The invention concerns a method implemented in a communication network comprising a source device including: a first symmetrical key for encrypting data to be transmitted to a display device connected to the network; and the first symmetrical key encrypted with a second symmetrical network key known only to at least one display device connected to the network. When the source device needs to renew its first symmetrical key to encrypt new data, it generates a random number, then it calculates a new symmetrical key based on the first symmetrical key and on the random number. It then encrypts the data to be transmitted with the new symmetrical key and transmits to a display device, via the network: the data encrypted with the new symmetrical key, the random number, and the first encrypted symmetrical key with the second symmetrical network key.