An Improved Robust Fuzzy Extractor

TL;DR

This paper introduces a new robust fuzzy extractor that enhances security by allowing secret key extraction even after the key has been used and observed by an adversary, improving upon previous methods.

Contribution

It presents a fuzzy extractor with post-application robustness capable of extracting more bits of a secret key than prior work, advancing secure key agreement methods.

Findings

Extracts up to (2m-n)/2 bits of the secret key.

Achieves robustness against active adversaries after key usage.

Improves the previous bound of (2m-n)/3 bits.

Abstract

We consider the problem of building robust fuzzy extractors, which allow two parties holding similar random variables W, W' to agree on a secret key R in the presence of an active adversary. Robust fuzzy extractors were defined by Dodis et al. in Crypto 2006 to be noninteractive, i.e., only one message P, which can be modified by an unbounded adversary, can pass from one party to the other. This allows them to be used by a single party at different points in time (e.g., for key recovery or biometric authentication), but also presents an additional challenge: what if R is used, and thus possibly observed by the adversary, before the adversary has a chance to modify P. Fuzzy extractors secure against such a strong attack are called post-application robust. We construct a fuzzy extractor with post-application robustness that extracts a shared secret key of up to (2m-n)/2 bits (depending on error-tolerance and security parameters), where n is the bit-length and m is the entropy of W. The previously best known result, also of Dodis et al., extracted up to (2m-n)/3 bits (depending on the same parameters).