Microsoft Corporation

US rdurga@microsoft.com

Security DISPATCH / HTTPAPI Working Group Agentic AI MCP A2A JCS Ed25519 Zero Trust Enterprise agent ecosystems routinely bridge tool invocations and task delegations across heterogeneous protocols, including Model Context Protocol (MCP), Agent-to-Agent (A2A), and OpenAPI/REST. During protocol translation, safety invariants, schema constraints, and mutability flags are frequently stripped—enabling cross-protocol capability downgrade attacks—while commercial settlement gateways erroneously bill on HTTP 200 transport responses rather than verified state-commit durability. This document specifies a cryptographic Cross-Protocol Semantic Equivalence Proof (CPSEP) and an 8-state Commit-Bound Metering Receipt (CBMR) protocol.

Introduction and Problem Statement When an A2A task is translated by a proxy into an MCP tool call and subsequently into a REST request, no cryptographic proof exists that the translated request preserved the tighter permission bounds, rate limits, and JSON Schema invariants of the originating envelope. Furthermore, inter-organization agent exchanges lack a non-repudiable, commit-bound metering receipt tied to Write-Ahead Log (WAL) finality, leading to billing disputes on aborted or rolled-back agent invocations. This specification complements the foundational execution-trace verification architecture defined in draft-durga-agenttrust-trace-verification-00 by introducing deterministic, fail-closed wire semantics specifically addressing Cross-Protocol Semantic Equivalence & Metering.

Conventions and Definitions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 ( ) when, and only when, they appear in all capitals, as shown here.

Semantic Equivalence Proof and Commit-Bound Metering Receipt Envelope Protocol translation proxies and settlement gateways MUST emit and verify a JCS-canonicalized Semantic Metering Envelope carried in the Agent-Semantic-Metering HTTP header or JSON-RPC _meta block:

Normative Fail-Closed Verification Rules Conforming verifiers and relying-party gateways MUST enforce the following deterministic invariants before releasing any downstream side effect:

1. ZERO_DOWNGRADE_INVARIANT: Any translation across A2A, MCP, or REST that widens parameter bounds, drops a readOnly/idempotent constraint, or strips a required human-approval flag MUST set the corresponding bit in downgrade_bitmask, and the receiving endpoint MUST reject any envelope where downgrade_bitmask != 0.
2. COMMIT_BOUND_SETTLEMENT_FINALITY: A commercial metering gateway MUST NOT transition a billable agent transaction to SETTLED unless wal_commit_state equals COMMITTED_VERIFIED and is backed by a countersigned receipt from the target execution engine.
3. AUTOMATIC_COMPENSATION_ZERO_BILLING: Transactions terminating in ABORTED_PRE_COMMIT, TIMED_OUT, or COMPENSATED_ROLLED_BACK MUST settle to zero billable units.

Strict Parser Determinism and Wire-Level Input Hardening When bridging requests across Model Context Protocol (MCP), Agent-to-Agent (A2A), and OpenAPI/REST gateways, parser differentials between TypeScript, Python, and Rust runtimes create exploitable semantic gaps. Conforming CPSEP proxies and CBMR settlement verifiers MUST enforce five pre-canonicalization parser invariants:

1. DUPLICATE_KEY_REJECTION: The JSON parser MUST reject any CPSEP/CBMR envelope or translated tool-argument payload containing duplicate member names prior to AST construction, preventing cross-protocol parameter smuggling where an upstream MCP proxy inspects the first key while a downstream REST service executes the second.
2. IEEE754_FINITE_SAFE_INTEGER_BOUNDS: The downgrade_bitmask and all metered token/unit counters MUST be exact JSON integers within [0, 2^53 - 1]. Boolean values (true/false coerced to 1/0 in Python), negative values, and non-finite IEEE-754 floats (NaN, Infinity, -Infinity, or 1e1000 exponent overflow) MUST cause immediate fail-closed rejection.
3. CANONICAL_LOWERCASE_HEX_AND_SAFE_LONE_SURROGATES: All sha256: digests (source_ast_merkle_root, target_ast_merkle_root, prev_receipt_hash, kid) MUST match ^sha256:[0-9a-f]{64}$, and strings containing unpaired UTF-16 surrogates (U+D800..U+DFFF) MUST be rejected prior to RFC 8785 JCS serialization.
4. MINIMAL_BASE64URL_AND_PUBLIC_JWK_HYGIENE: Signatures MUST use canonical unpadded Base64url encoding, and any verification key catalog MUST reject JWKs containing private key parameters (d, p, q, k).
5. EVIDENCE_TO_RULE_BYTE_BINDING: When a CBMR receipt is evaluated inside an external Verdict Record () for commercial settlement or dispute resolution, the settlement rule MUST bind directly to the digest-verified CBMR envelope bytes rather than unverified summary claims.

Implementation Status and Empirical Benchmark Evaluation The cross-protocol semantic equivalence and commit-bound metering mechanisms specified in this document are backed by open-source reference implementations and reproducible benchmark datasets:

• Open-Source MCP/A2A & Telemetry Verifiers: Strict canonicalization, duplicate-key rejection, safe-integer metering bounds, and cross-protocol transport verification are implemented across agentrust-io/cmcp, agentrust-io/ca2a, agentrust-io/agentrust-telemetry, and GenAI-Security-Project/agent-control-standard ().
• Core Rust Evidence Engine: Deterministic RFC 8785 JCS receipt hashing and isotonic confidence scoring are implemented in Rust in xerj-org/xerj (engine/crates/xerj-common/src/hub_evidence.rs, ).
• Empirical Benchmark Results: In reproducible evaluations over the 100-scenario OWASP ASI01-ASI10 benchmark () and the companion Technical Report (), Merkle-DAG delta proofs reduce cross-version re-verification overhead by 65.9%, the CPSEP downgrade gate blocks 100% of capability-widening translations across MCP/A2A/REST, and the 8-state WAL commit-bound metering machine achieves 0.0% over-billing across aborted and rolled-back transactions.

Security Considerations All pre-image objects MUST be canonicalized using RFC 8785 JSON Canonicalization Scheme (JCS) prior to signature generation or verification under RFC 8032 Ed25519. Any clock skew exceeding 60 seconds, digest mismatch, or missing cryptographic binding MUST cause the verifier to fail closed and emit an auditable security rejection receipt. For the normative test vector in , the Ed25519 test key seed is 2a2a...2a (32 bytes of 0x2a), yielding raw 32-byte public key 197f6b23e16c8532c6abc838facd5ea789be0c76b2920334039bfa8b3d368d61 (kid = sha256:b600306cfa76723fdec395e53a9b3d9fdb78b1e2d7a23c32fcbcd2dc6d0c4092), and the full RFC 8785 JCS-canonicalized envelope (710 bytes) hashes to SHA-256 c3c30e610159c85b7ea4c29c7e70ea278373e646fe618b596447279b1c74e611. This specification cross-references the companion Internet-Drafts (, , , ), external Verdict Record evidence binding (), Remote Attestation (), Merkle Tree domain separation (), and the 100-scenario OWASP ASI01-ASI10 benchmark ().

IANA Considerations This document requests IANA registration of the media type application/agent-metering-receipt+json and the registry IETF Agent Metering Commit States (RESERVED, VALIDATED, EXECUTING, COMMITTED_VERIFIED, ABORTED_PRE_COMMIT, TIMED_OUT, COMPENSATED_ROLLED_BACK, SETTLED), alongside the following header fields:

• HTTP/MCP Header Field: Agent-Semantic-Proof
• HTTP/MCP Header Field: Agent-Metering-Receipt

Normative References Informative References