Method, cryptographic system and security module for descrambling content packets of a digital transport stream

A Cryptographic system for descrambling content packets, with at least header data, included in a digital transport stream, with conditional access messages, said system comprising:

• an extractor for extracting at least one variable from the digital transport stream;
• a security module provided with a processor for processing said conditional access messages and extracting control words therefrom;
• a descrambler for running a deciphering algorithm using an initialization vector as a first input and the control word as a second input;
• secure transferring elements for exchanging information at least between said security module and said descrambler;
• and
• a computing unit for generating the initialization vector, or intermediate data to determine this initialization vector, as a result of a cryptographic function based on at least said variable extracted from the digital transport stream and processed by the processor into the security module.

A security module adapted to be in communication with a multimedia unit used for descrambling content packets using a descrambler processing a deciphering algorithm, said content packets, having at least header data, and included in a digital transport stream with conditional access messages, said security module comprising:

• a first interface connectable through a first channel between the security module and the incoming stream for inputting at least said conditional access messages with at least one variable provided by data extracted from said digital transport stream and a second interface connectable through at least one secured channel with the multimedia unit for exchanging information between the security module and the multimedia unit;
• a processor for processing a variable from said digital transport stream, for checking conditional access rights and for decrypting and extracting control words from conditional access messages;
• and
• a computing unit to generate an initialization vector, or intermediate data for determining said initialization vector, using a cryptographic function that inputs at least said variable processed by the processor.