Method allowing a secure counter to be achieved in and on-board computer system having access to a chip card, comprising the following operating phases: (i) storage, in a non-volatile memory of the chip card: a. of a counting function FC, b. of a counter Cpt, and c. of a private key Cf that is stored in the read-protected portion of the memory zone, the counter Cpt and the private key Cf being accessible only to the counting function FC; (ii) execution, on reception by the chip card of a counter request made by a requesting entity ER of the on-board computer system, wherein the requesting entity is off the chip card, of the following operations: a. the modification, by the counting function FC, of the counter Cpt, ensuring the uniqueness of the counter value returned in response to a given request; b. the calculation, by the counting function FC, of a signature on the basis of the value of the counter Cpt such as modified in step (a) and of the private key Cf, and c. the transmission, by the counting function FC, of a response to the requesting entity ER, said response including the value of the counter Cpt such as modified in step (a) and the signature calculated in step (b); and (iii) the execution, on reception by the on-board system of the response to the counter request, of a verification of the signature contained in this response. (i) storage, in a non-volatile memory of the chip card: a. of a counting function FC, b. of a counter Cpt, and c. of a private key Cf that is stored in the read-protected portion of the memory zone, the counter Cpt and the private key Cf being accessible only to the counting function FC; a. of a counting function FC, b. of a counter Cpt, and c. of a private key Cf that is stored in the read-protected portion of the memory zone, (ii) execution, on reception by the chip card of a counter request made by a requesting entity ER of the on-board computer system, wherein the requesting entity is off the chip card, of the following operations: a. the modification, by the counting function FC, of the counter Cpt, ensuring the uniqueness of the counter value returned in response to a given request; b. the calculation, by the counting function FC, of a signature on the basis of the value of the counter Cpt such as modified in step (a) and of the private key Cf, and c. the transmission, by the counting function FC, of a response to the requesting entity ER, said response including the value of the counter Cpt such as modified in step (a) and the signature calculated in step (b); and a. the modification, by the counting function FC, of the counter Cpt, ensuring the uniqueness of the counter value returned in response to a given request; b. the calculation, by the counting function FC, of a signature on the basis of the value of the counter Cpt such as modified in step (a) and of the private key Cf, and c. the transmission, by the counting function FC, of a response to the requesting entity ER, said response including the value of the counter Cpt such as modified in step (a) and the signature calculated in step (b); and (iii) the execution, on reception by the on-board system of the response to the counter request, of a verification of the signature contained in this response.