Denial of Service (DoS) attacks are serious threats for network societies. For dealing with DoS attacks, Jakobsson and Juels first proposed the notion of useful client puzzles (UCPs) which simultaneously decrease servers' burden and increase clients'. In ACM CCS'04, Diament, Lee, Keromytis, and Yung introduced the decryption-based UCPs. In this paper, we give a general framework for constructing such UCPs which is based on identity-based cryptography along with well-analyzed symmetric key authenticated encryption techniques (without random oracles). By using this framework, we can flexibly construct various UCPs according to different types of system requirements. We also give some instantiations: the first is a UCP based on the Boneh-Boyen Identity based encryption scheme, with provable security in the standard model. Another one is a UCP based on the Boneh-Gentry-Hamburg identity-based encryption scheme, which doesn't require pairings. Categories and Subject Desc...