Jianchang Lai

Efficient identity-based online/offline encryption and signcryption with short ciphertext

The technique of online/offline is regarded as a promising approach to speed up the computation of encryption, because the most part of computation, such as pairing over points on elliptic curve and exponentiation in groups, can be pre-computed in the offline phase without knowing the message to be encrypted and/or recipient’s identity. The online phase only requires light computation, such as modular multiplication. In this paper, we propose two novel identity-based online/offline schemes: a full secure identity-based online/offline encryption scheme and an identity-based online/offline signcryption scheme. Compared to the other schemes in the literature, our schemes achieve the shortest ciphertext size in both offline and online phases and demonstrate the best performance in offline computation. Our schemes are applicable to devices with limited computation power. They are proven secure in the random oracle model.

Improved Identity-Based Online/Offline Encryption

The notion of online/offline encryption was put forth by Guo, Mu and Chen (FC 2008), where they proposed an identity-based scheme called identity-based online/offline encryption (IBOOE). An online/offline encryption separates an encryption into two stages: offline and online. The offline phase carries much more computational load than the online phase, where the offline phase does not require the information of the message to be encrypted and the identity of the receiver. Subsequently, many applications of IBOOE have been proposed in the literature. As an example, Hobenberger and Waters (PKC 2014) have recently applied it to attribute-based encryption. In this paper, we move one step further and explore a much more efficient variant. We propose an efficient semi-generic transformation to obtain an online/offline encryption from a tradition identity-based encryption (IBE). Our transformation provides a new method to separate the computation of receiver’s identity into offline and online phases. The IBOOE schemes using our transformation saves one group element in both offline and online phases compared to other IBOOE schemes in identity computing. The transformed scheme still maintains the same level of security as in the original IBE scheme.

Iterated Random Oracle: A Universal Approach for Finding Loss in Security Reduction

The indistinguishability security of a public-key cryptosystem can be reduced to a computational hard assumption in the random oracle model, where the solution to a computational hard problem is hidden in one of the adversarys queries to the random oracle. Usually, there is a finding loss in finding the correct solution from the query set, especially when the decisional variant of the computational problem is also hard. The problem of finding loss must be addressed towards tighter reductions under this type. In EUROCRYPT 2008, Cash, Kiltz and Shoup proposed a novel approach using a trapdoor test that can solve the finding loss problem. The simulator can find the correct solution with overwhelming probability 1, if there exists a trapdoor test for the adopted hard problem. The proposed approach is efficient and can be used for many Diffie-Hellman computational assumptions. The only limitation is the requirement of a trapdoor test that must be found for the adopted computational assumptions. In this paper, we introduce a universal approach for finding loss, namely Iterated Random Oracle, which can be applied to all computational assumptions. The finding loss in our proposed approach is very small. For

Fully privacy-preserving and revocable ID-based broadcast encryption for data access control in smart city

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Oblivious Keyword Search with Authorization

queries to the random oracle, the success probability of finding the correct solution from the query set will be as large as 1i¾?/i¾?64 compared to

Identity-based revocation system: Enhanced security model and scalable bounded IBRS construction with short parameters

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