An Authorization Evidence Challenge for High-Risk Agent Actions

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."¶

This Internet-Draft will expire on 9 February 2027.¶

Copyright Notice

Copyright (c) 2026 IETF Trust and the persons identified as the document authors. All rights reserved.¶

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document.¶

Table of Contents

1. Introduction

Evidence formats describe artifacts such as receipts, permits, attestations, and logs. They do not by themselves define the live interaction that follows when a relying party finds the presented evidence insufficient. Without a common challenge, each agent protocol invents its own vocabulary for missing evidence, acceptable presentations, freshness, status checking, and retry.¶

This document defines the challenge as an application data model, independent of HTTP, a particular agent protocol, and any one evidence format. The nearest prior art is OAuth step-up authentication [RFC9470], which lets a resource identify stronger authentication requirements. AE-CHALLENGE applies the interaction shape to authorization evidence about an exact proposed action.¶

A challenge is a refusal with information. It is not an authorization decision, reservation, capability, ownership transfer, or promise of execution. Satisfying it causes the relying party to evaluate a new presentation under its live local policy.¶

2. Transport-Neutral Core Data Model

The AE-CHALLENGE-v1 data model is independent of its carrier. A transport binding MUST preserve every member that the relying party uses when registering and later consuming the challenge. A binding MUST also provide authenticated integrity and peer identification, or carry the challenge in an authenticated envelope that provides those properties.¶

The JSON format defined by [RFC8259] is the reference serialization of the core model. A non-JSON binding MUST define an unambiguous mapping to the same members and value types.¶

{
  "@version": "AE-CHALLENGE-v1",
  "challenge_id": "7a3120d1-...",
  "nonce": "<single-use unpredictable value>",
  "action_digest": "sha256:<relying-party-derived digest>",
  "action_profile": "https://example.net/action/payment-v3",
  "audience": "https://gateway-b.example",
  "policy_id": "https://gateway-b.example/policies/high-risk-v4",
  "policy_digest": "sha256:<relying-party policy digest>",
  "required_evidence": [
    {
      "requirement_id": "human-approval",
      "type": "authorization_receipt",
      "profiles": ["https://example.net/profiles/receipt-v1"],
      "max_age_sec": 300,
      "status": "current"
    }
  ],
  "present_as": ["ep-aec-v1"],
  "obtain_hints": [
    {
      "requirement_id": "human-approval",
      "mechanism": "https://example.net/flows/approval-v1",
      "uri": "https://approver.example/tasks/123"
    }
  ],
  "expires_at": "2026-08-08T23:59:00Z"
}

3. HTTP Binding

An HTTP origin server that refuses an action because authorization evidence is missing, stale, or unverifiable SHOULD return 403 Forbidden as defined by [RFC9110]. The response body MUST be an application/problem+json Problem Details object as defined by [RFC9457]. Status 428 is not used: it is defined for requests that the origin server requires to be conditional, not for authorization-evidence negotiation.¶

HTTP/1.1 403 Forbidden
Content-Type: application/problem+json
Cache-Control: no-store

{
"type":"https://iana.org/assignments/http-problem-types#ae-required",
  "title": "Authorization Evidence Required",
  "status": 403,
  "detail": "Required evidence is unavailable.",
  "evidence_challenge": {
    "@version": "AE-CHALLENGE-v1",
    "challenge_id": "7a3120d1-...",
    "nonce": "...",
    "action_digest": "sha256:...",
    "action_profile": "https://example.net/action/payment-v3",
    "audience": "https://agent.example",
    "policy_id": "https://resource.example/policies/high-risk-v4",
    "policy_digest": "sha256:...",
    "required_evidence": [
      { "requirement_id": "human-approval",
        "type": "authorization_receipt",
        "max_age_sec": 300,
        "status": "current" }
    ],
    "present_as": ["ep-aec-v1"],
    "obtain_hints": [],
    "expires_at": "2026-08-08T23:59:00Z"
  }
}

The Problem Details type and title MUST have the values shown above. The HTTP status code and the optional status member MUST both be 403. The core challenge MUST appear in the evidence_challenge extension member. Human-readable detail text is advisory and MUST NOT be parsed for protocol semantics.¶

The origin server MUST send Cache-Control: no-store. A generic intermediary can still transform an HTTP response, so a client MUST validate the Problem Details object and the authenticated origin before acting on hints or presenting evidence.¶

4. Informative DMSC Gateway Profile

Agent gateways can carry the transport-neutral challenge during federation or handoff. The receiving gateway is the relying party: it derives the exact action, applies its own trust anchors and policy, and issues a challenge when evidence is missing, stale, or unverifiable. The sending gateway or agent can obtain evidence and return a new presentation using a DMSC-defined extension point or error carrier.¶

When a receiving gateway refuses an action because required authorization evidence is missing, stale, or unverifiable, it may return a structured evidence challenge identifying the exact action, outstanding evidence requirements, applicable freshness or status constraints, and supported presentation profiles. The challenge does not authorize the action or transfer admission ownership; conserved admission across gateway boundaries remains the separate requirement described in Section 7.8 of the proposed next revision of [DMSC-GAPS].¶

