Flat ORAM: A Simplified Write-Only Oblivious RAM Construction for Secure Processors

Oblivious RAM (ORAM) is a cryptographic primitive which obfuscates the access paŠerns 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 paŠern (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 ‘writeonly’ ORAM is sucient, and, more interestingly, is preferred as it can o‚er far more performance and energy eciency than a fully functional ORAM. In this work, we present Flat ORAM: an ecient write-only ORAM scheme which outperforms the closest existing write-only ORAM called HIVE. HIVE su‚ers from performance boŠlenecks while managing the memory occupancy information vital for correctness of the protocol. Flat ORAM resolves these boŠlenecks by introducing a simple idea of Occupancy Map (OccMap) which eciently manages the memory occupancy information resulting in far beŠer performance. Our simulation results show that, on average, Flat ORAM only incurs a moderate slowdown of 3× over the insecure DRAM for memory intensive benchmarks among Splash2 and 1.6× for SPEC06. Compared to HIVE, Flat ORAM o‚ers 50% performance gain on average and up to 80% energy savings.