be advantageous for facilitating the synchronization of the different encrypted packets formed by the data of the stream and for accelerating their processing by the access control devices in fact a control word is memorized during each crypto period for being used during the following crypto period fig3 shows an embodiment in which the processing unit receives control messages ecm containing at once the first and second control words cw 1 cw 2 and the first and second right execution parameters c 1 c 2 as in the example of fig1 the second access control device ca 2 includes a memory m 2 structured in a plurality of registers r 1 r 2 r 3 rn containing keys and each register is associated to a counter ct 1 ct 2 ctn in the application of the digital pay television the keys kr stored in the registers r 1 r 2 rn may be attributed each to a particular set bouquet of channels to a user group owning a given kind of account r 1 standard account r 2 premium account r 3 video on demand vod or also to a set of programs or specific events football world cup ski competitions tennis etc the granularity or level of detail of the services provided by means of the register keys kr is generally coarser than that provided by the rights stored in the security module sm these rights may limit for example a viewing duration of programs accessible to a user owning a.