Patent Publication Number: US-6668320-B1

Title: Transmission system

Description:
The invention relates to a transmission system for transmitting a multiplex signal from a transmitter to a receiver, said multiplex signal comprising first and second sections, said first section comprising encrypted data packets. 
     The invention further relates to a transmitter for transmitting a multiplex signal, a receiver for receiving a multiplex signal and a multiplex signal comprising a section. 
     A transmission system according to the preamble is known from the document Draft EN 301 192 V1.1.1, European Standard, “Digital Video Broadcasting (DVB); DVB specification for data broadcasting”. In modern digital broadcast systems a transmitter, e.g. a headend, typically transmits a large number of services (or channels) to a plurality of receivers, like for instance television sets or set-top boxes. Such a service can contain an audio/video stream, an interactive application (for example in the MHEG-5 format), other kinds of data or a combination of these elements. An MPEG-2 transport stream is a multiplex of a number of services. Typically, a transmitter transmits several transport streams to the set-top boxes. A set-top box can tune to a specific transport stream and is then able to retrieve information from the transport stream. Such a set-top box typically has only one tuner and is thus merely able to receive one single transport stream at a time. When a user wants to look at a television program, or wants to run an interactive application, or wants to access other kinds of data the set-top box or television set tunes to the corresponding transport stream and retrieves and processes the required data from the service as it is being broadcast at that moment. 
     In such systems it may be desirable that only a limited number of the users, e.g. only those who have paid or who belong to a certain group, have access to the services. Such conditional access to the services can be realised by encrypting the data, by transmitting the encrypted data to the receivers, and by supplying only to those receivers who are entitled to the data the decryption keys necessary for the decryption of the data. By means of the decryption keys the receivers can decrypt the data. For security reasons the decryption key has to be changed after a certain period of time or after the transmission of a certain amount of data. The transition to a new decryption key has to be synchronised between transmitter and receiver. 
     In the known transmission system a transport stream comprising a number of data services is broadcast by a transmitter to a plurality of receivers. Each data service includes a number of encrypted data packets which are embedded into a single data section. 
     An object of the invention is to provide a transmission system, wherein the receiver or set-top box is able to efficiently handle the decryption of the data packets. This object is achieved in the transmission system according to the invention, which is characterized in that said second section comprises decryption packets, said decryption packets comprising decryption keys, said data packets being decryptable by means of said decryption keys, at least a part of said first and second sections being identical, said first and second sections being linked together by said part. By concentrating the encrypted data packets and the therewith associated decryption packets in sections which are linked together by means of an identical part, the receiver can easily obtain the data packets and the decryption packets needed for decrypting the data packets by using a single filter which filters out all packets included in those sections which contain that part. 
     A first embodiment of the transmission system according to the invention is characterized in that said first and second sections comprise, respectively, first and second headers, said part being comprised in said headers. In general, section headers contain information about the content of the sections and can therefore be used advantageously to store information which indicates that the first and second sections are linked together. 
     A second embodiment of the transmission system according to the invention is characterized in that the data packets comprise IP packets. In this way data services defined at the IP level can be broadcasted securely using a DVB compliant infrastructure. 
     A third embodiment of the transmission system according to the invention is characterized in that the decryption keys are being retransmitted a number of times. By this measure the receivers are able to access the decryption keys quickly, so that a fast access to the data service is realised. 
     A fourth embodiment of the transmission system according to the invention is characterized in that the section comprises a header, said header comprising information indicating a change of the decryption key being used. By including this information in the header the receiver can determine exactly when to start using a new decryption key. 
     A fifth embodiment of the transmission system according to the invention is characterized in that the receiver comprises filter means for filtering out second and further occurrences of the same decryption key on the basis of the information. By this measure the receiver only has to deal with the first occurrence of a decryption key. This means for instance that, in the case where the decryption keys themselves are encoded, the receiver is relieved from decoding the second and further occurrences of the same decryption key. 
    
    
     The above object and features of the present invention will be more apparent from the following description of the preferred embodiments with reference to the drawing, wherein: 
     FIG. 1 shows a block diagram of a transmission system according to the invention. 
     FIG. 2 shows a structure diagram of a data packet, which data packet is comprised in a section. 
     FIG. 3 shows an example of a structure diagram of a SPI field. 
