Cycle ORAM: A Practical Protection for Access Pattern in Untrusted Storage

Abstract

Oblivious RAM (ORAM) is a cryptographic primitive to hide memory access pattern in untrusted storage, but the cost remains prohibitively expensive. This paper proposes cycle ORAM, an improvement of Ring ORAM for small client setting. It gains the most efficient bandwidth and client storage simultaneously. In the eviction, Cycle ORAM is the first scheme which does not need to evict all buckets in the target path. Instead, it only evicts the root bucket. Moreover, to preserve the stash size, the root bucket is expected to provide the largest empty storage by cycling shifting buckets in the server. On the other hand, to meet the security requirement, the randomness of blocks is carried out in the process of bucket reshuffle. Comparing to Ring ORAM, the simulation shows that stash size is smaller and the amortized bandwidth is 1.5× better. In addition, the size of buffer required in ORAM refreshment is reduced from O(logN) to almost a constant, where N is the number of distinct data blocks in the server.