TMO: A new class of attack on cipher misusing test infrastructure

We present a new class of scan attack on hardware implementation of ciphers. The existing scan attacks on ciphers exploit the Design for Testability (DfT) infrastructure of the implementation, where an attacker applies cipher inputs in the functional mode and then by switching to the test mode retrieves the secret key in the form of test responses. These attacks can be thwarted by applying a reset operation when there is a switch of mode. However, the mode-reset countermeasure can be thwarted by using only the test mode of a secure chip. In this work we show how a Test-Mode-Only (TMO) attack can overcome the constraints imposed by a mode-reset countermeasure and demonstrate TMO attacks on private key as well as public key ciphers.