The public key encryption with keyword search (PEKS) scheme recently proposed by Boneh, Di Crescenzo, Ostrovsky, and Persiano enables one to search encrypted keywords without compromising the security of the original data. In this paper, we address three important issues of a PEKS scheme, "refreshing keywords", "removing secure channel", and "processing multiple keywords", which have not been considered in Boneh et. al.'s paper. We argue that care must be taken when keywords are used frequently in the PEKS scheme as this situation might contradict the security of PEKS. We then point out the inefficiency of the original PEKS scheme due to the use of the secure channel. We resolve this problem by constructing an efficient PEKS scheme that removes secure channel. Finally, we propose a PEKS scheme that encrypts multiple keywords efficiently.

Abstract. In Asiacrypt2001, Boneh, Lynn, and Shacham [8] proposed a short signature scheme (BLS scheme) using bilinear pairing on certain elliptic and hyperelliptic curves. Subsequently numerous cryptographic schemes based on BLS signature scheme were proposed. BLS short signature needs a special hash function [6,1,8]. This hash function is probabilistic and generally inefficient. In this paper, we propose a new short signature scheme from the bilinear pairings that unlike BLS, uses general cryptographic hash functions such as SHA-1 or MD5, and does not require special hash functions. Furthermore, the scheme requires less pairing operations than BLS scheme and so is more efficient than BLS scheme. We use this signature scheme to construct a ring signature scheme and a new method for delegation. We give the security proofs for the new signature scheme and the ring signature scheme in the random oracle model.

In this paper, we construct an efficient "multi-receiver identity-based encryption scheme". Our scheme only needs one (or none if precomputed and provided as a public parameter) pairing computation to encrypt a single message for n receivers, in contrast to the simple construction that re-encrypts a message n times using Boneh and Franklin's identity-based encryption scheme, considered previously in the literature. We extend our scheme to give adaptive chosen ciphertext security. We support both schemes with security proofs under precisely defined formal security model. Finally, we discuss how our scheme can lead to a highly efficient public key broadcast encryption scheme based on the "subset-cover" framework.

Abstract. "Certificateless Public Key Cryptography" has very appealing features, namely it does not require any public key certification (cf. traditional Public Key Cryptography) nor having key escrow problem (cf. Identity-Based Cryptography). Unfortunately, construction of Certificateless Public Key Encryption (CLPKE) schemes has so far depended on the use of Identity-Based Encryption, which results in the bilinear pairing-based schemes that need costly operations. In this paper, we consider a relaxation of the original model of CLPKE and propose a new CLPKE scheme that does not depend on the bilinear pairings. We prove that in the random oracle model, our scheme meets the strong security requirements of the new model of CLPKE such as security against public key replacement attack and chosen ciphertext attack, assuming that the standard Computational Diffie-Hellman problem is intractable.

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