  In this paper, we present a novel ciphertext-policy attribute based
encryption (CP-ABE) scheme that offers a flexible access structure. Our
proposed scheme incorporates an access tree as its access control policy,
enabling fine-grained access control over encrypted data. The security of our
scheme is provable under the hardness assumption of the decisional
Ring-Learning with Errors (R-LWE) problem, ensuring robust protection against
unauthorized access. CP-ABE is a cryptographic technique that allows data
owners to encrypt their data with access policies defined in terms of
attributes. Only users possessing the required attributes can decrypt and
access the encrypted data. Our scheme extends the capabilities of CP-ABE by
introducing a flexible access structure based on an access tree. This structure
enables more complex and customizable access policies, accommodating a wider
range of real-world scenarios. To ensure the security of our scheme, we rely on
the decisional R-LWE problem, a well-established hardness assumption in
cryptography. By proving the security of our scheme under this assumption, we
provide a strong guarantee of protection against potential attacks.
Furthermore, our proposed scheme operates in the standard model, which means it
does not rely on any additional assumptions or idealized cryptographic
primitives. This enhances the practicality and applicability of our scheme,
making it suitable for real-world deployment. We evaluate the performance and
efficiency of our scheme through extensive simulations and comparisons with
existing CP-ABE schemes. The results demonstrate the effectiveness and
scalability of our proposed approach, highlighting its potential for secure and
flexible data access control in various domains.
