A New PUF Based Lock and Key Solution for Secure In-Field Testing of Cryptographic Chips

Scan-based side-channel attacks have become a new threat to cryptographic chips. Existing countermeasures require a secret test key to unlock the scan chain before in-field testing is allowed. However, test key disclosure poses tremendous risks to multiple crypto chips that share a common test key. We address this open problem of in-field testing by leveraging physical unclonable function (PUF) to make the derived test key unique to each chip. The PUF's response is invoked only once and hardened into a one-time programmable pad. The PUF response required by the designer to derive a test key of each crypto chip can only be recovered at the time of locking the scan chains without directly reading it out. The manufacturer can test the chip normally with no test time penalty before the passed chips are locked. The proposed solution is analyzed to be secure against all known scan-based side-channel attacks and the overhead incurred for the added security is negligibly small.