International Association
for Cryptologic Research

Authenticated PIR enables a server to initially commit to a database of $N$ items, for which a client can later privately obtain individual items with complexity sublinear in $N$, with the added guarantee that the retrieved item is consistent with the committed database. A crucial requirement is privacy with abort, i.e., the server should not learn anything about a query even if it learns whether the client aborts.

This problem was recently considered by Colombo et al. (USENIX '23), who proposed solutions secure under the assumption that the database is committed to honestly. Here, we close this gap, and present a solution that tolerates fully malicious servers that provide potentially malformed commitments. Our scheme has communication and client computational complexity $\mathcal{O}_{\lambda}(\sqrt{N})$, solely relies on the DDH assumption, and does not introduce heavy machinery (e.g., generic succinct proofs). Privacy with abort holds provided the server succeeds in correctly answering $\lambda$ validation queries, which, from its perspective, are computationally indistinguishable from regular PIR queries. In fact, server side, our scheme is exactly the DDH-based scheme by Colombo et al.