Token-Guided Flow Tracing for Auditable Zero-Knowledge Smart Contract Transfers

Abstract

Private transfers on public smart contract blockchains hide transaction values and private transfer links, but accountable privacy requires controlled auditability. Existing auditable zero-knowledge transfer systems make per-transaction audit information available to an authorized auditor, but their audit model may allow the auditor to inspect transactions beyond the flow connected to the authorized audit target. In such a model, granting audit capability for one investigation can expose unrelated transactions or allow tracing to continue farther than intended. In this paper, we propose token-guided flow tracing for auditable zero-knowledge smart contract transfers, enabling audit authorization to be scoped to a transaction flow and epoch range. The construction separates audit authorization from audit capability by introducing a tracing-token manager and an auditor. The auditor can open audit information only when it holds the auditor secret key, an epoch secret key for an authorized epoch, and a tracing token associated with the target transaction flow. Direction-specific tracing tokens confine tracing to the authorized direction, while epoch public keys and a binary key-derivation tree support compact authorization of audit intervals. We analyze security through ledger indistinguishability, transaction nonmalleability, balance, audit correctness, and restricted audit authorization. Restricted audit authorization is established in an honest authorization model that assumes the tracing-token manager and the auditor do not collude beyond explicit audit authorizations; under these assumptions, the auditor cannot open or trace transactions outside the authorized flow, direction, and epoch scope. Our implementation adds 16,349 transfer-circuit constraints, about 178,000 gas to each accepted transfer, and 448 bytes to the serialized transfer transaction, showing that scoped auditability can be added with moderate on-chain overhead.