Validating the Integrity of Polygon CDK Validium Zero-Knowledge Proofs

Abstract

Polygon Chain Development Kit (CDK) Validium is a Layer 2 blockchain scaling solution that processes transactions off-chain. It uses Polygon’s distinctive approach to Zero-Knowledge Proofs (ZKPs) implemented within their Zero-Knowledge Ethereum Virtual Machine (zkEVM). A key factor in its successful deployment is robustness, ensuring that users can trust their transactions will be processed accurately and promptly. This research concerns developing robust validation methodologies and comprehensive testing strategies targeting the “double-spending” problem within Polygon CDK Validium. We indicate theoretical scenarios where double-spending vulnerabilities could arise in Polygon CDK Validium by identifying how execution errors can combine with a specific category of flawed constraints to create vulnerabilities. When combined with what we classify as Invalid PIL Constraints For EVM Specification Vulnerabilities (IPCFESV), these errors can trigger problematic behaviours. We further illustrate how erroneous behaviour resulting from IPCFESV can lead to cascading involvement in withdrawal operations resulting in irreversible cross-layer double-spending. We also illustrate how a protocol anti-censorship mechanism bypasses standard validation checks, thereby intensifying reliance on constraint correctness. We then propose ways to determine the correct behaviour. We propose a method to utilise Polygon’s integration testing framework for generating execution traces for de-facto ERC-20 fungible token standard. The outcomes of this study will form the foundational basis for the subsequent development of practical testing and verification methods for Polygon CDK Validium. Implementation and empirical validation remain as future work.