Improving availability and confidentiality of shared data under the multi-cloud environment

Current providers of the cloud storage service often ensure the data confidentiality by encrypting the file content and guarantee the data integrity by verifying the hash value of the file. However, when the cloud storage service fails, the availability of the user data cannot be guaranteed and nor can the cloud sharing function of the user data be supported. In addition, users have to give the provider of the cloud storage service full trust in existing schemes. Once the provider system is hacked or becomes untrustworthy, the confidentiality of the user data will be threatened. In order to solve these problems, this paper proposes a scheme for securely storing and sharing data based on the proxy re-encryption algorithm in the multi-cloud environment. In this scheme, the multi-cloud storage is designed to prevent the failure of any single cloud, the symmetric encryption algorithm is used to encrypt user files, the encryption key is reliably distributed with the Shamir's threshold secret sharing scheme, and finally the proxy re-encryption algorithm is leveraged to support sharing of the encrypted data via clouds. The prototype of the scheme is implemented in the Java development environment and is evaluated under the simulated multi-cloud environment. Our experimental results show that, the time cost of the Shamir's secret partitioning process and symmetric encryption process almost can be negligible when the key size is as long as 386 bytes, and the proxy re-encryption process takes about 1.6 seconds in average.