Proofs of Retrievability (PoRs) are protocols which allow a client to store
data remotely and to efficiently ensure, via audits, that the entirety of that
data is still intact. A dynamic PoR system also supports efficient retrieval
and update of any small portion of the data. We propose new, simple protocols
for dynamic PoR that are designed for practical efficiency, trading decreased
persistent storage for increased server computation, and show in fact that this
tradeoff is inherent via a lower bound proof of time-space for any PoR scheme.
Notably, ours is the first dynamic PoR which does not require any special
encoding of the data stored on the server, meaning it can be trivially composed
with any database service or with existing techniques for encryption or
redundancy. Our implementation and deployment on Google Cloud Platform
demonstrates our solution is scalable: for example, auditing a 1TB file takes
just less than 5 minutes and costs less than $0.08 USD. We also present several
further enhancements, reducing the amount of client storage, or the
communication bandwidth, or allowing public verifiability, wherein any
untrusted third party may conduct an audit.

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