End-to-end encrypted file-sharing systems, such as Keybase Filesystem, enable users to share files stored in the cloud without revealing the contents to the servers. However, the servers still learn metadata, such as user identities and access patterns. Prior works have tried to remove such leakage but relied on strong trust assumptions. Metal (NDSS’20) is not secure against malicious servers. MCORAM (ASIACRYPT’20) offers confiden- tiality against malicious servers, but not integrity.
We present Titanium, a metadata-hiding file-sharing system with confidentiality and integrity against both malicious users and servers. Compared with MCORAM, the state-of-the-art with con- fidentiality (but not integrity) against malicious servers, Titanium additionally offers integrity. Experiments show that Titanium is 5× to 200× faster, or even more, than MCORAM.