A fault detector for detecting a fault in a digital processing circuit configured to transform an input data set to an output data set based on an energy conserving function. The fault detector includes an input sum of absolute squares circuit configured to determine an input sum of absolute squares value of the input data set, which has a predetermined length; an output sum of absolute squares circuit configured to determine an output sum of absolute squares value of the output data set; and an energy conservation check circuit configured to identify a fault in the digital processing circuit if a comparison based on the input sum of absolute squares value and the output sum of absolute squares value does not meet a predetermined energy conservation criteria.