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Scan chain-based DfT is the industry standard in use for testing manufacturing defects in the semiconductor industry to ensure the structural and functional correctness of chips. Fault coverage is significantly enhanced due to the higher observability and controllability of the internal latches. These ensuing benefits to testing, if misused, expose vulnerabilities that can be detrimental to the security aspects, especially in the context of crypto-chips that contain a secret key. Hence, it remains of paramount importance for a chip designer to secure crypto-chips against various scan attacks. A countermeasure is proposed in this paper that preserves the secrecy of an embedded key in a cryptographic IC running an AES implementation. A novel design involving a hardware unit is illustrated which circumvents differential scan attacks by essentially performing bit flips deterministically, using a pre-computed mask value. This helps secure the chip while retaining full testability. The controller logic directly depends on a mask determination algorithm that can defend against any scan attack with \(\mathcal {O}(1) \) theoretical complexity. Security analysis of our proposed defense procedure is performed in the framework of Discrete Event Systems (DES). The sequential scan circuit of an AES cryptosystem is modeled as a DES using Finite State Automata. A security notion, Opacity , is used to quantify and formally verify the security aspects of our controlled system which shows that the entropy of the secret key is preserved. A case study is performed which shows to mitigate state-of-the-art differential scan attacks successfully at a nominal extra overhead of 1.78%.

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