Evolve and Revoke: A Secure and Efficient Conditional Proxy Re-Encryption Scheme with Ciphertext Evolution

Abstract

Cloud data sharing is an important issue in modern times. To maintain the privacy and confidentiality of data stored in the cloud, encryption is an inevitable process before uploading the data. However, the centralized management and transmission latency of the cloud makes it difficult to support real-time processing and distributed access structures. As a result, fog computing and the Internet of Things (IoT) have emerged as crucial applications. Fog-assisted proxy re-encryption is a commonly adopted technique for sharing cloud ciphertexts. It allows a semi-trusted proxy to transform a data owner's ciphertext into another re-encrypted ciphertext intended for a data requester, without compromising any information about the original ciphertext. Yet, the user revocation and cloud ciphertext renewal problems still lack effective and secure mechanisms. Motivated by it, we propose a revocable conditional proxy re-encryption scheme offering ciphertext evolution (R-CPRE-CE). In particular, a periodically updated time key is used to revoke the user's access privileges while an access condition prevents a malicious proxy from re-encrypting unauthorized ciphertext. We also demonstrate that our scheme is provably secure under the notion of indistinguishability against adaptively chosen identity and chosen ciphertext attacks in the random oracle model. Performance analysis shows that our scheme reduces the computation time for a complete data access cycle from an initial query to the final decryption by approximately 47.05% compared to related schemes.