29-10-2012, 02:02 PM
A SEMINAR REPORT ON CONDITIONAL ACCESS SYSTEM
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ABSTRACT
Conditional access system (CAS) is the essential basis of a digital pay-TV system, which is used to prevent unauthorized access to audio/video data and ensure the benefit of the service providers. In the CAS, key distribution is an essential part, whether the distribution scheme succeeding or not can directly affect the security of the CAS. It is based on four-level key hierarchy .
The traditional role of conditional access is to ensure that viewers see only those programs that they have paid to view. In the digital environment, conditional access has evolved far beyond this role. Today's conditional access systems still support traditional pay TV revenue generation. In addition they enable TV operators to create and protect a unique gateway to the enhanced TV experience - a world of interactive services, including home shopping, games, sports, interactive advertising, and pay-per-view programming. Using today's conditional access systems, you can target programming, advertisements, and promotions to subscribers by geographic area, by market segment, or according to subscribers' personal preferences. You can take advantage of conditional access features to implement flexible program packaging options and support new ways of generating revenue.
INTRODUCTION
A Pay-TV System is a commercial TV system, which charge its subscriber fee for receiving the broadcasting program. Here, a Pay-TV System can be a digital cable TV system, such as the current local CATV system, or a Digital Broadcasting System (DBS).
A Pay-TV System may have many broadcasting channels to provide its service and those channels can be classified into two classes, i.e., the basic channels and the pay-channels. The basic channels are available to all the subscribers of the system. The pay-channels require to charge the subscriber for the receiving fee. The pay-channels can be refined into two classes, the subscription (Pay-Per-Channel, PPC) channels and the Pay-Per-View (PPV) channels. The receiving fee for each channel in subscription channels is counted according to a time unit, e.g. weekly or monthly. While the receiving fee for PPV channel is counted for program unit, e.g. a film. The basic channel broadcasts the unscra- mbled programs, while PPC and PPV programs must be scrambled when they are broadcast. A Pay-TV System need a function to permit that only the authorized subscribers, who have paid the receiving fee, can watch the TV program; while an unauthorized viewer, a viewer who is not a subscriber or being a subscriber but without paying the receiving fee, can see nothing. The needed function is called Conditional Access System (CAS). Thus CAS is the essential basic system to charge the subscriber for the subscribing fee. The CAS function can be worked only by using scrambling. Scrambling is a method using cryptographic algorithm with some secret encrypting keys to encrypt source program to make it unintelligible.
ARCHITECTURE EXPLANATION
IN SENDER END
The server chooses a random variable control word (CW) as the seed of a pseudo random generator(PRG). A pseudo random sequence generated by PRG can be used to scramble the transported stream (TS). At the same time, CW will be encrypted by authorization key (AK) to form Entitlement Control Message (ECM). And AK and other entitlement information will be encrypted together by Master Private Key (MPK) to form Entitlement Management Message (EMM). ECM, EMM and the scrambled TS stream will be multiplexed into a new TS stream and transferred to subscribers in the broadcasting channel. Here, the ECM and EMM should be transferred with their digital signature respectively in order to prevent the intruder from maliciously modifying. The Subscribers Management System (SMS) administrates the customers’ different access rights to protected services.
SECURITY
Because CAS directly relates with the benefit of service provider and the subscriber, the security of CAS is very important to both sides. Scrambling algorithm being easy to attack will damage the CAS and reduce the security, while key distribution is not enough secure which will leak the information of key. The security of CAS heavily depends on these two factors. Generally, the stream processing has higher real-time requirement in DTV broadcast,
CAS always adopts symmetric encryption algorithm in scramble the program stream. For DTV broadcast system, the scrambled program stream can be received by anyone connected with the broadcast network. So the pirates can subscribe to the scrambled program channel to get the plaintext as well as the accordingly scrambled program and take known-plaintext attack on the scrambled stream. This is dangerous for CAS security, but this can be improved by frequently changing the CWs or scrambling the program with s more complex symmetric encryption algorithm. For a typical CAS, CWs will be changed once per 5~20 seconds.
Comparing with the attack on scrambling algorithm, keys distribution for CWs is more
important. If a pirate gets the keys, which can decrypt Entitlement Control Message (ECM)
for CWs for scrambled program, CAS will be easily cracked even frequently changing CWs or encryption algorithm. A good CAS should be high security, efficiency in processing stream and flexible in dynamic management.
CAS IN PAY-TV SYSTEM
A pay-tv system is a commercial tv system, which charge its subscriber fee for receiving the broadcasting program. Here, a pay-tv system can be a digital cable tv system, such as the current local catv system, or a digital broadcasting system (dbs), such as koreasat dbs . A pay-tv system may have many broadcasting channels to provide its service and those channels can be classified into two classes, i.e., the basic channels and the pay-channels. The basic channels are available to all the subscribers of the system. The pay-channels require to charge the subscriber for the receiving fee. The pay-channels can be refined into two classes, the subscription (pay-per-channel, ppc) channels and the pay-per-view (ppv) channels. The receiving fee for each channel in subscription channels is counted according to a time unit, e.g. Weekly or monthly. Whle the receiving fee for ppv channel is counted for program unit, e.g. A film. The basic channel broadcasts the unscrambled programs, while ppc and ppv programs must be scrambled when they are broadcast. A pay-tv system need a function to permit that only the authorized subscribers, who have paid the receiving fee, can watch the tv program; while an unauthorized viewer, a viewer who is not a subscriber or being a subscriber but without paying the receiving fee, can see nothing. The needed function is called conditional access system (cas’. Thus cas is the essential basic system to charge the subscriber for the subscribing fee. The cas function can be worked only by using scrambling. Scrambling is a method using cryptographic algorithm with some secret encrypting keys to encrypt source program to make it unintelligible. Therefore, only the receiver withholding the decryption key can descramble the received scrambled program and reconstruct the original source, while a receiver without the decryption key cannot receive programs correctly. Hence the authorized subscribers need the decryption key to receive programs. And cas can charge the subscribing fee by managing those decryption keys well. The encrypting keys used in scrambling is called the scrambling key. Due to the fact that almost pay-tv systems use symmetric key cryptosystems, i.e., decryption key is the same as encryption key, to encrypt the program, we hereafter use scrambling key to represent encryption or decryption key. Thus, the authorized subscribers need also the scrambling key to descramble the received scrambled program.
Conditional Access Sub-System
A Conditional Access Sub-System (CASS) is a detachable security module which is used as part of the CA system in a receiver. It is also possible to embed the security module in the receiver itself, in which case each receiver will typically have its own secret individual address. Replacement of the CASS is one means of recovering from a piracy attack. Replacement of the CASS also enables new features to be added to the system as and when they are developed. For analogue systems and some digital systems, the CASS is typically a smart card [1]. For digital systems which use the Common Interface (see Section 3.6.), the CASS will be a PCMCIA3 module and this may have an associated smart card.