New scan-based attack using only the test mode

Scan attack is a threat to crypto-chips. An attacker can leverage the test mode of the chip and control the scan chains in order to reveal the secret key. One solution for this kind of attacks is to hamper the ability to switch the device from normal mode to test mode and corrupt the data in the scan cells. If the device is reset each time it switches the mode from normal to test, all existing attacks can be thwarted. We propose a new scan-based attack by controlling only the scan chains and demonstrate it on the AES hardware. The attack uses only the test mode of the hardware and it does not require switching between normal and test mode. The attack will work even in the presence of mode blocking countermeasure. The attack requires only 375 test vectors with an attack time complexity around 2^12.58.