    
    
     FIG. 1 shows a block diagram of a transmission system according to the invention. In such a transmission system a number of data signals  12  or compound signals  12 , which represent a number of data services, are transmitted by a transmitter  10  to a receiver  14 . The transmission system may comprise further receivers  14 . An end-user is able to control a receiver  14  by means of a input device  15 , like for instance a keyboard or a remote control. Selected services can be shown on a display device  17 . By means of the data signals  12  data packets, e.g. IP packets, can be broadcasted by the transmitter  10  to a plurality of receivers  14 . 
     In such a transmission system it may be desirable that only a limited number of the users of the receivers  14 , e.g. only those who have paid or who belong to a certain group, have access to the data services. Such conditional access to the data services can be realised by encrypting the data in the transmitter  10  and by transmitting the encrypted data to the receivers  14 . Furthermore, the decryption keys necessary for the decryption of the data are encrypted themselves and only those receivers  14  who are entitled to the data are able to decrypt the decryption keys. By means of the decryption keys the receivers  14  can then decrypt the data. The decryption keys may be retransmitted a number of times so that the receivers  14  are able to access the decryption keys quickly. 
     The transmitter  10  transfers the decryption keys to the receiver  14  in the form of so-called Entitlement Control Messages or ECMs. Such an ECM, which may be embedded in an IP packet, contains an encrypted version of a decryption key or control word. By decrypting the ECM in the receiver  14 , e.g. by means of a smart card which is included in the receiver  14 , the decryption key may be revealed if the receiver also holds a corresponding data service or entitlement. For the purpose of security the control word value changes often, e.g. after a certain period of time or after the transmission of a certain amount of data. A new ECM has to be transferred to the receiver, in case the control word value has changed. So with each conditionally accessible data service a stream of ECMs is associated. It may be required to retransmit an unchanged ECM several times in order to reduce to time it takes for a receiver  14  to access the service. (To access a service, the receiver  14  must first acquire the corresponding ECM.) Filter means  16  may be included in the receiver  14  for the purpose of filtering out second and further occurrences of the same decryption key. 
     An example of a transmission system for broadcasting data services in a computer network environment, e.g. the Internet, is known from the document RFC 1825, “Security Architecture for the Internet Protocol”, August 1995. In this document two modes for the encryption of IP packets are described: 
     Transport mode: the payload of an IP packet is encrypted. 
     Tunnel mode: an entire IP packet is encrypted, and a new IP header is placed before the encrypted packet. This mode is for example used to encrypt virtual-private-network traffic that passes through a non-trusted network. 
     In both modes a so-called ESP (Encapsulating Security Payload) header is included in the newly created IP packets containing the encrypted data. This ESP header starts with a 32 bit field called Security Parameter Index (SPI). Together with the destination address, the SPI defines among other things which keys to use for decryption, which decryption algorithm to use and how to apply the decryption algorithm. 
     A transmission system for broadcasting data services is known from the document Draft EN 301 192 V1.1.1, European Standard, “Digital Video Broadcasting (DVB); DVB specification for data broadcasting”. Such a known transmission system may for example be implemented in a cable television (CATV) network environment. In this environment the transmitter  10  comprises the headend of the CATV-network and the receivers  14  comprise the set-top boxes, television sets or PCs of the end-users. The data signals  12  comprise multiplex signals  12  which can be implemented in the form of MPEG-2 transport streams. An MPEG-2 transport stream is a multiplex of a number of so-called services. Such a service can contain an audio/video stream, an interactive application (for example in the MHEG-5 format), other kinds of data (for example IP packets) or a combination of these elements. Typically, a headend  10  transmits several transport streams  12  to the set-top boxes  14 . In this way, a large number of services (or channels) can be broadcast by the headend  10  to a plurality of set-top boxes  14 . 
     A set-top box  14  can tune to a specific transport stream  12  and is then able to retrieve information from the transport stream  12 . Such a set-top box  14  typically has only one tuner and is thus merely able to receive a single transport stream  12  at a time. When a user wants to look at a television program, or wants to run an interactive application, or wants to access other kinds of data the set-top box  14  tunes to the corresponding transport stream  12  and retrieves and/or processes the required data from the service as it is being broadcast at that moment. 
