Complex approach to assurance of information safety of digital control systems for nuclear power plants

The paper considers, implemented by the authors within the project of advanced digital controls system for NPP with the reactor VVER-1000, a system of non-authorized access protection (NAP), partially built up on the technology of active audit. The active audit technology is based on response of the system on deviation of current signature of APCS (Automated Process Control System) in nuclear power plant from stable rather than a certain signature of attack. The methodology of the active audit, not giving up classical methods of unauthorized access protection, completes these methods. It enables one to provide functional closeness of static fragments of the information system by use of the logic scheme 'everything is forbidden with the exception of what is allowed', applied to really appeared events in the information system. (authors)