Agent Payment Security Requires Separation of Duties, Not Just Signatures

A technical discussion between developers this week highlighted a critical gap in emerging agent payment standards: a signed mandate proves authorization at one point in time but not that the authorization remained valid at execution. Researchers argue that revoked consents, expired authentication, or changed limits between mandate capture and payment execution can invalidate what a signature alone appears to confirm. The principle of separation of duties demands that the party generating a payment receipt cannot also control the attestation chain used to verify it, otherwise the audit trail is testimony rather than evidence. Additionally, legitimate payment mutations such as currency conversion or fee deductions mean integrity checks must verify that only mandate-permitted fields were altered, not simply that bytes are unchanged. The framework, developed with input from John Frandsen of open-banking.io, has been published as a preprint on Zenodo (DOI 10.5281/zenodo.21262985).