Efficient Ciphertext-Policy Attribute-Based Encryption Constructions with Outsourced Encryption and Decryption

The invention of the Ciphertext-Policy Attribute-Based Encryption scheme opened a new perspective for realizing attribute-based access control systems without being forced to trust the storage service provider, which is the case in traditional systems where data are sent to the storage service provider in clear and the storage service provider is the party that controls the access to these data. In the Ciphertext-Policy Attribute-Based Encryption model, the data owner encrypts data using an attribute-based access structure before sending them to the storage service, and only users with authorized sets of attributes can successfully decrypt the generated ciphertext. However, Ciphertext-Policy Attribute-Based Encryption schemes employ expensive operations (i.e., bilinear pairings and modular exponentiations) and generate long ciphertexts and secret keys, which makes them hard to implement in real-life applications especially for resource-constrained devices. In this paper, we propose two Ciphertext-Policy Attribute-Based Encryption Key Encapsulation Mechanisms that can be provided as services in the cloud, minimizing the user's encryption and decryption costs without exposing any sensitive information to the public cloud provider. In the first scheme, the ABE Service Provider is considered fully untrusted. On the other hand, the second scheme requires the ABE Service Provider to be semi-trusted (Honest-but-Curious) and does not collude with illegitimate users. Both schemes are proved to be selectively CPA-secure in the random oracle. The theoretical and experimental performance results show that both our first and second schemes are more efficient than the reviewed outsourced CP-ABE schemes in terms of user-side computation, communication, and storage costs.