1. Mehmud Abliz, Taieb Znati, ACSAC (Dec., 2009)

2. Outline Introduction Desired properties Basic scheme Improvements to the basic scheme Analysis Related work Conclusion and future work

3. Introduction A denial of service (DoS) or a distributed denial of service (DDoS) attack is an attempt by malicious parties to prevent legitimate users from accessing a service, usually by depleting the resources of the server. The targeted resource of the attack can be bandwidth, CPU, memory, disk capacity, or combination of the above.

4. Introduction Cryptographic puzzles are proposed to defend against such denial of service attacks by better balancing the computational load of client and server. In a cryptographic puzzle scheme, a client is required to solve a cryptographic puzzle and submit the puzzle solution as proof of work before the server commits substantial resources to its request.

5. Introduction Cryptographic puzzles are first introduced by Dwork and Naor to combat junk e-mails. Resource disparity

6. Introduction In a hash reversal puzzle, a puzzle with difficulty d takes on average 2 d-1 hash operations to compute its solution. Say a server can handle 1000 requests per second, and a subset of clients have limited resources, and can perform 10 5 hash operations per second at maximum. Assuming these clients can spend at most 50% of their computational power on computing puzzles, then the server should allow at least 2 20 /0.5*10 5 = 20 seconds between sending a puzzle and receiving its solution, for a hash-reversal puzzle with difficulty d = 21.

7. Introduction An attacker with an ASIC designed for hash computation can perform 10 9 hash operations per second, so she can compute 20 sec /(2 20 /10 9 ) = 20,000 puzzles with the same difficulty during that 20-second period. This means an attacker with just one such ASIC component can easily overwhelm the server.

8. Introduction Guided tour puzzle Use the network round-trip delay as the unit work in the puzzle solving process, and require a client to complete a guided tour by visiting a predefined set of nodes, called tour guides, in a certain sequential visiting order.

9. Desired properties