We present several new and fairly practical public-key encryption schemes and prove them secure against adaptive chosen ciphertext attack. One scheme is based on Paillier's Decision Composite Residuosity (DCR) assumption [7], while another is based in the classical Quadratic Residuosity (QR) assumption. The analysis is in the standard cryptographic model, i.e., the security of our schemes does not rely on the Random Oracle model. We also introduce the notion of a universal hash proof system. Essentially, this is a special kind of non-interactive zero-knowledge proof system for an NP language. We do not show that universal hash proof systems exist for all NP languages, but we do show how to construct very ecient universal hash proof systems for a general class of group-theoretic language membership problems. Given an ecient universal hash proof system for a language with certain natural cryptographic indistinguishability properties, we show how to construct an ecient public-key encryption schemes secure against adaptive chosen ciphertext attack in the standard model. Our construction only uses the universal hash proof system as a primitive: no other primitives are required, although even more ecient encryption schemes can be obtained by using hash functions with appropriate collision-resistance properties. We show how to construct ecient universal hash proof systems for languages related to the DCR and QR assumptions. From these we get corresponding public-key encryption schemes that are secure under these assumptions. We also show that the Cramer-Shoup encryption scheme (which up until now was the only practical encryption scheme that could be proved secure against adaptive chosen ciphertext attack under a reasonable assumption, namely, the Decision...

In this paper we present a general framework for password-based authenticated key exchange protocols, in the common reference string model. Our protocol is actually an abstraction of the key exchange protocol of Katz et al. and is based on the recently introduced notion of smooth projective hashing by Cramer and Shoup. We gain a number of benefits from this abstraction. First, we obtain a modular protocol that can be described using just three highlevel cryptographic tools. This allows a simple and intuitive understanding of its security.

Abstract. We present a general framework for constructing two-message oblivious transfer protocols using a modification of Cramer and Shoup’s notion of smooth projective hashing (2002). Our framework is actually an abstraction of the two-message oblivious transfer protocols of Naor and Pinkas (2001) and Aiello et. al. (2001), whose security is based on the Decisional Diffie Hellman Assumption. In particular, this framework gives rise to two new oblivious transfer protocols. The security of one is based on the N’th-Residuosity Assumption, and the security of the other is based on both the Quadratic Residuosity Assumption and the Extended Riemann Hypothesis. When using smooth projective hashing in this context, we must deal with maliciously chosen smooth projective hash families. This raises new technical difficulties that did not arise in previous applications, and in particular it is here that the Extended Riemann Hypothesis comes into play. Similar to the previous two-message protocols for oblivious transfer, our constructions give a security guarantee which is weaker than the traditional, simulation based, definition of security. Nevertheless, the security notion that we consider is nontrivial and seems to be meaningful for some applications in which oblivious transfer is used in the presence of malicious adversaries. 1

We introduce the notion of multi-trapdoor commitments which is a stronger form of trapdoor commitment schemes. We then construct two very e#cient instantiations of multi-trapdoor commitment schemes, based on the Strong RSA Assumption and the recently introduced Strong Di#e-Hellman Assumption.

Deniable Authentication protocols allow a Sender to authenticate a message for a Receiver, in a way that the Receiver cannot convince a third party that such authentication (or any authentication) ever took place.

Secure two-party and multiparty computation has long stood at the center of the foundations of theoretical cryptography. Recently, however, interest has grown regarding the efficiency of such protocols and their application in practice. As a result, there has been significant progress on this problem and it is possible to actually carry out secure computation for non-trivial tasks on reasonably large inputs. Part of this research goal of making secure computation practical has also involved implementations. Such implementations are of importance for two reasons: first, they demonstrate the real efficiency of known and new protocols; second, they deepen our understanding regarding where the bottlenecks in efficiency lie. However, it is very hard to compare between implementations by different research groups since they are carried out on different platforms and using different infrastructures. In addition, most implementations have been carried out without the goal of code reuse, and so are not helpful to other researchers. The difficulty of beginning implementation projects is further compounded by the fact that existing cryptographic libraries (like openSSL, Bouncy Castle, and others) are tailored for tasks like encryption, authentication and key-exchange, and not for secure computation. We have