Practical client puzzles in the standard model

Client puzzles are cryptographic problems that are neither easy nor hard to solve. In this paper, we solve the problem of constructing cryptographic puzzles that are secure in the standard model and are very efficient. To prove the security of our puzzle, we introduce a new variant of the interval discrete logarithm assumption which may be of independent interest, and show this new problem to be hard under reasonable assumptions. Our experimental results show that, for 512-bit modulus, the solution verification time of our proposed puzzle can be up to 50x and 89x faster than that of the existing puzzles.