Ciphertext Outdate Attacks on the Revocable Attribute-Based Encryption Scheme with Time Encodings

TL;DR

This paper identifies a critical security flaw in a recent revocable attribute-based encryption scheme, showing that a cloud server can access plaintexts using revoked user credentials, and discusses security under weaker models.

Contribution

The paper reveals a significant security vulnerability in Xu et al.'s RABE scheme and analyzes its security in different models, highlighting the need for more secure revocation mechanisms.

Findings

Cloud server can access plaintexts using revoked credentials

Security vulnerability exists in Xu et al.'s RABE scheme

Security can be maintained in weaker models without invalidated credentials

Abstract

Cloud storage is a new computing paradigm that allows users to store their data in the cloud and access them anytime anywhere through the Internet. To address the various security issues that may arise in the cloud storage accessed by a large number of users, cryptographic encryption should be considered. Currently, researches on revocable attribute-based encryption (RABE) systems, which provide user revocation function and ciphertext update function by extending attribute-based encryption (ABE) systems that provide access control to ciphertexts, are actively being studied. Recently, Xu et al. proposed a new RABE scheme that combines ABE and identity-based encryption (IBE) schemes to efficiently handle ciphertext update and user revocation functionality. In this paper, we show that there is a serious security problem in Xu et al.'s RABE scheme such that a cloud server can obtain the plaintext information of stored ciphertexts by gathering invalidated credentials of revoked users. Additionally, we also show that the RABE scheme of Xu et al. can be secure in a weaker security model where the cloud server cannot obtain any invalidated credentials of revoked users.