EMILIA Protocol, Inc.

US team@emiliaprotocol.ai

sec AI agents evidence challenge step-up When a relying party refuses a consequential agent action because authorization evidence is missing, stale, or unverifiable, the agent needs a machine-readable description of what remains necessary. This document defines a transport-neutral Authorization Evidence Challenge data model bound to the relying party's exact action. The challenge identifies outstanding evidence requirements, freshness and status constraints, acceptable presentation profiles, and retry state. It authorizes nothing, transfers no admission ownership, and provides no promise that a later request will execute. The document also defines an HTTP binding using 403 Forbidden and RFC 9457 Problem Details, and describes an informative gateway-handoff profile for DMSC-style federation. The gateway profile communicates evidence requirements; it does not solve conserved admission or double-admission across independently operated gateways.

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 , 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.

Requirements Language 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.

Terminology

Relying party:

The component that decides whether evidence satisfies its requirements for a proposed action.

Presenter:

The agent, gateway, or other component that receives a challenge and may later present evidence.

Exact action:

The relying-party-derived material action that would be admitted, including every field its selected action profile treats as consequential.

Challenge:

A single-use, expiring description of evidence still required for one exact action.

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 is the reference serialization of the core model. A non-JSON binding MUST define an unambiguous mapping to the same members and value types. ", "action_digest": "sha256:", "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:", "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" } ]]>

Action and Policy Binding @version MUST equal AE-CHALLENGE-v1. A recipient that does not implement this version MUST refuse automated processing. challenge_id is an opaque correlation identifier. Authorization decisions MUST NOT depend on its global uniqueness. nonce MUST be unpredictable, non-empty, and single-use. The relying party MUST bind the complete challenge body to the durable nonce record before exposing the challenge. action_digest MUST be computed by the relying party from the exact action it would admit. It MUST NOT be copied from the presenter or from presented evidence. action_profile MUST be present and identifies the canonicalization or mapping profile used to derive the digest. It is descriptive to the presenter; it does not let the presenter select or weaken that profile. If two parties cannot establish the same material action under an understood profile, the challenge cannot repair the disagreement and the relying party MUST refuse automated admission. audience identifies the component that is expected to answer the challenge. A binding or authenticated envelope MUST let the presenter establish the challenge issuer. A relying party processing a returned challenge MUST verify the issuer and audience before using any obtain hint or disclosing evidence. policy_id and policy_digest identify the local policy state from which the requirements were derived. They do not transfer policy authority to the presenter. At evaluation time the relying party MUST use its authenticated live policy and MUST NOT use a policy supplied in the response.

Evidence Requirements and Presentation Each required_evidence entry MUST contain a requirement_id and type. Requirement identifiers are unique within one challenge and correlate requirements with acquisition hints and diagnostics. Type and profile identifiers are opaque protocol identifiers; a recipient MUST NOT infer security semantics from string prefixes or substring matches. An entry MAY contain profiles, proof_predicates, max_age_sec, and status. Profiles are alternatives acceptable for that requirement. max_age_sec is measured at the relying party's evaluation time using the issuance or observation time defined by the selected evidence profile. status equal to current requires a separately authenticated status result satisfying the relying party's freshness policy; it does not mean that absence from an unauthenticated revocation list is sufficient. present_as lists alternative presentation profiles. The order expresses the relying party's preference, not a security ranking. Supporting a presentation profile does not imply support for every evidence type named by the challenge. The presenter selects one mutually supported profile and includes enough information for the relying party to match every component to a requirement. obtain_hints are untrusted routing hints. Following a hint, authenticating to its URI, or receiving an "approved" response confers no authority at the relying party. Evidence obtained through a hint remains subject to native verification, exact-action matching, freshness and status checks, policy evaluation, and one-time consumption. Unknown transports MAY omit obtain hints entirely.

Lifecycle, Replay, and Retry expires_at MUST be an absolute timestamp. The relying party MUST refuse an expired challenge before policy evaluation. Challenge expiry does not extend the validity of any evidence and evidence expiry does not extend the challenge. Evaluation is ordered: version and structure; authenticated issuer and audience; durable body binding; expiry; action agreement; nonce consumption; native evidence verification; and local policy evaluation. Malformed input that cannot be associated with a registered challenge is refused without consuming a nonce. Once a structurally valid presentation has been matched to a registered challenge, the nonce MUST be consumed before evidence evaluation so concurrent or action-swapped retries cannot reuse it. If evidence remains missing, stale, or unverifiable, the relying party MAY issue a follow-up challenge. A follow-up MUST use a fresh challenge identifier and nonce, MUST remain bound to the same locally derived action digest and action profile, and SHOULD list only the requirements that remain unsatisfied. It is a new refusal, not a continuation of the consumed nonce. Unknown acquisition status, timeout, transport failure, or an unverifiable response is indeterminate. It MUST NOT be interpreted as evidence satisfaction or permission to retry an action whose effect may already have occurred.

Extensibility Recipients MUST ignore an unknown member unless its name appears in the optional critical array. A recipient that does not understand every member named by critical MUST refuse automated processing. A specification defining an extension MUST state whether that extension is safe to ignore and how it is bound to the registered challenge body.

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 . The response body MUST be an application/problem+json Problem Details object as defined by . Status 428 is not used: it is defined for requests that the origin server requires to be conditional, not for authorization-evidence negotiation. 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.

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 . 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.

Gateway Conformance Case Gateway A forwards a proposed action to Gateway B. Gateway B derives the exact action under its pinned action profile and refuses because a current approval artifact is missing. B returns an AE-CHALLENGE bound to B's action digest and local evidence requirement. A obtains and presents the artifact. B verifies it under B's trust anchors, matches it to the same action, and reevaluates B's policy. The case passes only if A never treats the challenge as authorization and B refuses an action-digest mismatch, stale status, replayed nonce, or unsupported presentation profile. A second case attempts concurrent admission of one single-use right at both gateways. AE-CHALLENGE alone MUST NOT be reported as passing that case. The case passes only when a separate conserved-admission mechanism establishes exclusivity, or when the receiving gateway refuses or returns indeterminate because exclusivity cannot be established.

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.

IANA Considerations IANA is requested to add the following entry to the "HTTP Problem Types" registry established by . That registry uses the Specification Required policy defined by ; 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.

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.

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.

References Normative References Informative References

Relationship to Existing Mechanisms CAID is one mechanism for deriving and mapping exact material actions. AEB defines executor-side evidence verification, refusal, and one-time consumption. Bounded Capability Receipts define durable single-domain reservation and consumption. 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.

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 .