Cloud storage is in widespread use by individuals and enterprises but introduces a wide array of attack vectors. A basic step for users is to encrypt their data, yet it is not obvious what security properties are required for such encryption. Furthermore, cloud storage providers often use techniques such as data deduplication for improving efficiency which restricts the application of semantically-secure encryption. Generic security goals and attack models have thus far proved elusive: primitives are considered in isolation and protocols are often proved secure under ad hoc models for restricted classes of adversaries.