We analyze information leaks in the lookup mechanisms of structured peer-to-peer (P2P) anonymous communication systems and how these leaks can be used to compromise anonymity. We show that the techniques used to combat active attacks on the lookup mechanism dramatically increase information leaks and the efficacy of passive attacks, resulting in a tradeoff between robustness to active and passive attacks.We study this tradeoff in two P2P anonymous systems: Salsa and AP3. In both cases, we find that, by combining both passive and active attacks, anonymity can be compromised much more effectively than previously thought, rendering these systems insecure for most proposed uses. Our results hold even if security parameters are changed or other improvements to the systems are considered. Our study, therefore, shows the importance of considering these attacks in P2P anonymous communication. ACM Reference Format:Mittal, P. and Borisov, N. 2012. Information leaks in structured peer-to-peer anonymous communication systems.