HTTP AAuth Headers

This document defines two HTTP response headers — AAuth-Requirement and AAuth-Error — and profiles HTTP Message Signatures ([RFC9421]) for request authentication, with keying material conveyed via the Signature-Key header ([I-D.hardt-httpbis-signature-key]). A server uses AAuth-Requirement to require pseudonymous or verified agent identity, to request user interaction, or to signal that approval is pending. AAuth-Error conveys structured error information. Both headers use extensible registries for their values.