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A modular approach to constructing cryptographic protocols leads to simple designs but often inefficient instantiations. On the other hand, ad hoc constructions may yield efficient protocols at the cost of losing conceptual simplicity. We suggest a new design paradigm, structure-preserving cryptography, that provides a way to construct modular protocols… (More)

We propose a new methodology for rational secret sharing leading to various instantiations (in both the two-party and multi-party settings) that are simple and efficient in terms of computation , share size, and round complexity. Our protocols do not require physical assumptions or simultaneous channels, and can even be run over asynchronous, point-to-point… (More)

We introduce signatures where signers can only sign messages that conform to some policy, yet privacy of the policy is maintained. We provide definitions and show that policy-based signatures provide a framework which yields a unified view of many other existing types of signatures that now appear as special cases. We also show how still other primitives… (More)

Electronic cash (e-cash) refers to money exchanged electronically. The main features of traditional cash are usually considered desirable also in the context of e-cash. One such property is off-line trans-ferability, meaning the recipient of a coin in a transaction can transfer it in a later payment transaction to a third person without contacting a central… (More)

We extend the notion of verifiable random functions (VRF) to constrained VRFs, which generalize the concept of constrained pseudorandom functions, put forward by Boneh and Waters (Asiacrypt'13), and independently by Kiayias et al. (CCS'13) and Boyle et al. (PKC'14), who call them delegatable PRFs and functional PRFs, respectively. In a standard VRF the… (More)

Verifiable encryption allows one to encrypt a signature while preserving its public verifiability. We introduce a new primitive called commuting signatures and verifiable encryption that extends this in multiple ways, such as enabling encryption of both signature and message while proving validity. More importantly, given a ciphertext, a signer can create a… (More)

We propose an efficient blind certification protocol with interesting properties. It falls in the Groth-Sahai framework for witness-indistinguishable proofs, thus extended to a certified signature it immediately yields non-frameable group signatures. We use blind certification to build an efficient (offline) e-cash system that guarantees user anonymity and… (More)

We introduce the notion of automorphic signatures, which satisfy the following properties: the verification keys lie in the message space, messages and signatures consist of elements of a bilinear group, and verification is done by evaluating a set of pairing-product equations. These signatures make a perfect counterpart to the powerful proof system by… (More)

In 2008, Groth and Sahai proposed a general methodology for constructing non-interactive zero-knowledge (and witness-indistinguishable) proofs in bilinear groups. While avoiding expensive NP-reductions, these proof systems are still inefficient due to the number of pairing computations required for verification. We apply recent techniques of batch… (More)

We define a general model for consecutive delegations of signing rights with the following properties: The delegatee actually signing and all intermediate delegators remain anonymous. As for group signatures, in case of misuse, a special authority can open signatures to reveal the chain of delegations and the signer's identity. The scheme satisfies a strong… (More)