An algorithm for finding carriers of amplitude-modulated electromagnetic emanations in computer systems

This paper presents an algorithm for identifying carriers of amplitude-modulated electromagnetic (EM) emanations from computer systems. Computer systems create EM emanations across the RF spectrum making it difficult, error-prone, and time-consuming to find the relatively few emanations that expose sensitive information. One of the most common and simplest mechanisms for information leakage occurs when the amplitude of an existing strong signal (e.g. a processor or memory clock) is amplitude modulated by a system activity. If the system activity can be linked to sensitive information, this results in information leakage. We present an algorithm for finding these AM modulated signals, demonstrate the algorithm's performance on several different types of processors and systems (desktops, laptops, and smartphones), and compare the results to an exhaustive manual search. We also verify that all signals identified by the algorithm can be traced to plausible unintentional modulation mechanisms to illustrate that these signals can potentially cause information leakage. This algorithm can be an important tool for hardware designers to quickly identify circuits that are leaking sensitive information.