Emerging Attacks on Logic Locking in SFQ Circuits and Related Countermeasures

Hardware security in single flux quantum (SFQ) circuits is an emerging research area due the increasing demand of data centers operating with sensitive information and the limited number of fabrication facilities, where certain stages can be compromised. An existing logic locking technique in SFQ circuits, that is based on supplying additional current with mutual inductance to change the circuit operation, is analyzed in this article. Two new attacks on this logic locking technique are proposed. In the first attack, the locked OR cell is reset by applying a specific input combination. As a result, the overall range of correct key values can be increased by approximately 18.8 and 32.9% depending on the location of the generated mutual inductance. In the second attack, an overproduction threat model is proposed, which significantly decreases the number of measurements needed to reveal the secret key from exponential to linear scale. As a case study, a 4-to-2 priority encoder optimized for SFQ applications is designed and logic locked. The proposed attack is performed on this locked design. Finally, potential countermeasures for each attack type are discussed with the corresponding tradeoffs.