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File sharing systems like Dropbox offer insufficient privacy since a compromised server can see the file content in the clear. Though encryption can hide such content from the servers, metadata leakage remains significant. It is promising to develop a file sharing system that hides such metadata--including user identities and file access patterns.

Metal is the first file sharing system that hides such metadata from malicious users with a latency of only a few seconds. The core of Metal is a new two-server multi-user ORAM scheme, which is secure against malicious users, together with metadata-hiding access control and file sharing.

Compared with the state-of-the-art malicious-user file sharing scheme PIR-MCORAM (which does not hide user identities), Metal hides the user identities and is 500x faster (in terms of amortized latency) or 10^5x faster (in terms of worst-case latency).

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