Hashing it out in public: common failure modes of DHT-based anonymity schemes

Abstract

We examine peer-to-peer anonymous communication systems that use Distributed Hash Table algorithms for relay selection. We show that common design flaws in these schemes lead to highly effective attacks against the anonymity provided by the schemes. These attacks stem from attacks on DHT routing, and are not mitigated by the well-known DHT security mechanisms due to a fundamental mismatch between the security requirements of DHT routing's put/get functionality and anonymous routing's relay selection functionality. Our attacks essentially allow an adversary that controls only a small fraction of the relays to function as a global active adversary. We apply these attacks in more detail to two schemes: Salsa and Cashmere. In the case of Salsa, we show that an attacker that controls 10% of the relays in a network of size 10,000 can compromise more than 80% of all completed circuits; and in the case of Cashmere, we show that an attacker that controls 20% of the relays in a network of size 64000 can compromise 42% of the circuits.