Patent ID: 8121289

Claim:
A cryptographic method, wherein a first entity has a secret encryption key of an encryption scheme accepting a plurality of decryption keys, and a plurality of receiving entities have respective decryption keys, each decryption key incorporating a respective key index, the method comprising an encryption operation including the steps of: assigning a value to at least one variable element; and computing, using a processor selected between a general-purpose processor, a specific circuit and a programmed logic component, a cryptogram from data to be transmitted, at least each variable element value and the secret encryption key, the method further comprising a revocation operation for at least one decryption key, the revocation operation being incorporated in an encryption operation carried out by assigning to said at least one variable element a value depending on the key index of one of said decryption keys, wherein the secret encryption key includes an element g 1 of a cyclic group G 1 of order p and an integer γ chosen between 1 and p−1, where p is a prime number, wherein the secret encryption key is associated with a public key w of a cyclic group G 2 of order p, in the form w=g 2 γ , where g 2 is an element of group G 2 , and wherein each decryption key having an integer index x lying between 1 and p−1 includes two elements A=g 1 1/(γ+x) and B=h 1 1/(γ+x) of group G 1 , h 1 being a generator element of group G 1 such that g 1 =h 1 α where α is an integer exponent lying between 1 and p−1.