Updatable Block-Level Message-Locked Encryption

Deduplication is a widely used technique for reducing storage space of cloud service providers. Yet, it is unclear how to support deduplication of encrypted data securely until the study of Bellare et al. on message-locked encryption (Eurocrypt 2013). Since then, there are many improvements such as strengthening its security, reducing client storage, etc. While updating a (shared) file is common, there is little attention on how to efficiently update large encrypted files in a remote storage with deduplication. To modify even a single bit, existing solutions require the trivial and expensive way of downloading and decrypting the large ciphertext. We initiate the study of updatable block-level message-locked encryption. We propose a provably secure construction that is efficiently updatable with O(log vertical bar F vertical bar) computational cost, where vertical bar F vertical bar is the file size. It also supports proofof-ownership, a nice feature which protects storage providers from being abused as a free content distribution network.