Verifiable Message-Locked Encryption

One of today’s main challenge related to cloud storage is to maintain the functionalities and the efficiency of customers’ and service providers’ usual environments, while protecting the confidentiality of sensitive data. Deduplication is one of those functionalities: it enables cloud storage providers to save a lot of memory by storing only once a file uploaded several times. But classical encryption blocks deduplication. One needs to use a “message-locked encryption” (MLE), which allows the detection of duplicates and the storage of only one encrypted file on the server, which can be decrypted by any owner of the file. However, in most existing scheme, a user can bypass this deduplication protocol. In this article, we provide servers verifiability for MLE schemes: the servers can verify that the ciphertexts are well-formed. This property that we formally define forces a customer to prove that she complied to the deduplication protocol, thus preventing her to deviate from the prescribed functionality of MLE. We call it deduplication consistency. To achieve this deduplication consistency, we provide (i) a generic transformation that applies to any MLE scheme and (ii) an ElGamal-based deduplication-consistent MLE, which is secure in the random oracle model.