SyferLock presents a one-time password system, GridCode, that allows an unaided human to authenticate, reducing the cost of deployment. The one-time password system is a human computable challenge-response protocol which they claim defends against key-logging, replay, and brute force attacks, among others. We evaluate the security of the GridCode one-time password system and challenge these claims. We identify weak preimage resistance and character independence as key weaknesses of the GridCode system, leading to a variety of attacks. Our analysis indicates their scheme is akin to providing an adversary the ability to perform a brute force attack on a user's password in parallel without significant effort, lowering the effort required to recover a strong user password. Given a small number of challengeresponse pairs, an adversary can recover a user's password (e.g., 2-4 pairs), and additional secret (e.g., 1 pair). Copyright 2011 ACM.