Patent ID: 7697686

Claim:
A processing unit for digital audio/video data encrypted with one or several control-words coming from control messages, the processing unit including a deciphering unit for deciphering the audio/video data, a decompression unit, an input/output interface of the processed audio/video data and a communication device for communicating with a security module associated with the processing unit, wherein the security module is configured for: receiving a control message containing a control word, obtaining a common key and generating a first cryptogram made up of a set including the common key, the control word and a digest calculated from the common key and the control word using a unidirectional hash function, the set being encrypted by a personal key of the deciphering unit, and transmitting the first cryptogram to the deciphering unit, the deciphering unit including an encryption unit provided with at least the personal key and the common key, the deciphering unit configured for: decrypting the first cryptogram using the personal key, verifying integrity of the common key and the control word by comparing the digest calculated by the security module with a digest calculated by the deciphering unit using the received common key and control word, loading the common key in the encryption unit of the deciphering unit and authorizing the deciphering of the audio/video data with the control word, when the verification has been successful, deciphering the audio/video data and encrypting the audio/video data by the encryption unit using the common key, and transmitting said re-encrypted audio/video data to the decompression unit, the decompression unit including a decryption unit provided with at least one personal key and the common key, the decompression unit configured for: generating and transmitting a random number to the security module and the decompression unit further configured for selecting the common key specific to the audio/video data, for generating a second cryptogram made up of a set including at least the common key and the random number, the set being encrypted by the personal key of the decryption unit and for transmitting the second cryptogram to the decryption unit, decrypting the second cryptogram received from the security module using the personal key of the decryption unit and verifying the integrity of the random number by comparing the received random number with the previously generated random number, and loading the common key into the decryption unit and decrypting the audio/video data with the common key when the verification is successful.