PEB Separation and State Migration: Unmasking the New Frontiers of DeFi AML Evasion

TL;DR

This paper reveals fundamental limitations of current AML systems in DeFi due to structural decoupling mechanisms like PEB separation and state-mediated value migration, advocating for semantics-based approaches.

Contribution

It formalizes two key mechanisms that undermine transfer-layer attribution and demonstrates the need for execution semantics in AML systems.

Findings

Transfer-layer observation is neither attribution-complete nor causally closed.

Heuristic flow tracing cannot resolve attribution ambiguity under structural decoupling.

Structural mechanisms like PEB separation challenge existing forensic methods.

Abstract

Transfer-based anti-money laundering (AML) systems monitor token flows through transaction-graph abstractions, implicitly assuming that economically meaningful value migration is sufficiently encoded in transfer-layer connectivity. In this paper, we demonstrate that this assumption, the bedrock of current industrial forensics, fundamentally collapses in composable smart-contract ecosystems. We formalize two structural mechanisms that undermine the completeness of transfer-layer attribution. First, we introduce Principal-Execution-Beneficiary (PEB) separation, where intent originators, transaction executors (e.g., MEV searchers), and ultimate beneficiaries are functionally decoupled. Second, we formalize state-mediated value migration, where economic coupling is enforced through invariant-driven contract state transitions (e.g., AMM reserve rebalancing) rather than explicit transfer continuity. Through a real-world case study of role-separated limit order execution and a constructive cross-pool arbitrage model, we prove that these mechanisms render transfer-layer observation neither attribution-complete nor causally closed. We further argue that simply expanding transfer-layer tracing capabilities fails to resolve the underlying attribution ambiguity inherent in structurally decoupled execution. Under modular composition and open participation markets, these mechanisms are structurally generative, implying that heuristic-based flow tracing has reached a formal observational boundary. We advocate for a paradigm shift toward AML based on execution semantics, focusing on the restitution of economic causality from atomic execution logic and state invariants rather than static graph connectivity.