Cryptology ePrint Archive: Report 2019/662

Generalized Proofs of Knowledge with Fully Dynamic Setup

Christian Badertscher and Daniel Jost and Ueli Maurer

Abstract: Proofs of knowledge (PoK) are one of the most fundamental notions in cryptography. The appeal of this notion is that it provides a general template that an application can suitably instantiate by choosing a specific relation. Nonetheless, several important applications have been brought to light, including proofs-of-ownership of files or two-factor authentication, which do not fit the PoK template but naturally appear to be special cases of a more general notion of proofs of knowledge or possession. One would thus expect that their security properties, in particular privacy and soundness, are simply derived as concrete instantiation of a common generalized PoK concept with well understood security semantics. Unfortunately, such a notion does not exist, resulting in a variety of tailor-made security definitions whose plausibility must be checked on a case-by-case basis.

In this work, we close this gap by providing the theoretical foundations of a generalized notion of PoK that encompasses dynamic and setup-dependent relations as well as interactive statement derivations. This novel combination enables an application to directly specify relations that depend on an assumed setup, such as a random oracle, a database or ledger, and to have statements be agreed upon interactively and dynamically between parties based on the state of the setup. Our new notion is called agree-and-prove and provides clear semantics of correctness, soundness, and zero-knowledge in the above generalized scenario.

As an application, we first consider proofs-of-ownership of files for client-side file deduplication. We cast the problem and some of its prominent schemes in our agree-and-prove framework and formally analyze their security. Leveraging our generic zero-knowledge formalization, we then devise a novel scheme that is provably the privacy-preserving analogue of the well-known Merkle-Tree based protocol. As a second application, we consider two-factor entity authentication to showcase how the agree-and-prove notion encompasses proofs of ability, such as proving the correct usage of an abstract hardware token.

Category / Keywords: foundations /

Original Publication (with major differences): IACR-TCC-2021

Date: received 4 Jun 2019, last revised 17 Sep 2021

Contact author: daniel jost at cs nyu edu, christian badertscher at iohk io

Available format(s): PDF | BibTeX Citation

Version: 20210917:142046 (All versions of this report)

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