TWORAM: Round-Optimal Oblivious RAM with Applications to Searchable Encryption

We present TWORAM, the first efficient round-optimal oblivious RAM (ORAM) scheme. TWORAM provides oblivious access of a memory index y in exactly two rounds: The client prepares an encrypted query encapsulating y and sends it to the server. The server accesses memory obliviously and returns encrypted information containing the desired value M[y]. The cost of TWORAM is only a multiplicative factor of security parameter higher than the tree-based ORAM schemes such as the path ORAM of Stefanov et al. (CCS, 2013). TWORAM gives rise to interesting applications, and in particular to the first fully-secure searchable symmetric encryption scheme where search is sublinear and search pattern is not leaked—access pattern can also be concealed if we assume the documents are stored in the obliviously accessed memory M.