A first entity (1) includes a secret encryption key (g1, γ) of an encryption diagram which can accept a plurality of encryption keys. A plurality of decoders have respective encryption keys, each incorporating a respective key index. The method comprises an encryption operation, wherein a value is taken as a variable element (s) and a ciphertext (C′) is calculated from the data to be transmitted (K) and at least from the value taken as the variable element and the secret encryption key. An operation for revoking a decryption key is integrated into an encryption operation performed with a value of the variable element (s) based on the index key (x′) of one of the decryption keys, which will be used during an operation for tracking illegitimate decoders.