Providing trust to multi-cloud storage platforms through the blockchain

Cloud storage services are currently a commodity that allows users to store data persistently, access the data from everywhere, and share it with friends or co-workers. The number of cloud services is growing rapidly but with low interoperability between them; consequently, managing and sharing files between users of different cloud storage is very difficult. To address this problem, specialized cloud aggregator systems emerged that provide users a global view of all files in their accounts and enable file sharing between users from different clouds. To remove the need to trust the cloud providers, Crypto Cloud solution provides the full encryption of stored data, allowing to use multiple cloud storage providers to securely store files. However, in Crypto Cloud, there is a central server which is responsible for managing metadata about users, clouds, files, and permission. The general problem is that if the server is attacked, the integrity of files and public keys can be compromised. The Crypto Cloud system was created with the assumption that the server does not act maliciously. In this dissertation, we propose a solution that, through the use of the blockchain, is able to provide integrity of metadata without relying on the server. This is achieved by extending Crypto Cloud with secure metadata management using the blockchain. We focused on the management of the users' identities and how to provide metadata integrity without relying on the central server. We built a prototype and tested it with real use cases such as the addition of a user, or creation/reading of files. While more complex, the new client removes the trust from the central server and is, therefore, a step towards more decentralized and secure cloud storage systems.