Patent ID: 8144867

Claim:
A duration computing method in a security module inserted into an apparatus including an internal clock, the security module having a decryption time memory and a current time memory, the apparatus receives a digital data stream encrypted via control words contained in a control message ECM, the method comprising: receiving data coming from the internal clock of the apparatus including a current temporal information, storing the current temporal information into the current time memory in the security module, receiving the control message ECM to decrypt at least one control word, reading the decryption time memory representing a previous temporal information at a moment of processing a previous control message ECM, processing the control message ECM when the current temporal information is temporally ahead of the previous temporal information, and storing the current temporal information into the decryption time memory, verifying during the reception of the current temporal information that the information is temporally ahead of the previous temporal information, wherein the security module contains rights serving to verify access conditions contained in the control message ECM together with the control words, and the rights contained in the security module authorize the decryption of the control message ECM when the current temporal information representing date and time is temporally ahead of the previous temporal information, wherein the apparatus is connected to a management center, the current temporal information of the apparatus is updated by messages received from the management center, and wherein the apparatus is a digital video recorder PVR for digital pay television programs.