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We introduce the notion of security-mediated certificateless (SMC) cryptography. This allows more lightweight versions of mediated cryptography while maintaining the ability for instantaneous revocation of keys. Moreover, our solutions avoid key escrow, which has been used in all previous mediated cryptography algorithms. We provide a model of security against a fully-adaptive chosen ciphertext attacker, who may be a rogue key generation centre or any coalition of rogue users. We present a generic construction and also a concrete algorithm based on bilinear pairings. Our concrete scheme is more efficient than the identity-based mediated encryption scheme of Baek and Zheng in PKC 2004 which is provably secure in a comparable security model. In addition, our proposals can be easily extended to support distributed security mediators. Keywords: security-mediated cryptography, certificateless cryptography. [ABSTRACT FROM AUTHOR]

Ring signature is a group-oriented signature in which the signer can spontaneously form a group and generate a signature such that the verifier is convinced the signature was generated by one member of the group and yet does not know who actually signed. Linkable ring signature is a variant such that two signatures can be linked if and only if they were signed by the same person. Recently, the first short linkable ring signature has been proposed. The short signature length makes it practical all of a sudden to use linkable ring signature as a building block in various cryptographic applications. However, we observed a subtle and yet imperative blemish glossed over by their security model definition which, if not carefully understood and properly handled, could lead to unanticipated security threats. Inspired by the recent refinement of security definitions in conventional ring signatures, we formalize a new and better security model for linkable ring signature schemes that takes into account realistic adversarial capabilities. We show that the new model is strictly stronger than all existing ones in the literature. Under our new model, we propose a new short linkable ring signature scheme, improved upon the existing scheme. Keywords: ring signature, linkable ring signature, short signature. [ABSTRACT FROM AUTHOR]

A number of e-lottery schemes have been proposed; however, none of them can satisfy all the identified requirements. In particular, some of them require a certain subset of players to remain online or the existence of a trusted third party (TTP) in order to generate the winning number(s) and some suffer from the forgery ticket attack. In this paper, we propose a new e-lottery scheme based on Verifiable Random Function that can satisfy all the identified requirements without the presence of TTP, yet the result of this generation is publicly verifiable. Keywords: e-lottery, verifiable random function, applied cryptography. [ABSTRACT FROM AUTHOR]

Identity-based (ID-based) cryptosystems eliminate the need for validity checking of the certificates and the need for registering for a certificate before getting the public key. These two features are desirable especially for the efficiency and the real spontaneity of ring signature, where a user can anonymously sign a message on behalf of a group of spontaneously conscripted users including the actual signer. In this paper, we propose a novel construction of ID-based ring signature which only needs two pairing computations for any group size. The proposed scheme is proven to be existential unforgeable against adaptive chosen message-and-identity attack under the random oracle model, using the forking lemma for generic ring signature schemes. We also consider its extension to support the general access structure. Keywords: Identity-based signature, ring signature, bilinear pairings, efficiency, real spontaneity, general access structure, anonymity [ABSTRACT FROM AUTHOR]

Delegation of authorities is a common practice in various organizations. The way delegation is performed can be quite complicated. To capture possible delegation structures, the concept of delegation network is proposed, so that anyone can be convinced of who obtained delegation from whom in order to produce the final proxy signature. In this paper, we consider the delegation network for identity-based (ID-based) scenario. Since the public key is just a string denoting the user's identity, certificate management is simplified. Proxy signature schemes have been devised to delegate signing authorities. We show that a trivial attempt of extending an existing ID-based proxy signature may result in an insecure scheme. After that we propose a building block of our ID-based delegation network, which is an ID-based proxy signature supporting batch verifications. Our proposed ID-based delegation network is flexible in the sense that the whole delegation network does not need to be known in advance. Our proposal is provably secure under the random oracle model. Keywords: Delegation network, identity-based cryptography, proxy signature, batch verification, bilinear pairings [ABSTRACT FROM AUTHOR]

We present a bandwidth-efficient off-line anonymous e-cash scheme with traceable coins. Once a user double-spends, his identity can be revealed and all his coins in the system can be traced, without resorting to TTP. For a security level comparable with 1024-bit standard RSA signature, the payment transcript size is only 512 bytes. Security of the proposed scheme is proven under the q-strong Diffie-Hellman assumption and the decisional linear assumption, in the random oracle model. The transcript size of our scheme can be further reduced to 192 bytes if external Diffie-Hellman assumption is made. Finally, we propose a variant such that there exists a TTP with the power to revoke the identity of a payee and trace all coins from the same user, which may be desirable when a malicious user is identified by some non-cryptographic means. Keywords: E-cash, Coin-traceability, Bilinear Pairing. [ABSTRACT FROM AUTHOR]

In threshold ring signature schemes, any group of t entities spontaneously conscript arbitrarily n - t entities to generate a publicly verifiable t-out-of-n signature on behalf of the whole group, yet the actual signers remain anonymous. The spontaneity of these schemes is desirable for ad-hoc groups such as mobile ad-hoc networks. In this paper, we present an identity based (ID-based) threshold ring signature scheme. The scheme is provably secure in the random oracle model and provides trusted authority compatibility. To the best of authors' knowledge, our scheme is the first ID-based threshold ring signature scheme which is also the most efficient (in terms of number of pairing operations required) ID-based ring signature scheme (when t = 1) and threshold ring signature scheme from pairings. Keywords: Threshold ring signature, identity-based signature, bilinear pairings, anonymity, spontaneity. [ABSTRACT FROM AUTHOR]

This paper gives a solid and inspiring survey of ID-based ring signatures from a number of perspectives. It is well known that ID-based cryptosystems provide some advantages that traditional public key infrastructure (PKI) cannot achieve. What advantages do ID-based ring signature schemes possess that PKI-based schemes do not? Many ID-based ring signature schemes have been proposed. What is the design philosophy behind existing ID-based ring signature schemes? This paper summarizes the study of ID-based ring signature schemes in the literature, investigates their relationships with other existing cryptographic schemes, describes the extension of ID-based ring signature schemes and the related supporting protocol, reviews the state-of-the-art and discusses a number of interesting open problems. Keywords: Identity based cryptography, ring signature, spontaneous anonymous group signature, PKI, bilinear pairings. [ABSTRACT FROM AUTHOR]