Decoupling Trust in Byzantine CRDTs: Fine-grained Post-Compromise Handling without Breaking Causality

Abstract: Conflict-free Replicated Data Types (CRDTs) provide strong eventual consistency without coordination, but classical approaches assume benign participants. In Byzantine settings, convergence is typically enforced through agreement on update validity, often relying on identity-based filtering. However, such approaches struggle in post-compromise scenarios, where a previously correct participant becomes malicious: retroactive exclusion of its updates may break causal dependencies and invalidate subsequent computations. In this paper, we decouple identity-based trust from content-based trust and introduce a fine-grained trust model that combines both dimensions. Building on deterministic reconstruction, our approach allows replicas to preserve previously accepted updates while enabling selective inclusion or exclusion based on both the originating identity (e.g., public keys) and the semantics of individual updates. Trust decisions can incorporate application-level policies, enabling precise control over the impact of each update on the system state. Our approach preserves causal consistency and enables robust and flexible handling of both Byzantine and faulty behavior in decentralized CRDT systems.