A computer system with multiple security levels, the system comprising a high-power processing device (130), a low-power processing device (110), and an interface unit (120) comprising functions for moving classified information between the high-power device (130) and the low-power device (110) according to formal rules for confidentiality and/or integrity. Additional security aspects, e.g. availability, may readily be accommodated. A method for implementing multiple levels of security along a number of independent security axes on the system is also disclosed.