A first circuit is authenticated using a second circuit. A first datum and a second datum are stored in the second circuit. The second datum corresponds to an application of a first function to the first datum and a third datum. The second circuit sends the second datum to the first circuit. The first circuit decrypts the second datum and sends a fourth datum representative of a result of the decrypting to the first second circuit for authentication. The second circuit verifies a correspondence between the first datum and the fourth datum.