A sub-module of a digital circuit can contain secret elements such as keys and algorithms. These elements need to be kept secret all along the product life: production distrubution and end use. In particular testing is a critical step, since conventional techniques allow external access to the mode of operation of sub-module. Such conventional techniques include structural scan test, which allows dowloading of all the internal registers of the circuit, and functional application-like tests in which the sub-module receives stimuli such as it would receive during operation, and outputs respective signals which reveal information on the function of the sub-module by linking the expected output signals to the input signals.