Conditionally Input-Revealing 2PC and Fuzzy Password-Authenticated Key Exchange

Yao's famous protocol for secure 2-party computation, based on garbled circuits, is well-known to be insecure against an actively corrupt garbler. We introduce a new and extremely simple variant of Yao's protocol that is fully secure against active adversaries, for a certain class of functions that we call conditionally input-revealing.

We then show how to use this new protocol as the basis for fuzzy password authenticated key exchange (fuzzy PAKE). In fuzzy PAKE, two parties each hold a low-entropy secret (e.g., a password), and they interact to obtain a secure high-entropy key if and only if the passwords are sufficiently close.'' Our new fuzzy PAKE protocol supports completely arbitrary predicates for password closeness''. Compared to prior fuzzy PAKE protocols, ours is roughly $2\times$ cheaper in communication, computation, and round complexity.