  Oblivious RAM (ORAM) protocols are powerful techniques that hide a client's
data as well as access patterns from untrusted service providers. We present an
oblivious cloud storage system, ObliviSync, that specifically targets one of
the most widely-used personal cloud storage paradigms: synchronization and
backup services, popular examples of which are Dropbox, iCloud Drive, and
Google Drive. This setting provides a unique opportunity because the above
privacy properties can be achieved with a simpler form of ORAM called
write-only ORAM, which allows for dramatically increased efficiency compared to
related work. Our solution is asymptotically optimal and practically efficient,
with a small constant overhead of approximately 4x compared with non-private
file storage, depending only on the total data size and parameters chosen
according to the usage rate, and not on the number or size of individual files.
Our construction also offers protection against timing-channel attacks, which
has not been previously considered in ORAM protocols. We built and evaluated a
full implementation of ObliviSync that supports multiple simultaneous read-only
clients and a single concurrent read/write client whose edits automatically and
seamlessly propagate to the readers. We show that our system functions under
high work loads, with realistic file size distributions, and with small
additional latency (as compared to a baseline encrypted file system) when
paired with Dropbox as the synchronization service.
