  Tax returns contain key financial information of interest to third parties:
public officials are asked to share financial data for transparency, companies
seek to assess the financial status of business partners, and individuals need
to prove their income to landlords or to receive benefits. Tax returns also
contain sensitive data such that sharing them in their entirety undermines
privacy. We introduce a zero-knowledge tax disclosure system (zkTax) that
allows individuals and organizations to make provable claims about select
information in their tax returns without revealing additional information,
which can be independently verified by third parties. The system consists of
three distinct services that can be distributed: a tax authority provides tax
documents signed with a public key; a Redact & Prove Service enables users to
produce a redacted version of the tax documents with a zero-knowledge proof
attesting the provenance of the redacted data; a Verify Service enables anyone
to verify the proof. We implement a prototype with a user interface, compatible
with U.S. tax forms, and demonstrate how this design could be implemented with
minimal changes to existing tax infrastructure. Our system is designed to be
extensible to other contexts and jurisdictions. This work provides a practical
example of how distributed tools leveraging cryptography can enhance existing
government or financial infrastructures, providing immediate transparency
alongside privacy without system overhauls.
