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Hash tables are fundamental data structures that optimally answer membership queries. Suppose a client stores n elements in a hash table that is outsourced at a remote server so that the client can save space or achieve load balancing. Authenticating the hash table functionality, i.e., verifying the correctness of queries answered by the server and ensuring the integrity of the stored data, is crucial because the server, lying outside the administrative control of the client, can be malicious.

We design efficient and secure protocols for optimally authenticating membership queries on hash tables: for any fixed constants 0 < ε < 1 and κ > 1/ε, the server can provide a proof of integrity of the answer to a (non-)membership query in constant time, requiring O(n^ε/log^κε--1 n) time to treat updates, yet keeping the communication and verification costs constant. This is the first construction for authenticating a hash table with constant …