Hellō Cloudflare This document defines five HTTP header fields for use with HTTP Message Signatures as defined in RFC 9421. The Signature-Key request header distributes public keys used to verify signatures, with eight initial key distribution schemes: pseudonymous inline keys (hwk), self-issued key delegation via JWK Thumbprint JWTs (jkt-jwt), identified signers with JWKS URI discovery (jwks_uri), direct JWKS fetch (jwks), JWT-based delegation (jwt), self-issued JWTs (self- jwt), X.509 certificate chains (x509), and references to previously cached assertions (cached). The Accept-Signature-Scheme and Accept- Signature-Alg response headers state the schemes and algorithms a server accepts, so a client can select both before it signs. The Signature-Error response header provides structured error information when signature verification fails, and the Signature-Key-Cache response header issues a cache identifier by which a caller can reference a previously presented assertion instead of resending it. Together, these mechanisms enable flexible trust models ranging from privacy-preserving pseudonymous verification to horizontally-scalable delegated authentication and PKI-based identity chains.