Activation of logic encrypted chips: Pre-test or post-test?

Logic encryption has been a popular defense against Intellectual Property (IP) piracy, hardware Trojans, reverse engineering, and IC overproduction. It protects a design from these threats by inserting key-gates that break the functionality when controlled by wrong keys. Researchers have taken multiple attempts in breaking logic encryption and leaking its secret key, while they also proposed difficult-to-break logic encryption techniques. Mainly, state-of-the-art logic encryption techniques pursue two different models that differ in when the manufactured chips are activated by loading the secret key on the chip's memory: activation prior to manufacturing test (pre-test) versus subsequent to manufacturing test (post-test). In this paper, we shed light on the interaction between manufacturing test and logic encryption. We assess and compare the pre-test and post-test activation models not only in terms of the impact of logic encryption on test parameters such as fault coverage, test pattern count and test power consumption, but also in terms of the impact of manufacturing test on the security of logic encryption. We outline a test data mining attack that can successfully determine the logic encryption key of a pre-test activated chip by utilizing the test data.