A method of monitoring execution of computer instructions includes receiving data items representing real-time measurements of side-channel information emanating from execution of computer instructions, each data item forming a value of a corresponding dimension of a side-channel information vector, receiving, for two or more of the dimensions of the side-channel vector, classifiers that assign the corresponding values of a side-channel vector to classes, and classifying the data items in accordance with the received classifiers, wherein an orthogonal distance of the data item from the classifier indicates a confidence value of the classification, generating a combined a confidence value for the side-channel information vector a, and outputting a signal if a confidence value indicates affiliation to a selected one of the two classes with a predetermined probability. The method conducts a self-test by generating a combined confidence value based to ensure correct outputting of the confidence value.