A Guaranteed Secure Scan Design Based on Test Data Obfuscation by Cryptographic Hash

Design-for-testability (DfT) techniques have been widely adopted into the integrated circuit (IC) design process to facilitate manufacture testing. The scan-based DfT architecture is a popular DfT feature that provides full testability for the circuit under test. However, this turns into a double-edged sword for some ICs such as cryptographic chips because scan design could be used as a side channel to access the intermediate encryption results, with which the cipher key can be deduced easily. To resist such scan-based side-channel attacks, many countermeasures are proposed to obfuscate the test data in scan chain. Unfortunately, most of the obfuscation logic, due to the performance and resource constraints, cannot be proven to be irreversible and hence suffers a high risk for the correct test data being derived from the obfuscated output. In this article, we propose to utilize the cryptographic hash module for some post-processing of the test responses in order to secure the scan design. Our approach has several clear advantages over existing ones. First, the security is guaranteed based on the preimage resistance of cryptographic hash function and the introduced salt information and data collection scheme. Second, it incurs low overhead because the hash module is normally available on the IC, in particular, those where security is important. Finally, full testability is retained as we are not modifying any test input. We present the implementation of the proposed secure design, report the experimental results, and demonstrate that our approach can resist all known scan-based side-channel attacks with negligible overhead while maintaining the testability and other testing performances.