In our previous work [12], [13], we introduced generalized feed-forward shift registers (GF~(2)SR, for short) to apply them to secure and testable scan design, where we considered the security problem from the viewpoint of the complexity of identifying the structure of GF~(2)SRs. Although the proposed scan design is secure in the sense that the structure of a GF~(2)SR cannot be identified only from the primary input/output relation, it may not be secure if part of the contents of the circuit leak out. In this paper, we introduce a more secure concept called strong security such that no internal state of strongly secure circuits leaks out, and present how to design such strongly secure GF~(2)SRs.
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