementing of the method for it is then not necessary to estimate the instant tdi+1 of a start of the segment Pi,+1 ;
- the activating of the replacing of the key TEKj by the key TEKj+i only if the instant ttej+1 is outside the slot ]tdi+1 ; tdi+1 + TSTKM[ resolves the problem while at the same time enabling the replacing of the keys during a segment.

These embodiments moreover have the following advantages:
- comparing the instants tc and ttej+1 facilitates the implementation of the
method because it is then not necessary to estimate the instant tdi+1 of
a start of the segment Pi+1;
- activating the replacing of the key TEKj by the key TEKj+1 only if the
instant ttej+1 is outside the interval ]tdi+i ; tdi+1 + TSTKM[ resolves the problem while at the same time making it possible to keys during asegment.

An object of the invention is also another method for sending an enciphered multimedia content in bursts, the method comprising:
a) the choice of a duration T for successive crypto-periods,
b) the immediately replacing of a current encipherment key TEKj by a new current key TEKj+1 at each end of a crypto-period,
c) the enciphering of immediately consecutive segments Pi of the multimedia content with the current enciphering key, each segment Pi starting at a respective instant tdi and having a constant minimum duration of play TB,
d) the building of a burst Si, the duration of reception of which is shorter than the duration of play of the segment Pi, the burst Si containing the enciphered segment Pi and a cryptogram of each current key used to encipher the segment Pi wherein the duration of play of each segment is greater than or equal to a minimum duration TB common to all the segments.

Furthermore, in this method, the duration T is chosen to verify the following conditions:
- T/TB = r/q, where r and q are non-zero natural integers that are mutually prime numbers, and
- 1/q > TSTKM/TB where the duration TSTKM is greater than or equal to the time needed so that a receiver deciphers the cryptogram of a current key contained in the burst Si and strictly smaller than the chosen duration T.

In the above method, the duration T initially chosen is not any unspecified value. On the contrary, this duration T is chosen as being a rational number r/q verifying the two conditions set forth here above. When T verifies these two conditions, it ensures that the instant tej+1 will never occur in the time slot ]tdi; tdi + TSTKM[- Hence, in the above method, the duration T is constant and it is not necessary to lengthen or shorten it dynamically to prevent an instant tej+1 from occurring in the slot ]tdi; tdi + TSTKM[- However, the freedom of choice of the duration T is restricted.

The embodiments of this method may have the following characteristic:
- the duration of play of each segment Pi is equal to the duration TB.
These embodiments facilitate the implementation of the method.

An object of the invention is also a carrier for the recording of information containing instructions for the execution of one of the above methods when these instructions are executed by an electronic computer.

An object of the invention is also a device for sending an enciphered multimedia content in bursts, the device comprising:
a) a memory in which a duration T for successive crypto-periods is recorded,
b) a synchronizer capable of replacing an enciphering current key TEKj by a new current key TEKj+1 at each end of a crypto-period,
c) a scrambler capable of enciphering immediately consecutive segments Pj of the multimedia content with the current enciphering key, each segment Pj starting at a respective instant tdi,
d) a generator of a burst Si whose duration of reception is shorter than the duration of play of the segment Pi, the burst Si containing the enciphered segment Pj and a cryptogram of each current key used to encipher the segment Pi.

Furthermore, the generator and the synchronizer are capable of exchanging a synchronization signal to delay the replacing of the current key TEKj by the current key TEKj+1 until after the instant tdi + TSTKM or to bring this replacing forward to the instant tdi or before the instant tdi.

Finally, an object of the invention is also another device for sending an enciphered multimedia content by bursts, the device comprising:
a) a memory in which a duration T for successive crypto-periods is recorded,
b) a synchronizer capable of immediately replacing an enciphering current key TEKj by a new current key TEKj+1 at each end of a crypto-period,
c) a scrambler capable of enciphering immediately consecutive segments Pi of the multimedia content with the current enciphering key, each segment Pi starting at a respective instant tdi, and having a duration of play greater than or equal to a minimum duration TB common to all the segments,
d) a generator of a burst Si whose duration of reception is shorter than the duration of play of the segment Pi, the burst S, containing the enciphered segment Pf and a cryptogram of each current key used to encipher the segment Pi.

