Abstract: A secure microprocessor is provided with reduced vulnerability to attack. In the past, the secure operation of such processors has been overcome by observing the behavior of the clock used by the processor. Such observations, and the prediction of subsequent clock pulses therefrom, are prevented by modulating the clock by a substantially random function to provide an unpredictable stream of clock pulses. The secure processor is responsive to the unpredictable stream of clock pulses for processing data in accordance with a security algorithm.

Abstract: A signal amplitude distribution analyzer measures the amplitude probability density function of electrical signals, and in particular, the amplitude probability distribution of noise signals. Such measurements may be used to determine the "Gaussianicity" of noise signals, that is, a measurement of how closely the amplitude distribution of noise signals corresponds to theoretical values derived from the Gaussian probability distribution density function. This theoretical density function represents the relative percentage of time that a noise signal is at a given amplitude. The invention gives an approximation of this function by measuring the amount of time a noise signal is between a window of two adjustable voltage levels. This is accomplished by producing an output voltage proportional to the amount of time a noise signal amplitude falls within the window of values defined by the two adjustable voltage levels.