Paper 2020/118

InfoCommit: Information-Theoretic Polynomial Commitment and Verification

Saeid Sahraei and Salman Avestimehr

Abstract

We introduce InfoCommit, a protocol for polynomial commitment and verification. InfoCommit consists of two phases. An initial commitment phase and an evaluation phase. During the commitment phase, the verifier and the prover engage in a private two-party computation algorithm so that the verifier extracts a private verification key. In the evaluation phase, the verifier is interested in learning the evaluations of the polynomial at several input points. InfoCommit has four main features. Firstly, the verifier is able to detect, with high probability, if the prover has responded with evaluations of the same polynomial that he has initially committed to. Secondly, InfoCommit provides rigorous privacy guarantees for the prover: upon observing the initial commitment and the response provided by the prover to $m$ evaluation requests, the verifier only learns $O(m^2)$ symbols about the coefficients of the polynomial. Thirdly, the verifiability guarantee is unconditional and without the need for a trusted party, while ``bounded storage" is the only assumption underlying the privacy of the algorithm. In particular, both properties hold regardless of the computation power of the two parties. Lastly, InfoCommit is doubly-efficient in the sense that in the evaluation phase, the verifier runs in $O(\sqrt{d})$ and the prover runs in $O(d)$, where $d-1$ is the degree of the polynomial.