Forward-Secure Puncturable Identity-Based Encryption for Securing Cloud Emails

As one of the most important manners of personal and business communications, cloud emails have been widely employed due to its advantages of low-cost and convenience. However, with the occurrence of large-scale email leakage events and the revelation of long-term monitoring of personal communications, customers are increasingly worried about the security and privacy of their sensitive emails. In this paper, we first formalize a new cryptographic primitive named forward-secure puncturable identity-based encryption (fs-PIBE) for enhancing the security and privacy of cloud email systems. This primitive enables an email receiver to individually revoke the decryption capacity of a received email that was encrypted, while retaining the decryption capacity of those unreceived ones. Consequently, those received emails remain secure even if the secret key is comprised. Thus, it provides more practical forward secrecy than traditional forward-secure public key encryption, in which the decryption capacity of those received and unreceived emails is revoked simultaneously. Besides, we propose a concrete construction of fs-PIBE with constant size of ciphertext, and prove its security in the standard model. We present the performance analysis to demonstrate its merits.