A Don’t Trust the Cloud, Verify: Integrity and Consistency for Cloud Object Stores

More and more data is outsourced to the cloud, and collaborating on a shared resource using cloud services has become easier than ever. Programmers work together on online source code repositories, global project teams produce technical deliverables, and friends share their photo albums. Nevertheless, the clients need to trust the cloud provider as they rely on it for the confidentiality and correctness of their data. Encryption may preserve the confidentiality of data but cannot prevent inadvertent or malicious data modifications. This work shows how to protect the integrity and consistency of data on an untrusted cloud storage service accessed by multiple clients. With a single client only, the client may locally keep a short cryptographic hash value of the outsourced data. Later, this can be used to verify the integrity of the data returned by the cloud storage service. However, with multiple disconnected clients, no common synchronization, and no communication among the clients, neither hashing