【正文】 tion fields and cost effectiveness. The result is an exiting overview of a mercially very important field summarizing the recent advances in Conditional Access technology.INTRODUCTIONAdvances in the area of Conditional Access technology are diverse and in various stages of evolution. The key element is the constant drive for cost effective protection of PayTV services.This is widely believed to be of increasing importance for the rapidly emerging Digital TV market. Due to the difficult operational environment of a PayTV service provider,Conditional Access technology is leading the way in consumer oriented secure technology. New developments in securing access to broadcast services will eventually be adopted in other security applications. This effect has been demonstrated in Smartcard and secure microprocessor technology which 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 overallprotection 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.OVERVIEWThe Conditional Access technology as used in PayTV applications uses a large 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 onlyEMM’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 theactual implementation is called a tamper resistant device. A variety of implementations exist, but the most widely used is the smartcard based solution.SCRAMBLINGThe 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 onanalogue 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 adequatefor 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.ENCRYPTIONIn 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 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.COMPONENTSThe advances in Conditional Access te