數字電視有條件系統的研究-資料下載頁

【導讀】原本功能簡單，目標單一的數字電視接收機也變成了圖

　　

【正文】 ich found its first significant market in conditional Access for broadcast services. The same technology was then applied to a range of other uses. This paper first introduces the general principles of Conditional Access technology in PayTV applications. It then continues to discuss the evolution of such technology from analogue TV to digital TV. In these descriptions a clear view emerges on the constantly changing areas of security concern in order to improve the overall protection of the PayTV services. The article is based on a System oriented approach (1) and tries to place earlier descriptions of recent Conditional Access technological developments (3) in a wider historical context. OVERVIEW The Conditional Access technology as used in PayTV applications uses a large 6 variety of implementations for the same basic principle. This section introduces the basic principles of such a system. The basic service is scrambled (. rendered unusable to non authorized viewers)based on a parameter monly referred to as the Control Word. The Control Word needs to be transmitted to the properly entitled receivers such that it can be used to convert the scrambled data back to its original form. This is achieved by first encrypting the Control Word with Ek(cw)= C and using the inverse process DK(C)=cw in the receiver. Some signalling mechanism exists in addition to the cryptogram C to allow the receiver to select the proper decryption algorithm DK(). As the distribution of decryption algorithms is made part of the entitlement distribution, not all receivers have full knowledge about the algorithm DK(). The cryptogram C is called an Entitlement Control Message. The distribution of information on the decryption algorithm DK()is usually referred to as an Entitlement Management Message. The decryption process should be implemented in such a way that it is not possible to retrieve the details of OK()by any means. This ensures that only EMM’s contain the secret DK() information and it is not possible to copy the DK()from a properly enabled receiver to another (none authorised) one. This property relies on the mechanical construction of the decryption process and the actual implementation is called a tamper resistant device. A variety of implementations exist, but the most widely used is the smartcard based solution. SCRAMBLING The method used to render the basic service contents useless for an unauthorized receiver is called scrambling. This process operates on the actual broadcast audio and video information. The scrambling of analogue TV signals was based on analogue manipulation of the TV signals. Well known methods are Video inversion, Sync Removal, Line Cut and Rotate, Line Shuffling and binations of these basic techniques. The strength of these methods can be made adequate for the purpose and good enough to block brute force attacks. Digital TV signals need to be scrambled using digital methods. Several algorithms are available for this purpose such as the Data Encryption Standard (DES), the DVB Common Scrambling Algorithm and the Multi2 Algorithm. The DES algorithm is widely used by panies in the USA. European legislation mandates the support of the DVBCSA in all Digital TV receivers in Europe. The Multi2 system is adopted in Japan. All these digital scrambling methods are considered as significantly stronger than the analogue ones. All Digital TV scrambling methods use the transport mechanisms defined in MPEG2 (2) to perform accurate Control Word switching using the two bit scrambling control field to signal the actual Control Word usage. ENCRYPTION In order to transmit the Control Word and the Entitlements to the relevant receivers, certain information needs to be inserted into the broadcast signal. The actual encryption method uses proprietary algorithms, which are shared in both analogue TV and digital TV signals. Similar algorithms are used in the construction of the EMM data. The security of these encryption processes is well known and some early attacks (. playback of EMM messages) are now well know. The application restrictions are sufficiently understood that the security is based on the strength of the encryption 7 process. Typical encryption methods are based on DESlike schemes often in bination with public key based ponents. There is sufficient literature on this subject to make attacks at this level not feasible. In addition to a better understanding of the PayTV application constraints, significant advances have been made in terms of addressing efficiency. A wide variety of addressing schemes and operational modes are now available to the PayTV operator. This allows modern Conditional Access systems a great deal of flexibility to efficiently support a very wide range of applications. The operation of such a flexible system demands a sophisticated control system, which also has to deal with larger subscriber numbers and rapidly increasing number of services and service packages. In most modern Conditional Access systems, the plexity resides in the control system. COMPONENTS The advances in Conditional Access technology can best be demonstrated by introducing the various ponents which have been introduced into the market. Most of these ponents have seen significant evolution and functional changes. The relevant ponents are the Integrated Receiver Decoder, the smartcard, the Conditional Access Module and the SimulCrypt operational model. These elements are described in the following paragraphs. Integrated Receiver Decoder The Integrated Receiver decoder (IRD) is the oldest part in PayTV applications. It serves to receive the scrambled TV signals and convert it into a format suitable to viewing on a ordinary TV set. The IRD, sometimes called settop box, is a simple device which resides in the v