AGTP Agent Certificate Extension

1.1. The Identity Gap in Base AGTP

The AGTP base specification requires every request to carry Agent-ID, Principal-ID, and Authority-Scope headers. These headers are self-asserted: an AGTP client declares its identity and scope, and the server logs the declaration. In the base spec, there is no transport-layer mechanism to verify that the declared Agent-ID corresponds to a registered agent, that the Principal-ID is accurate, or that the Authority-Scope does not exceed what was granted.¶

This is a deliberate design choice in the core spec: self-asserted identity with mandatory logging provides a useful baseline and enables broad adoption. For many deployments, anomaly detection and audit trails over self-asserted headers are sufficient.¶

For higher-stakes deployments -- financial transactions, healthcare operations, legal actions, multi-organization agent federations -- the self-assertion model is insufficient. Infrastructure needs to verify agent identity and enforce scope at the transport layer without parsing application payloads.¶

1.2. The Agent Certificate Extension

The AGTP Agent Certificate Extension provides cryptographic identity binding at the transport layer. An AGTP Agent Certificate is an X.509 v3 certificate with agent-governance-specific extensions. It is presented during TLS mutual authentication, enabling the server and any AGTP-aware infrastructure component to verify the agent's identity and authority scope from the certificate alone, without inspecting the request headers or body.¶

This document specifies:¶

• The AGTP Agent Certificate schema and X.509 v3 extension fields¶

• The certificate issuance and renewal protocol¶

• The authority scope commitment mechanism for O(1) per-request scope enforcement¶

• Session-level revocation propagation via AGTP NOTIFY¶

• The AGTP Certificate Transparency Log (AGTP-CTL)¶

1.3. Scope

This extension is OPTIONAL. Core AGTP implementations that do not implement this extension remain fully compliant with [AGTP]. The extension is required only for Trust Tier 1 agent identity verification and for SEP-enforced scope constraint without application-layer access.¶

3.1. Certificate Structure

The AGTP Agent Certificate is an X.509 v3 certificate per [RFC5280] with the following subject fields and extensions:¶

4.1. Eligibility

Certificate Signing Requests (CSRs) for AGTP Agent Certificates MUST only be accepted for agents in Active lifecycle state in the AGTP registry. A governance platform MUST verify the agent's lifecycle state at CSR submission time and MUST reject CSRs for agents in Suspended, Revoked, or Deprecated state.¶

4.2. Issuance Steps

1. The governance platform generates a key pair for the agent (or accepts a CSR with an agent-generated key pair).¶

2. The governance platform populates the certificate subject fields and all AGTP-specific extensions from the agent's Birth Certificate and registry record.¶

3. The governance platform verifies that the proposed authority-scope- commitment does not exceed the scope granted in the agent's Birth Certificate. If it does, the CSR MUST be rejected.¶

4. The governance platform signs the certificate using its issuing CA key per [RFC5280].¶

5. The governance platform submits the certificate to the AGTP-CTL and obtains a Signed Certificate Timestamp (SCT).¶

6. The SCT is embedded in the agtp-ctl-sct extension and the certificate is delivered to the agent.¶

7. The governance platform publishes the new certificate to the agent's registry record, triggering a registry state update.¶

4.3. Certificate Validity

AGTP Agent Certificates SHOULD have a validity period of no more than 90 days. Short validity periods limit the exposure window of a compromised certificate and reduce reliance on revocation mechanisms. Renewal SHOULD begin at 80% of the validity period.¶

Certificate renewal carries forward the predecessor's subject-agent-id and activation-certificate-id unchanged. The renewed certificate receives a new serial number, new validity period, and a new SCT.¶

5.1. Mutual Authentication

AGTP connections using the Agent Certificate Extension MUST use TLS 1.3 mutual authentication. The agent presents its AGTP Agent Certificate as the client certificate during the TLS handshake.¶

The server verifies the client certificate chain against the issuing CA trust anchors. Following successful handshake:¶

1. The server extracts the subject-agent-id extension value and verifies it matches the Agent-ID header on the first request.¶

2. The server extracts the principal-id extension value and verifies it matches the Principal-ID header on the first request.¶

3. The server extracts the authority-scope-commitment extension value and uses it to verify Authority-Scope header tokens on each request.¶

Any mismatch between certificate extension values and AGTP header values MUST cause the server to return 401 Unauthorized and MUST be logged.¶

5.2. Scope Enforcement at SEPs

A SEP operating with the Agent Certificate Extension verifies Authority- Scope at O(1) cost per request:¶

1. At session establishment, the SEP extracts the authority-scope- commitment from the client certificate. (One-time per session.)¶

2. On each request, the SEP checks whether the Authority-Scope header tokens are covered by the commitment. (O(1) per request after session setup.)¶

3. If any header token is not covered by the commitment, the SEP returns 451 Scope Violation without forwarding the request to the application layer.¶

This enables governance enforcement at line rate without application- layer parsing.¶

6.1. Revocation Events

An AGTP Agent Certificate MUST be revoked when any of the following occur:¶

• The agent's lifecycle state transitions to Revoked or Deprecated¶

• The Birth Certificate's certificate_hash is invalidated¶

• The agent's authority-scope-commitment requires modification¶

• The principal requests revocation¶

• A trust violation is detected¶

6.2. Session-Level Revocation Propagation

Standard certificate revocation (CRL, OCSP) operates on polling cycles, leaving a window during which revoked certificates may still be used. For agent systems, this window is unacceptable for high-stakes operations.¶

AGTP Agent Certificate revocation MUST be propagated to active sessions via AGTP NOTIFY broadcast:¶

