Third-Party Private DFA Evaluation on Encrypted Files in the Cloud

Motivated by the need to outsource file storage to untrusted clouds while still permitting limited use of that data by third parties, we present practical protocols by which a client (the third-party) can evaluate a deterministic finite automaton (DFA) on an encrypted file stored at a server (the cloud), once authorized to do so by the file owner. Our protocols provably protect the privacy of the DFA and the file contents from a malicious server and the privacy of the file contents (except for the result of the evaluation) from an honest-but-curious client (and, heuristically, from a malicious client). We further present simple techniques to detect client or server misbehavior.