A first entity generates an initial value IV, and iterates a one-way function F on V a given number of times so as to obtain a final value FV. A second entity (possibly equal to the first) burns X into a device D. Device D has means to iterate the function F. Device D later receives an alleged n-th proof value PV, where n is a positive integer, and verifies PV by iterating the function F on PV n t