A system for increasing a tamper resistance of a digital data processing unit, comprises a first unit (901) comprising processing means (906) for processing the received digital data in dependence on values looked up in at least one look-up table (916) based on values in the received digital data. The system further comprises a second unit (902) comprising means (912) for computing at least one value for inclusion in the digital data causing the first unit (901) to look up at least one predetermined value in the look-up table (916) when processing the digital data. It comprises an inserter (910) for including the at least one value in the digital data, and an output (908) for transmitting the digital data to the first unit.