Proofs of Retrievability via Hardness Amplification

Proofs of Retrievability (PoR)

, introduced by Juels and Kaliski [JK07], allow the client to store a file

F

on an untrusted server, and later run an efficient audit protocol in which the server proves that it (still) possesses the client’s data. Constructions of PoR schemes attempt to minimize the client and server storage, the communication complexity of an audit, and even the number of file-blocks accessed by the server during the audit. In this work, we identify several different variants of the problem (such as bounded-use vs. unbounded-use, knowledge-soundness vs. information-soundness), and giving nearly optimal PoR schemes for each of these variants. Our constructions either improve (and generalize) the prior PoR constructions, or give the first known PoR schemes with the required properties. In particular, we

Formally prove the security of an (optimized) variant of the bounded-use scheme of Juels and Kaliski [JK07], without making any simplifying assumptions on the behavior of the adversary.

Build the first unbounded-use PoR scheme where the communication complexity is linear in the security parameter and which does not rely on Random Oracles, resolving an open question of Shacham and Waters [SW08].

Build the first bounded-use scheme with

information-theoretic

security.

The main insight of our work comes from a simple connection between PoR schemes and the notion of

hardness amplification

, extensively studied in complexity theory. In particular, our improvements come from first abstracting a purely information-theoretic notion of

PoR codes

, and then building nearly optimal PoR codes using state-of-the-art tools from coding and complexity theory.