The aim of this invention is to propose a flexible solution to the risk represented by the interception of data by an unauthorized party during the transmission of said data between a broadcast center and a specialized decryption/decompression circuit such as is used in a Pay-TV decoder. This aim is achieved by a data transmission method involving a broadcast center or diffusion center, a management center and a multimedia unit, the latter comprising at least one unique key and a security module having a transport key.