Furthermore, in this sender, the chosen duration T contained in the memory verifies the following conditions:
- T/TB = r/q, where r and q are natural non-zero integers that are mutually prime numbers, and
- 1/q > TSTKM/TB

The embodiments of the sending device may comprise the following characteristic:
- the sending device is capable of limiting the choice of the duration T to
a duration T that meets both conditions.

The invention will be understood more clearly from the following description given purely by way of a non-exhaustive example and made with reference to the appended drawings, of which:
- Figure 1 is a schematic illustration by means of timing diagrams of a particular synchronization between segments Pi and a change of crypto-period,
- Figure 2 is a timing diagram schematically illustrating a problem of the prior art,
- Figure 3 is a schematic illustration of the architecture of a system for transmitting enciphered multimedia contents using device for sending by bursts,
- Figure 4 is a flowchart of a method for sending a multimedia content by bursts by means of the sending device of figure 3,
- Figure 5 is a timing diagram illustrating the synchronization of the segments P, with the changes of crypto-periods obtained by means of the method of figure 4,
- Figures 6 and 7 are flowcharts of other embodiments of methods for sending an enciphered multimedia content by bursts.

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

Here below in the description, characteristics and functions well known to those skilled in the art are not described in detail.

Figure 3 represents a system 20 for the transmission of multimedia contents in bursts. This system 20 comprises a sender 22 for sending a multimedia content in bursts to mobile receivers. To simplify the illustration, only three mobile receivers 24 to 26 have been shown. The mobile receivers 24 to 26 are linked to the sender device 22 by means of an information transmission network 28. The receivers 24 to 26 are linked to this network 28 by means of wireless links, respectively 30 to 32, so as to enable mobility.

Each receiver 24 to 26 is equipped with a screen 34 and at least one speaker 36 so that it can perceptibly and comprehensibly display the multimedia content received. For example, the receivers 24 to 26 are mobile telephones.

For example, the network 28 is a mobile telephony network.

The sender 22 comprises a port 40 through which is received the cleartext multimedia content to be sent out in enciphered form. This port 40 is connected to the input of a 42 for compressing the multimedia content. One output of the module 42 is connected to one input of a scrambler 44 capable of enciphering of compressed multimedia content. To this end, the scrambler 44 uses a current key TEKj contained in a memory 46. An output of the scrambler 44 is connected to an input of a generator of bursts 48. This generator 48 also has a buffer memory 49 in which there is registered a segment Pi of the enciphered multimedia content as well as other different pieces of information to be transmitted in the form of a single burst. The other pieces of information incorporated in a burst comprise especially at least one STKMj message and possibly other pieces of information such as identifiers of burst, channels and the like. One output of the generator 48 is connected to an input of a broadcaster 50 capable of broadcasting the bursts generated by the generator 48 on the network 28.

The sending device 22 also has a generator 52 of keys capable of generating a new key TEKj for each new crypto-period. One output of this generator 52 is connected to one input of a synchronizer 54 and to one input of a builder 56 of STKMj messages. The builder 56 is capable of building the message STKMj containing a cryptogram of the key TEKj. One output of the builder 56 is connected to one input of the synchronizer 54.

The synchronizer 54 is capable of replacing the key TEKj currently registered in the memory 46 by a new key TEKj+1 at the appropriate time.

The synchronizer 54 is also capable of sending the generator 48 the message STKMj corresponding to the key TEKj currently used by the scrambler 44 to encipher the current segment of the multimedia content.

In this embodiment, the generator 48 is also capable of sending a synchronization signal to the synchronizer 54. For example, this synchronization signal informs the synchronizer about the end of the preparation of a burst and the start of the preparation of the next burst.

For example, the sender 22 is made out of one or more programmable electronic computers capable of executing the instructions recorded on an information recording carrier. For example, the synchronizer 54 and the generator 48 are made out of respective electronic computers capable of working in parallel. To this end, here, the sender 22 is connected to a memory 60 containing instructions and the information needed to execute at least one of the sending methods described with reference to figures 4, 7 and 8.

