• Journal of the Korea Institute of Information and Communication Engineering
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• v.20 no.6
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• pp.1109-1122
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• 2016

Certificateless public key cryptography is a technique that can solve the certificate management problem of a public key cryptosystem and clear the key escrow issue of ID-based cryptography using the public key in user ID. Although the studies were actively in progress, many existing schemes have been designed without taking into account the safety of the secret value with the decryption key exposure attacks. If previous secret values and decryption keys are exposed after replacing public key, a valid private key can be calculated by obtaining the partial private key corresponding to user's ID. In this paper, we propose a new security model which ensures the security against the key exposure attacks and show that several certificateless public key encryption schemes are insecure in the proposed security model. In addition, we design a certificateless public key encryption scheme to be secure in the proposed security model and prove it based on the DBDH(Decisional Bilinear Diffie-Hellman) assumption.

• Journal of the Institute of Electronics Engineers of Korea CI
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• v.46 no.1
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• pp.46-55
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• 2009

We study the problem of searching documents containing each of several keywords (conjunctive keyword search) over encrypted documents. A conjunctive keyword search protocol consists of three entities: a data supplier, a storage system such as database, and a user of storage system. A data supplier uploads encrypted documents on a storage system, and then a user of the storage system searches documents containing each of several keywords. Recently, many schemes on conjunctive keyword search have been suggested in various settings. However, the schemes require high computation cost for the data supplier or user storage. Moreover, up to now, their securities have been proved in the random oracle model. In this paper, we propose efficient conjunctive keyword search schemes over encrypted documents, for which security is proved without using random oracles. The storage of a user and the computational and communication costs of a data supplier in the proposed schemes are constant. The security of the scheme relies only on the hardness of the Decisional Bilinear Diffie-Hellman (DBDH) problem.

• Journal of the Korea Institute of Information Security & Cryptology
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• v.25 no.3
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• pp.531-546
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• 2015

A non-interactive key exchange protocol provides an efficiency of overall system by eliminating additional communication. However, traditional non-interactive key exchange protocols without updating a private key fail to provide forward secrecy, since there is no usage of ephemeral key for randomness of session key. In 2012, Sang et al. proposed a certificateless non-interactive key exchange(CL-NIKE) protocol, but they do not prove the security of the protocol and it does not provide forward secrecy. In this paper, we propose a new CL-NIKE protocol and it's security model. Then we prove the proposed protocol is secure under the security model based on DBDH(Decision Bilinear Diffie-Hellman) assumption. Moreover, we propose a CL-NIKE protocol with forward secrecy which updates user's private key by using multilinear map and prove it's security.