Ideally, electronic circuits are insensitive to interfering influences such as fluctuations in the supply voltage [lacuna] in the clock frequency or environmental influences such as light and temperature. Unfortunately, this is not the case in practice. Thus, every electronic circuit operates without error only in a particular tolerance range for said parameters. One problem found in this context is the definition of the tolerance limits for the aforementioned parameters. Should the combined effective tolerances of various parameters need to be taken into account, it becomes even more difficult.
Making circuits entirely insensitive to interfering influences is associated with an extremely high level of cost or is technically infeasible or is not at all possible physically. This property of electronic circuits is utilized maliciously by hackers who wish to influence the output signals from the circuit consciously and who deliberately bring about faults. Particularly in the case of smart cards and comparable applications, this is the reason why the need to protect the information and circuits on these media against unauthorized access is very high. To bring about a state for smart cards which ensures the greatest possible security, simply placing a tight restriction on the operating conditions is not a satisfactory solution.
This has been the solution to date, however. The environmental influences such as supply voltage, frequency, light and temperature are monitored using sensors. If one of the parameters slips into a range which is not tolerated, the chip is disconnected. One difficulty found in this context is defining the tolerated range. Firstly it should be as large as possible and secondly error-free, reliable operation should be ensured within the tolerated range.
In addition, the sensors for, by way of example, light, temperature, voltage and clock frequency cannot detect the environmental conditions directly on or in the circuit, since they are normally implemented so as to be physically isolated from the circuit which is to be monitored. This means that the measured result delivered by the sensors does not reflect the environmental and operating conditions which are effective directly in the circuit. The environmental and operating conditions in the circuits can differ very greatly from those on the sensors.