A robust scan-based side-channel attack method against HMAC-SHA-256 circuits

A scan-based side-channel attack is still a real threat against a crypto circuit as well as a hash generator circuit, which can restore secret information by exploiting the scan data obtained from scan chains inside the chip during its processing. In this paper, we propose a scan-based attack method against a hash generator circuit called HMAC-SHA-256. Our proposed method restores the secret information by finding out the correspondence between the scan data obtained from a scan chain and the internal registers in the target HMAC-SHA-256 circuit, even if the scan chain includes registers other than the target hash generator circuit and an attacker does not know well the hash generation timing. Experimental results show that our proposed method successfully restores two secret keys of the HMAC-SHA-256 circuit in at most 6 hours.