An apparatus for detecting tampering with a clock of a state-machine, comprising, a master state-machine having master states and driven by a master clock, the master states being switchable responsive to events, and an auxiliary state-machine having auxiliary states and driven by an auxiliary clock synchronous with the master clock, the auxiliary states being switchable responsive to a signal generated based at least on said events, consequently establishing a correspondence between the master states and the auxiliary states, thus ensuing that subsequent to tampering with the master clock the correspondence between the master states and the auxiliary states become discordant, thereby indicating that the master clock has been tampered with.