A method and cryptographic system for descrambling content packets, included within a digital transport stream, having conditional access messages and header data. The method and system require a deciphering algorithm, processed by a descrambler of a multimedia unit, using an initialization vector as a first input and a control word as a second input. The multimedia unit is connectable to a security module for processing the conditional access messages and extracting control words therefrom. The initialization vector, or intermediate data to determine the initialization vector, is the result of a cryptographic function based on at least one variable extracted from the digital transport stream and processed by the security module.