International Association
for Cryptologic Research

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 $\\mathsf{M}[y]$. 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 $\\mathsf{M}$.