Sleeping dogs lie on a bed of onions but wake when mixed

We introduce and investigate sleeper attacks and explore them in the context of anonymous communication, especially mix networks. Sleeper attacks can make use of the interference inherent to mix protocols. Simply by leaving his own messages in a mix network an adversary can learn about the communication of network users. Sleeper attacks can be combined with epistemic attacks, statistical disclosure, or other attacks to be made even more effective. We use sleeper attacks to disprove the common belief that mix networks are necessarily more secure than onion routing networks. Finally we use our results to disprove another commonly held belief about computer security in general, that it is always conservative to prove security against the strongest possible adversary.