A method for violating the security of a cryptographic coprocessor, such as a smart card, by disrupting the execution of one or more machine instructions. For example, the attacker carefully inspects the signals generated by the instruction sequence of the circuits. At the precise moment that a compare or jump instruction is executing, a high-speed external disturbance is applied, or the internal clock frequency is increased. This effectively blocks the instruction's execution, perhaps bypassing a critical authentication routine. Countermeasures for such an attack might be circuits that generate random patterns that cannot be easily analyzed. See cryptographic coprocessor.