Hardware implementation of cryptographic algorithms is 6 subject to various attacks. It has been previously demonstrated that scan 7 chains introduced for hardware testability open a back door to potential 8 attacks. Here, we propose a scan-protection scheme that provides testing 9 facilities both at production time and over the course of the circuit's life. 10 The underlying principles to scan-in both input vectors and expected 11 responses and to compare expected and actual responses within the 12 circuit. Compared to regular scan tests, this technique has no impact 13 on the quality of the test or the model-based fault diagnosis. It entails 14 negligible area overhead and avoids the use of an authentication test 15 mechanism.