ApertoID-Signature: HTTP Request Signing for AI Agent Identity

This document defines the ApertoID-Signature HTTP header field, which enables AI agents to cryptographically prove their identity on each HTTP request. The agent signs the request method, target URL, body hash, and identity metadata using an Ed25519 private key whose corresponding public key is published in DNS via the ApertoID protocol [APERTOID-DNS]. The mechanism provides request-level identity verification, action binding (the signature is tied to the specific method and URL), and replay protection via timestamps and nonces.