In particular, AE-CHALLENGE does not prevent Gateway A and Gateway B from admitting the same single-use right. It does not copy, fence, or relinquish consumption state and does not establish which gateway owns admission during a partition. A DMSC handoff profile MUST solve those properties separately. If the receiving gateway cannot establish the required exclusivity, its safe result is refusal or indeterminate, not acceptance based on the challenge.¶

5. Security Considerations

Challenge forgery and evidence exfiltration. A forged challenge cannot authorize an action, but it can redirect a presenter, induce unnecessary approval work, or solicit sensitive evidence. Presenters MUST authenticate the relying party and audience before following obtain hints or disclosing evidence. A bare JSON object copied outside its authenticated carrier has no origin authenticity.¶

Action substitution. The relying party computes the challenge action digest from the action it would execute and recomputes that binding before admission. A digest supplied by the presenter or an action profile selected by the presenter defeats the purpose of the protocol.¶

Replay and denial of service. Durable body-bound nonce registration is required before challenge exposure. Consumption before evidence evaluation prevents concurrent replays, but an attacker who can submit a structurally valid response can consume a challenge. Deployments therefore need authenticated presenters, rate limits, short expiries, and a cheap path to issue a replacement challenge.¶

Information disclosure. Evidence requirements, policy identifiers, action profiles, and obtain hints can reveal sensitive policy structure. A relying party SHOULD disclose only the minimum information needed for remediation. The HTTP binding requires Cache-Control: no-store.¶

Stale facts and uncertain effects. A satisfied challenge proves only that the relying party's evidence requirement was met at evaluation time. It does not prove that external facts remain true, that an action executed, or that an uncertain action is safe to retry. Status rechecking, execution custody, effect observation, and reconciliation remain separate controls.¶

Gateway split brain. An authenticated challenge from a receiving gateway does not transfer a single-use authorization or its consumption ledger. Implementations MUST NOT claim cross-gateway double-admission prevention merely because both gateways implement this challenge.¶

6. IANA Considerations

IANA is requested to add the following entry to the "HTTP Problem Types" registry established by [RFC9457]. That registry uses the Specification Required policy defined by [RFC8126]; this request does not require IETF Review or Standards Action.¶

Type URI:

https://iana.org/assignments/http-problem-types#ae-required¶

Title:

Authorization Evidence Required¶

Recommended HTTP status code:

403¶

Reference:

This document.¶

This revision withdraws the earlier request for a new application/authorization-evidence-challenge+json media type. The HTTP binding reuses the registered application/problem+json media type. Non-HTTP bindings define their own carriage without changing the core data model.¶

7. Interoperability Questions for Review

This revision resolves the transport layering, HTTP status and media type, gateway-ownership boundary, and acquisition-protocol coupling in revision -02. Review is requested on the following remaining choices:¶

• Whether evidence type, presentation profile, and action profile identifiers should be required to be absolute URIs or may remain opaque identifiers governed by the carrying protocol.¶
• Whether consuming the nonce after structural and action matching but before evidence verification is the correct common replay rule for both task-oriented and message-oriented agent protocols.¶
• Which Agent2Agent and AgentProto extension or error carriers can preserve the complete challenge, authenticated issuer, and audience without treating the challenge as a task result or authorization.¶
• Whether a DMSC gateway profile should carry the core object directly or reference it by an authenticated, digest-bound URI.¶

8. Changes since -02

• Separates the transport-neutral core model from transport bindings.¶
• Replaces HTTP 428 and the proposed dedicated media type with HTTP 403 and RFC 9457 application/problem+json.¶
• Requests an HTTP Problem Types registry entry under Specification Required rather than a standards-tree media type.¶
• Removes the protocol-specific EP-APPROVAL acquisition flow from the core and defines generic, non-authoritative obtain hints.¶
• Defines action-profile, requirement-correlation, extension, nonce consumption, and authenticated-carriage rules.¶
• Adds an informative DMSC gateway profile and conformance cases while explicitly excluding admission transfer and double-admission prevention.¶

Appendix A. Relationship to Existing Mechanisms

CAID [CAID] is one mechanism for deriving and mapping exact material actions. AEB [AEB] defines executor-side evidence verification, refusal, and one-time consumption. Bounded Capability Receipts [BCR] define durable single-domain reservation and consumption. AEC [AEC] is one possible presentation profile. None is required by the core challenge, and the challenge does not extend any of them across independently operated admission domains.¶

Appendix B. Implementation Status

The EMILIA Protocol repository contains a same-team reference implementation of the AE-CHALLENGE-v1 core lifecycle, including relying-party-derived action binding, policy-derived requirements, durable body-bound registration, single-use consumption, expiry, follow-up challenges, and action-swap refusal. Revision -03 adds a serializer and parser for the RFC 9457 HTTP binding and tests that pin status 403, application/problem+json, Cache-Control: no-store, and the evidence_challenge extension.¶

This is same-team implementation evidence. No independent implementation or interoperability is claimed. The implementation does not yet provide a DMSC carrier or conserved-admission mechanism, and its current local evidence and presentation identifiers do not resolve the identifier-governance question in Section 7.¶

Author's Address