Breaking undercover

This paper reports two attacks on Undercover, a human authentication scheme against passive observers proposed at CHI 2008. The first attack exploits nonuniform human behavior in responding to authentication challenges and the second one is based on information leaked from authentication challenges or responses visible to the attacker. The second attack can be generalized to break two alternative Undercover designs presented at Pervasive 2009. All the attacks exploit design flaws of the Undercover implementations.