A CCA-Secure Collusion-Resistant Identity-Based Proxy Re-Encryption Scheme

Cloud storage enables its users to store confidential information as encrypted files in the cloud. A cloud user (say Alice) can share her encrypted files with another user (say Bob) by availing proxy re-encryption services of the cloud. Proxy Re-Encryption (PRE) is a cryptographic primitive that allows transformation of ciphertexts from Alice to Bob via a semi-trusted proxy, who should not learn anything about the shared message. Typically, the re-encryption rights are enabled only for a bounded, fixed time and malicious parties may want to decrypt or learn messages encrypted for Alice, even beyond that time. The basic security notion of PRE assumes the proxy (cloud) is semi-trusted, which is seemingly insufficient in practical applications. The proxy may want to collude with Bob to obtain the private keys of Alice for later use. Such an attack is called collusion attack, allowing colluders to illegally access all encrypted information of Alice in the cloud. Hence, achieving collusion resistance is indispensable to real-world scenarios. Realizing collusion-resistant PRE has been an interesting problem in the ID-based setting. To this end, several attempts have been made to construct a collusion-resistant IB-PRE scheme and we discuss their properties and weaknesses in this paper. We also present a new collusion-resistant IB-PRE scheme that meets the adaptive CCA security under the decisional bilinear Diffie-Hellman hardness assumption in the random oracle model.