     FIG. 2 shows a structure diagram of a data packet, which data packet is comprised in a first section. DVB has defined 6 protocol stacks for data broadcasting. One of these protocol stacks is the so-called multi-protocol encapsulation or MPE. In this case, there is a one-to-one mapping between a data packet and an MPEG section, and an MPEG section is usually transmitted in several MPEG-2 transport stream packets. The MPEG sections are of a special type as defined by the protocol layers DSM-CC private data and DVB multi protocol encapsulation. In the structure diagram shown in FIG. 2 the data packet is an IP packet which comprises an IP header  22 , an ESP header  24  and an encrypted IP payload or data field  26 . The data fields  26  can be decrypted by means of a changing decryption key  48 . The ESP header comprises a so-called Security Parameter Index (SPI) field. An MPE section header  20  indicates that the data packet is embedded in a MPE section. The ECMs may also be embedded in a similar structure. The packet containing an ECM is referred to as a decryption packet. Preferably the decryption packets are included in a second section which is linked to the first section which contains the data packets. The first and second sections are linked together by means of at least a part of the first and second sections, which part is identical for both sections. By concentrating the encrypted data packets and the therewith associated decryption packets in sections which are linked together the receiver can easily obtain the data packets and the decryption packets needed for decrypting the data packets by using a single filter which filters out all packets containing said identical part. Preferably the identical part is included in the section headers  20 . For example, the table_id and/or the MAC_address parts of the MPE section header  20  as defined in Table 3 in the document Draft EN 301 192 V1.1.1, European Standard, “Digital Video Broadcasting (DVB); DVB specification for data broadcasting”) can be used for this purpose. 
     FIG. 3 shows an example of a structure diagram of a SPI field. This is the only mandatory field for all different IP encryption methods used in the Internet, so this field does not change. Its purpose is to identify (together with the destination address) which keys and algorithms the receiver  14  should use to decrypt the data. In this example the first 14 bits of the SPI field are used to store an ECM stream reference  30 , which provides the association between the ECM stream and the encrypted data. The fifteenth bit is used to store information  32  indicating a change of the decryption key. By means of this information  32  control word changes can be synchronised in the transmission system. The synchronisation bit  32  toggles value when the control word used to encrypt the data has changed value. This allows the receiver  14  to determine exactly when to start using a new control word value. The sixteenth bit  34  of the SPI field is reserved for future use. The last sixteen bits of the SPI field are used to store an identification  36  of the conditional access system which is used by the transmission system. 
     The information  32  indicating a change of the decryption key may also be stored in a multi-bit counter. 
     It takes time for the smart card in the receiver  14  to process an ECM. So to avoid the need of extensive buffering of encrypted data, a new ECM should be transmitted some time before the corresponding control word is actually used. The receiver  14  must store the new control word, while it is still using the old control word to decrypt the data. The same ECM is normally retransmitted several times in order to reduce the service access time. To descramble a service, first an ECM has to be decoded to obtain the control word. Without transmission errors it is sufficient that ECMs contain a single control word. A more robust system is however obtained if ECMs contain two consecutive control words. If there are two or more consecutive control words in an ECM, retransmissions of the same ECM increase the probability that a control word is received in time if ECMs are possibly lost due to transmission errors. 
     In the MPE section header  20  a so-called payload_scrambling_control field (two bits) has been defined to indicate the change to another control word (see Table 3 in the document Draft EN 301 192 V1.1.1, European Standard, “Digital Video Broadcasting (DVB); DVB specification for data broadcasting”). For IP packets that contain data, these synchronisation bits indicate that the content is scrambled (11) or not (00). For IP packets that transfer an ECM, the bits indicate that the control word is even (01) or uneven (10). These synchronisation bits can be used by the filter means  16 , which preferably is a hardware filter, to filter out multiple instances of the same ECM so that these don&#39;t have to be removed by software. If encryption is used, the filter means  16  alternates between (1×) and (×1). Initially the filter means  16  uses a filter 1×. This means that when the first control word (which is uneven) has passed the filter means  16  start using a filter ×1. Second and further occurrences of the first control word are filtered out by the filter means  16 . The first occurrence of the second control word (which is even) passes the filter means  16 , after which the filter means  16  start using the filter 1×. Second and further occurrences of the second control word are filtered out by the filter means  16 . The first occurrence of the third control word can now pass the filter means  16 , and so forth.