Cryptanalysis of a CP-ABE scheme with policy in normal forms

•We cryptanalyzed Rao and Dutta's CP-ABE scheme by mounting a CPA.•A registered node in VANET can decrypt a ciphertext with unsatisfied DNF policy.•Root cause of the attack exists in all CP-ABE schemes with normal forms policy.•We propose a solution to serve as remarks in designing a secure CP-ABE scheme.

In 2013, Rao and Dutta constructed an efficient attribute based access control mechanism for vehicular ad hoc network (VANET) based on a newly proposed ciphertext-policy attribute-based encryption (CP-ABE) scheme. As the CP-ABE scheme views access policy in normal forms, the length of ciphertext is independent against the number of attributes in the policy besides having constant number of pairing operations for both encryption and decryption functions. In this paper, we cryptanalyze Rao and Dutta's CP-ABE scheme by mounting a chosen plaintext attack to demonstrate that a registered node in VANET can (eavesdrop the conversation to) decrypt a ciphertext with unsatisfied disjunctive normal form policy. Since the security of Rao and Dutta's proposed attribute based access control mechanism for VANET relies on the proposed CP-ABE scheme, our attack indicates that the proposed access control mechanism is insecure. Subsequently, the root cause of the attack and possible solutions are presented to serve as important remarks in designing a secure CP-ABE scheme.