Abstract. We identify and fill some gaps with regard to consistency (the extent to which false positives are produced) for public-key encryption with keyword search (PEKS). We define computational and statistical relaxations of the existing notion of perfect consistency, show that the scheme of [7] is computationally consistent, and provide a new scheme that is statistically consistent. We also provide a transform of an anonymous IBE scheme to a secure PEKS scheme that, unlike the previous one, guarantees consistency. Finally we suggest three extensions of the basic notions considered here, namely anonymous HIBE, public-key encryption with temporary keyword search, and identity-based encryption with keyword search.

A multi-signature scheme enables a group of signers to produce a compact, joint signature on a common document, and has many potential uses. However, existing schemes impose key setup or PKI requirements that make them impractical, such as requiring a dedicated, distributed key generation protocol amongst potential signers, or assuming strong, concurrent zero-knowledge proofs of knowledge of secret keys done to the CA at key registration. These requirements limit the use of the schemes. We provide a new scheme that is proven secure in the plain public-key model, meaning requires nothing more than that each signer has a (certified) public key. Furthermore, the important simplification in key management achieved is not at the cost of efficiency or assurance: our scheme matches or surpasses known ones in terms of signing time, verification time and signature size, and is proven secure in the random-oracle model under a standard (not bilinear map related) assumption. The proof is based on a simplified and general Forking Lemma that may be of independent interest.

Abstract. We study an adaptive variant of oblivious transfer in which a sender has N messages, of which a receiver can adaptively choose to receive k one-after-the-other, in such a way that (a) the sender learns nothing about the receiver's selections, and (b) the receiver only learns about the k requested messages. We propose two practical protocols for this primitive that achieve a stronger security notion than previous schemes with comparable efficiency. In particular, by requiring full simulatability for both sender and receiver security, our notion prohibits a subtle selective-failure attack not addressed by the security notions achieved by previous practical schemes. Our first protocol is a very efficient generic construction from unique blind signatures in the random oracle model. The second construction does not assume random oracles, but achieves remarkable efficiency with only a constant number of group elements sent during each transfer. This second construction uses novel techniques for building efficient simulatable protocols.