An expressive ciphertext-policy attribute-based encryption with outsourced decryption in blockchain

Ciphertext-Policy Attribute-Based Encryption (CP-ABE) allows data to be encrypted under an access policy, which is specified as a logical combination of attributes. However, CP-ABE schemes usually use expensive operations (i.e. modular exponentiations and bilinear pairings) to generate long ciphertexts and secret key, which makes them difficult to implement in practical applications, especially for resource constrained devices. Fortunately, with the development of cloud computing and blockchain technology, users can outsource most of the decryption costs to proxy, while the underlying plaintext is still confidential. In this scheme, we proposed an expressive ciphertext-policy attributebased encryption with outsourced decryption in blockchain. Suppose that ABE ciphertexts are stored in the InterPlanetary File System (IPFS), the user can provide the proxy with a single transformation key that allows he/she to translate any ABE ciphertext satisfied by that users attributes into a few ciphertexts which can support user to verify and the a summarized ElGamalstyle ciphertext. The scheme is selectively RCCA-secure in the random oracle model for large message spaces.