  Oblivious RAM (ORAM) is a cryptographic primitive which obfuscates the access
patterns to a storage thereby preventing privacy leakage. So far in the current
literature, only `fully functional' ORAMs are widely studied which can protect,
at a cost of considerable performance penalty, against the strong adversaries
who can monitor all read and write operations. However, recent research has
shown that information can still be leaked even if only the write access
pattern (not reads) is visible to the adversary. For such weaker adversaries, a
fully functional ORAM turns out to be an overkill causing unnecessary
overheads. Instead, a simple `write-only' ORAM is sufficient, and, more
interestingly, is preferred as it can offer far more performance and energy
efficiency than a fully functional ORAM.
  In this work, we present Flat ORAM: an efficient write-only ORAM scheme which
outperforms the closest existing write-only ORAM called HIVE. HIVE suffers from
performance bottlenecks while managing the memory occupancy information vital
for correctness of the protocol. Flat ORAM resolves these bottlenecks by
introducing a simple idea of Occupancy Map (OccMap) which efficiently manages
the memory occupancy information resulting in far better performance. Our
simulation results show that, on average, Flat ORAM only incurs a moderate
slowdown of $3\times$ over the insecure DRAM for memory intensive benchmarks
among Splash2 and $1.6\times$ for SPEC06. Compared to HIVE, Flat ORAM offers
$50\%$ performance gain on average and up to $80\%$ energy savings.
