Informatica

This paper studies the generic construction of certificate-based signature (CBS) from certificateless signature (CLS). This paper proposes a new generic conversion from CLS to CBS which is more intuitive, simpler, and provably secure without random oracles than the current one. To develop the security proof, we put forth one novel CLS security model which features a previously neglected but nontrivial attack and hence captures the CLS security notion more comprehensively. We show that many existing CLS schemes can be proved secure in the current model by slightly modifying its original security proof. Following this conversion, many provably secure CBS schemes can be constructed from the corresponding existing CLS schemes.

Key-insulated cryptography is an important technique to protect private keys in identity-based (IB) cryptosytems. Despite the flurry of recent results on IB key-insulated encryption (IBKIE) and signature (IBKIS), a problem regarding the security and efficiency of practicing IBKIE and IBKIS as a joint IB key-insulated signature/encryption scheme with a common set of parameters and keys remains open. To deal with the above question, we propose an identity-based key-insulated signcryption (IBKISC) scheme. Compared with the Sign-then-Encrypt (StE) and Encrypt-then-Sign (EtS) using IBKIE and IBKIS in the standard model, our proposed IBKISC scheme is the fastest with the shortest ciphertext size.

The notion of concurrent signatures was introduced by Chen, Kudla and Paterson in their seminal paper in Eurocrypt 2004. In concurrent signature schemes, two entities can produce two signatures that are not binding, until an extra piece of information (namely the keystone) is released by one of the parties. Upon release of the keystone, both signatures become binding to their true signers concurrently. In ICICS 2005, two identity-based perfect concurrent signature schemes were proposed by Chow and Susilo. In this paper, we show that these two schemes are unfair. In which the initial signer can cheat the matching signer. We present a formal definition of ID-based concurrent signatures which redress the flaw of Chow et al.'s definition and then propose two simple but significant improvements to fix our attacks.

This paper proposes a threshold key escrow scheme from pairing. It tolerates the passive adversary to access any internal data of corrupted key escrow agents and the active adversary that can make corrupted servers to deviate from the protocol. The scheme is secure against threshold adaptive chosen-ciphertext attack. The formal proof of security is presented in the random oracle model, assuming the decision Bilinear Diffie-Hellman problem is computationally hard.

This paper introduces a new concept of convertible user designating confirmer partially blind signature, in which only the designated confirmer (designated by the user) and the user can verify and confirm the validity of given signatures and convert given signatures into publicly verifiable ones. We give a formal definition for it and propose a concrete provably secure scheme with a proof of security and a brief analysis of efficiency. Assuming the intractabilities of the Discrete Logarithm Problem and the ROS-Problem, the proposed scheme is unforgeable under adaptive chosen-message attack.