The working of the sender 22 shall now be described in greater detail by means of the method of figure 4 and the timing diagram of figure 5.

Initially, before any sending of a multimedia content during a step 80, the duration T of the crypto-periods is chosen and then for example recorded in the memory 60. In this first embodiment, the duration T chosen may be any unspecified duration. In other words, there is no restriction on the choice of duration T except that it should be greater then TSTKM

Then, during the sending of an enciphered multimedia content, at a step 82, the generator 48 sends a synchronization signal to the synchronizer 54 to tell it that it must now start preparing a new burst. For example, the generator 48 sends this signals when it has finished generating the previous burst and before generating the next burst.

Then, at a step 86, the next burst to be broadcast is prepared. For example, in an operation 86, the module 42 compresses the current segment Pi of the multimedia content and then the scrambler 44 enciphers this compressed segment, using to this end the key currently recorded in this memory 46. As and when the segment is enciphered, it is recorded in the buffer memory 49.

Then, during an operation 90, when the quantity of information elements recorded in the memory 49 goes beyond a predefined threshold, the generator 48 starts the construction of the burst Si containing the segment Pi. In particular, at the operation 90, the generator 48 associates the following in one and the same burst:
- the compressed and enciphered segment Pj,
- a burst identifier,
- a message STKMj if the segment Pi has been enciphered only by means of the key TEKj or two messages STKMj and STKMj+1 if the segment P, has been enciphered successively by means of the key TEKj and the key TEKj+1.

Once the preparation of the burst Si has been terminated, it is transmitted to the broadcaster 50 which, in a step 92, broadcasts it to all the receivers connected to the sender 22 by means of the network 28.

Then, the steps 82 to 92 are reiterated in a loop for each segment P, of the multimedia content to be broadcast.

In parallel with the steps 82 to 92, the synchronizer 54 manages the change of crypto-period. For example, at each start of a crypto-period Tj, in a step 100, the synchronizer 54 records the instant tej for starting this crypto-period.

Then, in a step 102, the generator 52 generates the key TEKj+1 which must be used at the next crypto-period Tj+1. At the step 102, once the key TEKj+1 has been generated, the builder 56 immediately builds the message STKMj+1 containing a cryptogram TEKj+1 of the key TEKj+1. The key TEKj+1 and the message STKMj+1 are transmitted to the synchronizer 54.

Then, the generator 48 sends the synchronization signal to the synchronizer 54. In response, the synchronizer 54 immediately carries out a step 104 for computing a difference At. The difference At is computed by means of the following relationship:
Δt = tc-tej,

where the instant tc is the current instant of the content, i.e. the cumulated duration of play of the segments or parts of segments of the content that are already enciphered, at the time when the synchronizer 54 receives the synchronization signal sent by the generator 48.

Then, at a step 106, the synchronizer 54 compares the difference At with the chosen duration T of the crypto-periods. This step is a particular embodiment of a comparison between the current instant tc and a theoretical instant ttej+1 of an end of the current crypto-period Tj that can be computed starting from an instant tej of a start of the crypto-period Tj and the chosen duration T.

If the difference At is strictly smaller than the duration T, it means that the end of the crypto-period has not yet been reached. In this case, at a step 108, the synchronizer 54 inhibits any replacement of the key TEKj recorded in the memory 46 up to the next indication of the generator 48 according to which the preparation of a new burst will soon begin. By operating this way, the synchronizer 54 prevents any replacement of the key recorded in the memory 46 during the encipherment of a segment. Thus, the synchronizer 54 ensures that no change of a key TEKj can occur during the encipherment of a segment Pi. This can therefore lead to extending the duration of the crypto-period Tj if necessary.

At the end of the step 108, the method returns to a state of waiting for a new synchronization message from the generator 48.

If not, i.e. if the difference At is greater than or equal to the duration T, it means that the crypto-period Tj us coming to an end or has almost come to an end. In this case, in a step 110, the synchronizer 54 replaces the key TEKj in the memory 46 by the key TEKj+1 before the start of encipherment of the segment Pi+i. At the step 110, the synchronizer 54 also transmits the message STKMj+i o the generator 48.

