Inserting malicious sub-circuits that may destroy a logical circuit or provide leakage of confidential information from a system containing the logical circuit demands detection of such sub-circuits followed their masking if possible. We suggest a method of finding a set of sequential circuit nodes in which Trojan Circuits (TC) may be inserted. The method is based on precisely calculated random estimations of controllability and observability of internal combinational part nodes and an evidence of existence of a transfer sequence of the length, not more preset value l for a set of internal states. Calculations of controllability and observability estimations and clarification of an evidence of existence of the transfer sequence are based on applying ROBDD (Reduced Ordered Binary Decision Diagram) operations. The techniques of masking TCs in working area and out of working area (out of the specification) are suggested.
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