A method for protecting multimedia data exchanged between a multimedia processing device and a security module managed by a management center, the multimedia processing device receiving an encrypted multimedia data, decrypting the encrypted multimedia data and converting the decrypted multimedia data, said multimedia processing device including a personal key, the method comprising obtaining an identifier pertaining to an entity representing a group of multimedia processing devices, calculating a security key formed by a one-way function based on the personal key of the processing multimedia device and the entity identifier, transmitting this security key and the entity identifier to the security module connected to said multimedia processing device, calculating the security key using a one-way function based on the personal key of said multimedia processing device and the entity identifier, and using the calculated security key to secure the data exchanged between the multimedia processing device and the security module.