Then, at a step 112, the synchronizer 54 inhibits every new change of the key recorded in the memory 46 until the next synchronization signal. At the end of the step 112, the method returns to the step 100 so as to memorise the instant tej+1 at which the synchronizer 54 had replaced the key in the memory 46 as a new instant of start of the current crypto-period Tj+1 .

The timing diagram of figure 5 makes it possible to understand the behavior and consequences of the method of figure 4 in greater detail. Figure 5 shows a time axis 120 on which the instants tdi of the starting of each of the segments Pf have been represented.
In a content played at normal speed on a receiver, the segments Pi are immediately consecutive so that the instant of the end of the playing of a segment corresponds to the instant of the start of the playing of the next segment. It will be noted that this is not necessarily the case on the sender side so that the encipherments of consecutive segments can be separated by a sufficiently lengthy time slot, for example, to enable the replacing of the encipherment key in the memory 46. The scrambler 44 can also be capable, at least for a certain period of time, of enciphering two multimedia content channels in parallel with, respectively, the keys TEKj and TEKj+1. In this case, the current key is replaced by sending the same multimedia content on these two channels and then by switching over from one channel to the other to pass instantaneously from the crypto-period Tj to the crypto-period Tj+1 without interrupting the stream of multimedia content. Other approaches are again possible.

On another time axis 122 plotted to the same scale, the instant tej+1 and a theoretical instant ttej+1 of the end of the crypto-period Tj have been shown.

The instant ttej+1 corresponds to the theoretical instant of an end of the crypto-period Tj computed by adding to the instant tej of the start of the crypto-period Tj the duration T chosen during the step 80.

Finally, instants tc1 and tC2 represented on the axis 120 correspond to two successive current-instants at which the generator 48 sends the synchronization signal to the synchronizer 54. For the readability of the figures, the instants tc1 and tc2 are represented as being prior respectively to the instants tdi and tdi+1.

In figure 5, the instant tc1 is prior to the instant ttej+1- In this context, at the step 106, the synchronizer 54 determines the fact that the difference Δt is strictly smaller than the duration T. It therefore prevents any new change of key in the memory 46 until the next instant tc2- The instant tc2 is after the instant ttej+1. Thus, the synchronizer 54 determines the fact that the difference Δt is strictly greater than the duration T. It therefore replaces the key TEKj by the key TEKj+1 at the end of the enciphering of the segment Pi and before the start of the enciphering of the segment Pj+1.

It will therefore be understood that the next segment, i.e. the segment PM is entirely enciphered through the use of the key TEKj+1. It can also be observed that while a replacing of the key TEKj should have occurred during the segment Pi, this replacing has actually occurred only at the end of the segment Pi. This corresponds to a lengthening of the duration of the crypto-period Tj. This lengthening of the duration of the crypto-period Tj is activated in response to a synchronization signal. More specifically, here, this lengthening of the duration of the crypto-period Tj is activated only if the instant ttej+1 occurs during a segment. If not, the duration of the crypto-period is equal to the chosen duration T. Acting in this way ensures that, whatever the duration T chosen, no replacing of a key TEKj can occur in the interval ]tdj; tdj + TSTKM[-

Figure 5 represents a time axis 124 plotted to the same scale on which the instants tsi for starting the reception of the bursts Sj are shown.

Figure 6 represents another embodiment of a method of sending an enciphered multimedia content in bursts. Initially, before the transmission of the multimedia content, at a step 130, an unspecified duration T is chosen for the crypto-periods.

Then, the steps 82 to 92 are repeated in a loop to transmit the multimedia content in bursts. At the same time, the synchronizer 54 manages the replacing off the key stored in the memory 46 by a new key. For example, in a step 132, the synchronizer 54 records the instant tej at which the current crypto-period Tj has begun. At the step 132, the synchronizer 54 also computes the instant ttej+1 at which the end of the current crypto-period Tj has occurred. For this computation, the synchronizer uses for example the instant tej as well as the duration T chosen at the step 130.

