Abstract
Identity-based encryption (IBE) eliminates the necessity of having a costly certificate verification process. However, revocation remains as a daunting task in terms of ciphertext update and key update phases as due to the lack of a certificate revocation list in this infrastructure. In this paper, we provide an affirmative solution to solve the efficiency problem incurred by revocation. We propose the first cloud-based revocable identity-based proxy re-encryption (CR-IB-PRE) scheme that supports user revocation but also delegation of decryption rights. No matter a user is revoked or not, at the end of a given time period the cloud acting as a proxy will re-encrypt all ciphertexts of the user under the current time period to the next time period. If the user is revoked in the forthcoming time period, he cannot decrypt the ciphertexts by using the expired private key anymore. We state that this primitive is applicable to many practical network applications, such as subscription-based cloud storage services. Comparing to some naive solutions which require a private key generator (PKG) to interact with non-revoked users in each time period, the new scheme provides definite advantages in terms of communication and computation efficiency. Our scheme only requires the PKG to publish a constant-size public string for each time period and meanwhile, the workload of ciphertexts update is off-loaded to the cloud server. More importantly, the scheme can be proven secure in the standard model.