Metal: A Metadata-Hiding File-Sharing System

Weikeng Chen (UC Berkeley), Raluca Ada Popa (UC Berkeley)

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

Zeria is the first file-sharing system that simultaneously
hides metadata such as user identities and file access patterns from malicious users with a latency of only several seconds. Zeria develops emph{a new two-server multi-user ORAM scheme}, which is secure against emph{malicious} users, and emph{an oblivious capability sharing scheme}, which hides even the number of files that a user shares. Compared with the state-of-the-art malicious-user file sharing scheme PIR-MCORAM, which leaks user identities, Name hides the user identities and is $boldsymbol{geq500times}$ faster (amortized) or $boldsymbol{geq10^5times}$ faster (worst-case).