Then, after each start of a crypto-period, in a step 134, the generator 52 and the builder 56 respectively give the key TEKj+1 and the message STKMj+1.

In parallel with the step 134, at a step 136, whenever the generator 48 sends the synchronizer 54 the synchronization signal, the synchronizer 54 sets up an instant tdi at which the next segment contained in the burst that will be prepared must start. For example, the synchronization signal is sent at each instant tdi of the start of a segment Pj. Thus, here the synchronization signal indicates the start of each segment.

Then, the synchronizer 54 proceeds to a step 138 in which it makes a check to see if the following condition is fulfilled:
| tdi - ttej+1| > TsTKM
where tdi is the starting instant recorded at the step 136.

The step 138 therefore consists in comparing the instant ttej+1 with an interval ]tdi; tdi + TSTKM[ set up from the synchronization signal onwards.

Should this condition be fulfilled, the synchronizer 54 immediately performs the step 140 for replacing the key TEKj by the new key TEKj+1 without waiting for the encipherment of the end of the current segment Pi.

If not, the synchronizer 54, at a step 142, delays the replacing of the key TEKj. For example, at the step 142, the synchronizer 54 introduces a delay strictly greater than tdi + TSTKM-ttej+1. After having introduced this delay at the step 142, the synchronizer proceeds to the step 140. At the end of the step 140, the method returns to the step 132.

Thus, in this method, unlike in the method of figure 4, a change of crypto-period can take place during a segment. However, the synchronizer 54 is capable of extending the duration of the crypto-period Tj if the instant ttej+1 occurs in the interval ]tdi; tdi + TSTKM[.

Figure 7 also illustrates another possible embodiment of the method. Before the start of the sending of the multimedia content, at a step 150, the duration T of the crypto-period is chosen to verify the following relationships:
T/TB = r/q, and
1/q>TsTKM/TB

where :
- r and q are natural non-zero integers that are mutually prime numbers,
and
- TB is a minimum duration that is smaller than or equal to the smallest
duration of play of a segment of the multimedia content.

The duration TB is therefore a minorant, independent of the index i, which reduces the duration of all the segments Pi of the multimedia content. It is assumed here that the duration of play of each segment Pi is equal to the durationTB. The duration TB is greater than TSTKM-

Such a choice of the duration T ensures that no change in crypto-period can occur in the interval ]tdi; tdi + TSTKM[-

For example, at the step 150, the sender 22 limits the choice of the duration T solely to the durations complying with the above relationships. For example, at the step 150, a graphic interface enabling only one operator to choose a duration T verifying these relationships is presented to an operator of this sender 22. The graphic interface is then generated by the synchronizer 54.

Then, at the sending of the multimedia content, each burst is prepared during a step 152. The step 152 is identical for example to the step 86.

Then, once the burst has been prepared, it is broadcast in a step 154 identical for example to the step 92.

The steps 152 and 154 are reiterated in a loop to transmit the multimedia content in bursts.

At the same time, at a step 156, at each start of a crypto-period, the generator 52 and the builder 56 respectively give the new key TEKj+1 and the new message STKMj+1.
Then, as soon as the instant ttej+1 occurs, at a step 158, the synchronizer 54 immediately replaces the key TEKj with the key TEKj+1 in the memory 46. At the step 158, the synchronizer 54 also sends the generator 48 the new message STKMj+1 corresponding to the key TEKj+1.

The steps 156 and 158 are reiterated in a loop. In this method, through the particular choice of the duration T, it is no longer necessary for the generator 48 to inform the synchronizer 54 of the start of the preparation of a new burst. Furthermore, here by means of the method of figure 7, the synchronizer 54 no longer has the function of lengthening or on the contrary of shortening the duration of a crypto-period. On the contrary, in this embodiment, the duration T of the crypto-period is constant. In other words, in this embodiment the synchronizer 54 is used solely to:
- limit the choice of the duration T, and
- replace keys in the memory 46 and at the same time transmit the new messages STKMj to the generator 48.

