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Circular All-Or-Nothing: Revisiting Data Protection Against Key Exposure

Rivest's seminal paper introduced the first All-Or-Nothing (AON) method transforming input data into ciphertext in a way that the decryption of the input data is only possible when the ciphertext is complete. The AON processing, when combined with data fragmentation and dispersal, protects against an attacker that was able to acquire the encryption key but could not gather all of the data fragments. The reinforcement in data protection comes at the cost of a significant decrease of performance as Rivest's proposal requires at least two rounds of data encryption. Recently, a new scheme named Bastion was introduced that protects fragmented data against key exposure using only a single round of data encryption combined with a linear post-processing transform. In this paper, we make three advancements in the state-of-the-art. First, we formulate a new security model for the all-or-nothing processing that depends not on the amount of data acquired by the attacker but on the number of compromised storage nodes. Second, we introduce the Circular All-Or-Nothing (CAON): an algorithm protecting encrypted data against a situation of key exposure. It transforms, fragments, and disperses data in a way that initial data is protected unless all of the storage nodes are compromised. It is faster than the Bastion's scheme as it reduces the number of exclusive-or operations made in addition to encryption by almost a half. We believe it can be integrated inside modern distributed storage systems or multi-cloud data solutions in order to reinforce the confidentiality level of the stored data at the cost of a very small, almost negligible, performance overhead. Third, we provide security properties that are easy to formalize under standard cryptographic hypotheses.