1. The governance platform issues a revocation event to the AGTP-CTL.¶

2. The governance platform broadcasts an AGTP NOTIFY to all infrastructure components holding active sessions for the revoked certificate's subject-agent-id:¶

{
  "method": "NOTIFY",
  "parameters": {
    "recipient": "infrastructure:broadcast",
    "content": {
      "event_type": "certificate_revoked",
      "subject_agent_id": "[agent-id]",
      "certificate_serial": "[serial]",
      "revocation_reason": "[reason]",
      "effective_at": "2026-04-01T00:00:00Z"
    },
    "urgency": "critical"
  }
}

1. Infrastructure components receiving this NOTIFY MUST immediately terminate all active sessions for the identified subject-agent-id. Session termination MUST occur before the next request is processed on the affected session.¶

2. The target revocation-to-termination latency is 30 seconds. This is materially shorter than standard CRL or OCSP cache-based models.¶

6.3. Session Manager Responsibilities

AGTP Session Managers in deployments using the Agent Certificate Extension MUST maintain a per-certificate-serial active session registry. On receiving a revocation NOTIFY, the Session Manager MUST terminate all sessions associated with the revoked serial before processing the next request on any affected session.¶

7.1. Purpose

The AGTP Certificate Transparency Log (AGTP-CTL) is an append-only, Merkle-tree-based log of all issued AGTP Agent Certificates. It provides tamper-evident public accountability for certificate issuance and revocation, enabling:¶

• Fleet-level analytics: population-wide trust score distributions, archetype frequencies, governance zone composition¶

• Anomaly detection: detection of certificates issued outside normal governance flows¶

• Audit reconstruction: verifiable history of certificate issuance and revocation for compliance¶

7.2. Log Structure

The AGTP-CTL follows the Certificate Transparency log structure defined in [RFC6962], adapted for agent governance metadata. Each leaf entry contains:¶

• Certificate serial number¶

• subject-agent-id¶

• principal-id¶

• governance-zone¶

• trust-tier¶

• archetype¶

• activation-certificate-id¶

• Issuance timestamp¶

• Revocation status (updated on revocation)¶

• Merkle leaf hash¶

The leaf hash covers all governance metadata fields. Any modification to a log entry is detectable by any party with access to the log.¶

7.3. Privacy Considerations

The principal-id field in the AGTP-CTL leaf entries MAY be pseudonymized to protect individual principal identity while maintaining audit integrity. Pseudonymous principal IDs MUST be resolvable by authorized parties (regulators, compliance auditors) via a trusted resolution service. The pseudonymization mapping MUST be maintained separately from the *RECOMMENDED public log.¶

8.1. Certificate Pinning

Deployments with strict security requirements MAY implement certificate pinning for known agents, rejecting connections from agents whose certificate serial or key does not match a pre-registered value. Certificate pinning interacts with renewal; pinned agents MUST update pins on each certificate renewal before the old certificate expires.¶

8.2. Scope Commitment Forgery

The authority-scope-commitment is an Ed25519 signature over the canonical scope token set, signed by the governance platform's issuing key. An attacker who compromises the issuing key can forge scope commitments. Issuing key compromise MUST trigger immediate revocation of all certificates issued by that key and issuance of replacement certificates from a new key pair. Issuing keys SHOULD be stored in hardware security modules.¶

8.3. Cross-Certificate Confusion

An agent MAY hold multiple certificates (e.g., during renewal overlap). Infrastructure MUST use the subject-agent-id extension value as the authoritative agent identifier, not the certificate subject CN, to prevent cross-certificate identity confusion during renewal windows.¶

8.4. IPR Notice

Certain mechanisms described in this document may be subject to pending patent applications by the author, specifically: the authority-scope-commitment mechanism and the session-level revocation propagation architecture. The licensor (Chris Hood / Nomotic, Inc.) is prepared to grant a royalty-free license to implementers for any patent claims covering these mechanisms, consistent with the IETF's IPR framework under the normative reference in [AGTP] Section 8.7.¶

9.1. X.509 Extension OID Registrations

This document requests registration of the following Object Identifiers in the IANA Private Enterprise Numbers registry or an appropriate OID arc for IETF use. Specific OID assignments are subject to IANA allocation:¶

10.1. Normative References

[AGTP]

Hood, C., "Agent Transfer Protocol (AGTP)", Work in Progress, Internet-Draft, draft-hood-independent-agtp-02, 2026, <https://datatracker.ietf.org/doc/html/draft-hood-independent-agtp-02>.

[RFC2119]

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/rfc/rfc2119>.

[RFC5280]

Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, <https://www.rfc-editor.org/rfc/rfc5280>.

[RFC8174]

Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

[RFC8555]

Barnes, R., Hoffman-Andrews, J., McCarney, D., and J. Kasten, "Automatic Certificate Management Environment (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019, <https://www.rfc-editor.org/rfc/rfc8555>.

10.2. Informative References

[RFC6749]

Hardt, D., Ed., "The OAuth 2.0 Authorization Framework", RFC 6749, DOI 10.17487/RFC6749, October 2012, <https://www.rfc-editor.org/rfc/rfc6749>.

[RFC6962]

Laurie, B., Langley, A., and E. Kasper, "Certificate Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013, <https://www.rfc-editor.org/rfc/rfc6962>.

[RFC9334]

Birkholz, H., Thaler, D., Richardson, M., Smith, N., and W. Pan, "Remote ATtestation procedureS (RATS) Architecture", RFC 9334, DOI 10.17487/RFC9334, January 2023, <https://www.rfc-editor.org/rfc/rfc9334>.