Cryptographic method, wherein a first entity (1; 3) has a secret encryption key (g 1, γ) of an encryption scheme accepting several decryption keys, and several receiving entities have respective decryption keys ({A, B, x}), each decryption key incorporating a respective key index (x), the method comprising an encryption operation comprising the following steps: - assigning a value to at least one variable element (s); and characterized in that it comprises the steps of: - computing a cryptogram (C') from data to be transmitted, from at least each value of a variable element (s), and from the secret encryption key, wherein the cryptogram (C') has a size that is constant and independent of the number of receiving entities, and comprises at least portions representative: • of the element C 1 = h 1 1/(γ+s) of the group G 1, where s is the value assigned to a variable element, taken as an integer lying between 1 and p-1; • of the value C 2 = s of said variable element; • of a value C 5 derived from the data to be transmitted and from a masking value computed in a cyclical group the method also comprising an operation of revocation of at least one decryption key incorporated in an encryption operation carried out by assigning to a variable element (s) a value that is a function of the key index (x') of one of the decryption keys ({A', B', x'}), at least certain of the revocation operations also including an update of at least one parameter (h 1, h 2) involved in the computation of the cryptogram (C') for a subsequent encryption operation, wherein the secret encryption key includes an element g 1 of a cyclical group G 1 of order p and a whole number 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 cyclical group G 2 of order p, in the form w = g 2 γ, where g 2 is an element of the group G 2, and wherein each decryption key with 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 the group G 1, h 1 being a generating element of the group G 1 such that g 1 = h 1 α where α is an integer exponent lying between 1 and p-1. - assigning a value to at least one variable element (s); and - computing a cryptogram (C') from data to be transmitted, from at least each value of a variable element (s), and from the secret encryption key, wherein the cryptogram (C') has a size that is constant and independent of the number of receiving entities, and comprises at least portions representative: • of the element C 1 = h 1 1/(γ+s) of the group G 1, where s is the value assigned to a variable element, taken as an integer lying between 1 and p-1; • of the value C 2 = s of said variable element; • of a value C 5 derived from the data to be transmitted and from a masking value computed in a cyclical group the method also comprising an operation of revocation of at least one decryption key incorporated in an encryption operation carried out by assigning to a variable element (s) a value that is a function of the key index (x') of one of the decryption keys ({A', B', x'}), at least certain of the revocation operations also including an update of at least one parameter (h 1, h 2) involved in the computation of the cryptogram (C') for a subsequent encryption operation, wherein the secret encryption key includes an element g 1 of a cyclical group G 1 of order p and a whole number 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 cyclical group G 2 of order p, in the form w = g 2 γ, where g 2 is an element of the group G 2, and wherein each decryption key with 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 the group G 1, h 1 being a generating element of the group G 1 such that g 1 = h 1 α where α is an integer exponent lying between 1 and p-1. • of the element C 1 = h 1 1/(γ+s) of the group G 1, where s is the value assigned to a variable element, taken as an integer lying between 1 and p-1; • of the value C 2 = s of said variable element; • of a value C 5 derived from the data to be transmitted and from a masking value computed in a cyclical group