Mitigating Distributed Service Flooding Attacks with Guided Tour Puzzles

Various cryptographic puzzle schemes have been proposed as defenses against Denial of Service (DoS) attacks. However, these schemes have two common shortcomings that diminish their effectiveness as a DoS mitigation solution. First, the DoS-resilience that these schemes provide is minimized when there is a large disparity between the computational power of malicious and legitimate clients. Second, the legitimate clients also have to perform the same heavy puzzle computations that do not contribute to any useful work from the clients’ perspective. In this article, we introduce guided tour puzzles (GTP), a novel puzzle scheme that addresses these shortcomings. GTP uses latency — as opposed to computational delay — as a way of forcing a sustainable request arrival rate on clients. Measurement results from a large-scale network test-bed shows that the variation in the puzzle solving times is significantly smaller than the puzzle solving time variation of computation-based puzzles. As attackers have much less control over the round-trip delays than they do over the computational power, a latency-based puzzle scheme, such as GTP, provides significantly better protection against strong attackers. Meanwhile, we show that GTP minimizes useless computations required for the client computers. We evaluate the effectiveness of guided tour puzzles in a realistic simulation environment using a large-scale Internet topology, and show that GTP provides a strong mitigation of DoS request flooding attacks and puzzle solving attacks. Keywords-denial of service; availability; tour puzzles; proof of work; client puzzles; cryptography.