Agent Passport System (APS): Cryptographic Identity, Faceted Authority Attenuation, and Governance for AI Agent Systems

This document specifies the Agent Passport System (APS), a protocol for cryptographic identity, faceted authority attenuation, and governance for AI agent systems. APS introduces Ed25519-based agent passports, scoped delegation chains with monotonic narrowing across seven constraint dimensions (scope, spend, depth, time, reputation, values floor, reversibility), cascade revocation, a three-signature policy chain (intent, evaluation, receipt), and signed receipts that record what an agent declared, what a policy engine decided, and what an enforcement boundary observed. Authority is modeled as an element of a product lattice, and delegation is a monotone function on that lattice, ensuring that delegated capabilities can only be attenuated, never amplified. APS also defines key rotation with identity continuity, and recognizes institutional governance primitives (charters, offices, approval policies, federation) that compose with the delegation lattice; their normative specification is companion work. The protocol is intended to complement current AI agent infrastructure, including MCP and A2A, with cryptographic identity, delegated authority, and verifiable enforcement evidence. A protocol binding is specified for MCP. Reference implementations are available in TypeScript and Python under Apache-2.0; see the Implementation Status section.