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In smart wards, data generated by wearables and monitoring devices are periodically transferred to the cloud server for long-term logging and subsequent access. Remote cloud storage inevitably raises security and access control challenges. Encryption can secure data but may severely impact the value generated by sharing data. More importantly, the privacy leakage caused by keyword searches is unacceptable for medical data. The designated-server public-key encryption with keyword search (dPEKS) can realize ciphertext-based search. However, existing dPEKS face an efficiency bottleneck as they only achieve one-to-one data sharing. In addition, cloud-controlled access control creates over-centralized power. In contrast, using blockchain to control who can search has the extra benefit that the blockchain can record access behavior for subsequent tracking. Therefore, this paper proposes a blockchain-assisted dPEKS (BC-dPEKS) scheme, which exploits a permissioned blockchain to perform trapdoor generation on behalf of data users and record data uploading and access for tracing. To the best of our knowledge, this is the first scheme to tightly integrate blockchain to change PEKS primitive to achieve one-to-many search. Formal security models, the corresponding security proofs, and comprehensive performance analysis are presented. [ABSTRACT FROM AUTHOR]

• Fast access revocation is a major challenge in cloud storage service. • The encryption key is embedded into a set of coefficients of a polynomial so-called access polynomial. • The scheme supports fast revocation by means of updating the access polynomial coefficients. • Blockchain is utilized to implement access control as a smart contract between data owner and users. • Attribute-based encryption (ABE) is used in response transactions to impose the access policy to the file as well as preserving user anonymity. This paper presents a novel collaboration scheme for secure cloud file sharing using blockchain and attribute-based encryption(ABE). Blockchain enables us to implement access control as a smart contract between data owner and users. Each data owner creates its own smart contract where in a data user can request to access a specific file by registering a transaction. In response transaction, the data owner sends the required credential to the user thereby enabling her/him to decrypt the intended file on the cloud storage. This scheme is decentralized, fault tolerant and secured against DoS attacks. The cipher-key , which is used for file encryption, is embedded into a set of coefficients of a polynomial so-called access polynomial. It is attached to the encrypted file on the cloud storage as a metadata. The data user can restore the cipher-key by means of the credential receiving in response transaction and access polynomial. The data owner uses ABE scheme in response transaction to impose her/him access policy to the file as well as preserving user anonymity. This scheme supports fast revocation of the user access by means of updating the access polynomial coefficients and without any communication overhead to non-revoked users. Through formal verification, we show that the scheme is secure in terms of secrecy of credential information and authentication of participants. Finally, the evaluation results show that our scheme is scalable with acceptable performance up to 20,000 users. [ABSTRACT FROM AUTHOR]