Attribute-Based Encryption (ABE) must provide an efficient revocation mechanism since a userâ??s private key can be compromised\nor expired over time.The existing revocable ABE schemes have the drawbacks of heavy computational costs on key updates and\nencryption operations,whichmake the entities for performing these operations a possible bottleneck inpractice applications. In this\npaper, we propose an efficientCiphertext-PolicyAttribute-Based Online/Offline Encryption with userRevocation (R-CP-ABOOE).\nWe integrate the subset difference method with ciphertext-policyABE to significantly improve key-update efficiency on the side of\nthe trusted party from O(r log(N/r)) to O(r), whereN is the number of users and r is the number of revoked users. To reduce the\nencryption burden for mobile devices, we use the online/offline technology to shift the majority of encryption work to the offline\nphase, and thenmobile devices only need to execute a fewsimple computations to create a ciphertext. In addition, we exploit anovel\ntrick to prove its selective security under the q-type assumption. Performance analysis shows that our scheme greatly improves the\nkey-update efficiency for the trusted party and the encryption efficiency for mobile devices.
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