Metal: A Metadata-Hiding File-Sharing System

File-sharing systems like Dropbox offer insufficient privacy because a compromised server can see the file contents in the clear. Although encryption can hide such contents from the servers, metadata leakage remains significant. The goal of our work is to develop a file-sharing system that hides metadata— including user identities and file access patterns. Metal is the first file-sharing system that hides such metadata from malicious users and that has a latency of only a few seconds. The core of Metal consists of a new two-server multi-user oblivious RAM (ORAM) scheme, which is secure against malicious users, a metadata-hiding access control protocol, and a capability sharing protocol. Compared with the state-of-the-art malicious-user filesharing scheme PIR-MCORAM (Maffei et al.’17), which does not hide user identities, Metal hides the user identities and is 500× faster (in terms of amortized latency) or 10× faster (in terms of worst-case latency).