Efficient Chosen-Ciphertext Security via Extractable Hash Proofs

We introduce the notion of an extractable hash proof system. Essentially, this is a special kind of non-interactive zero-knowledge proof of knowledge system where the secret keys may be generated in one of two modes to allow for either simulation or extraction. - We show how to derive efficient CCA-secure encryption schemes via extractable hash proofs in a simple and modular fashion. Our construction clarifies and generalizes the recent factoring-based cryptosystem of Hofheinz and Kiltz (Eurocrypt '09), and is reminiscent of an approach proposed by Rackoff and Simon (Crypto '91). We show how to instantiate extractable hash proof system for hard search problems, notably factoring and computational Diffie-Hell man. Using our framework, we obtain the first CCA-secure encryption scheme based on CDH where the public key is a constant number of group elements and a more modular and conceptually simpler variant of the Hofheinz-Kiltz cryptosystem (though less efficient). - We introduce adaptive trapdoor relations, a relaxation of the adaptive trapdoor functions considered by Kiltz, Mohassel and O'Neil (Eurocrypt 10), but nonetheless imply CCA-secure encryption schemes. We show how to construct such relations using extractable hash proofs, which in turn yields realizations from hardness of factoring and CDH.