1. Method for sending an enciphered multimedia content in bursts, this method
comprising:
a) the choice (80) of any duration T for successive crypto-periods,
b) the replacing (110; 140), by means of a synchronizer, of a current
encipherment key TEKj by a new current key TEKj+1 at each end of a crypto-
period,
c) the encipherment (88) of immediately consecutive segments Pi of the
multimedia content with the current enciphering key, each segment Pi
starting at a respective instant tdi,
d) the building (90), by means of a generator, of a burst Si for which the duration of reception is shorter than the duration of play of the segment Pi, the burst Si containing the enciphered segment Pi and a cryptogram of each current key used to encipher the segment Pi
characterized in that the replacing (110; 140) of the current key TEKj by the current key TEKj+1 to encipher the segment Pi is delayed until after an instant tdi + TSTKM brought forward to the instant tdi or before the instant tdi, in response to a synchronization signal exchanged (82) between the generator and the synchronizer, the duration TSTKM being greater than or equal to the time needed for a receiver to decipher the cryptogram of a current key contained in the burst S, and strictly smaller than the duration T chosen.

2. Method according to claim 1, wherein the method comprises:
- before the enciphering of a new segment Pi, the comparing (106) of a
current instant tc, established on the basis of the synchronization signal, with a theoretical instant ttei+1 of an end of the current crypto-period calculable from the instant tej and of the chosen duration T, the instant tej being the instant of a start of the current crypto-period, and
- if the instant tc is prior to the instant ttej+1, the enciphering (88, 108) of the totality of the segment Pi with the current key TEKj even if the end of the crypto-period Tj.occurs during the segment Pi, and
- if the instant tc is later than the instant ttej+1, the enciphering (88, 108) of the
totality of the segment Pi with the new current key TEKj+1;

3. Method according to claim 1, wherein the method comprises :
- before the enciphering of a new segment Pi during which there occurs a
theoretical instant ttej+1 of an end of the current crypto-period calculable from
the instant tej of a start of the current crypto-period and of the chosen
duration T, the comparing of the instant ttej+1 with an interval ]tdi; tdi + TSTKM[
established from the synchronization signal, and

c) a scrambler (44) capable of enciphering immediately consecutive segments Pi of the multimedia content with the current enciphering key, each segment Pi starting at a respective instant tdi,
d) a generator (48) of a burst Si whose duration of reception is shorter than the duration of play of the segment Pi, the burst Si containing the enciphered segment Pi and a cryptogram of each current key used to encipher the segment Pi.
characterized in that the generator (48) and the synchronizer (54) are capable of exchanging a synchronization signal to delay the replacing of the current key TEKj by the current key TEKj+1 until after the instant tdi + TSTKM or to bring this replacing forward to the instant tdi or before the instant tdi, the duration TSTKM being greater than or equal to the time needed for a receiver to decipher the cryptogram of a current key contained in the burst Si and strictly smaller than the chosen duration T.

8. Sender of a an enciphered multimedia content in bursts, this sender
comprising:
a) a memory (60) in which a duration T for successive crypto-periods is recorded,
b) a synchronizer (54) capable of immediately replacing an enciphering current key TEKj by a new current key TEKj+1 at each end of a crypto-period,
c) a scrambler (44) capable of enciphering immediately consecutive segments Pi of the multimedia content with the current enciphering key, each segment Pi starting at a respective instant tdi, and having a duration of play greater than or equal to a minimum duration TB common to all the segments,
d) a generator (48) of a burst Si whose duration of reception is shorter than the duration of play of the segment Pi, the burst Si containing the enciphered segment Pi and a cryptogram of each current key used to encipher the
segment Pi.
characterized in that the chosen duration T contained in the memory verifies the following conditions:
- T/TB = r/q, where r and q are natural non-zero integers that are mutually prime numbers, and
- 1/q > TSTKM/TB where the duration TSTKM is greater than or equal to the time needed for a receiver to decipher the cryptogram of a current key contained in the burst Si and strictly smaller than the chosen duration T.

9. Sender according to claim 8, wherein the sender is capable of limiting the
choice of the duration T to a duration T that/meets both conditions.