Updatable Block-Level Message-Locked Encryption

Deduplication is widely used for reducing the storage requirement for storage service providers. Nevertheless, it is unclear how to support deduplication of encrypted data securely until the study of Bellare et al. on message-locked encryption (MLE, Eurocrypt 2013). While updating (shared) files is natural, existing MLE solutions do not allow efficient update of encrypted files stored remotely. Even modifying a single bit requires the expensive way of downloading and decrypting a large ciphertext (then re-uploading). This paper initiates the study of updatable block-level MLE, a new primitive in incremental cryptography and cloud cryptography. Our proposed provably-secure construction is updatable with computation cost logarithmic in the file size. It naturally supports block-level deduplication. It also supports proof-of-ownership which protects storage providers from being abused as a free content distribution network. Our experiments show its practical performance relative to the original MLE and existing non-updatable block-level MLE.