Encrypted Key-Value Stores

Distributed key-value stores (KVS) are distributed databases that enable fast access to data distributed across a network of nodes. Prominent examples include Amazon's Dynamo, Facebook's Cassandra, Google's BigTable and LinkedIn's Voldemort. The design of secure and private key-value stores is an important problem because these systems are being used to store an increasing amount of sensitive data. Encrypting data at rest and decrypting it before use, however, is not enough because each decryption exposes the data and increases its likelihood of being stolen. End-to-end encryption, where data is kept encrypted at all times, is the best way to ensure data confidentiality. In this work, we study end-to-end encryption in distributed KVSs. We introduce the notion of an encrypted KVS and provide formal security definitions that capture the properties one would desire from such a system. We propose and analyze a concrete encrypted KVS construction which can be based on any unencrypted KVS. We first show that this construction leaks at most the operation equality (i.e., if and when two unknown queries are for the same search key) which is standard for similar schemes in the non-distributed setting. However, we also show that if the underlying KVS satisfies read your writes consistency, then the construction only leaks the operation equality of search keys that are handled by adversarially corrupted nodes-effectively showing that a certain level of consistency can improve the security of a system. In addition to providing the first formally analyzed end-to-end encrypted key-value store, our work identifies and leverages new and interesting connections between distributed systems and cryptography.