Multi-Party Quorum Authorization for High-Risk Agent Actions (EP-QUORUM)

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Table of Contents

1. Introduction

The base EP authorization receipt closes the gap between "is this actor authorized in general?" and "should this exact action happen, and which accountable human said yes?" by binding one named approver's device-held signature to one exact action (see [EP-RECEIPTS], Section 1). For the highest-consequence actions, one approver is not the right control. The discipline that governs nuclear release, large-value treasury movement, and production-credential change is the two-person rule: no single human -- however well-authenticated, however senior -- can unilaterally cause the action. Two or more distinct, accountable humans must each independently authorize, and the action proceeds only when all of them have.¶

As autonomous agents acquire credentials sufficient for irreversible operations, the two-person rule is exactly the control that lets an organization grant an agent real authority without creating a single point of failure: a compromised, misaligned, or prompt-injected agent cannot act alone, and neither can a single compromised or coerced approver. EP-QUORUM specifies how to express that control as a cryptographic predicate over EP signoffs and how to enforce it both at the moment each approver signs and at the moment the action would execute.¶

The base draft already contemplates multi-approver policies ([EP-RECEIPTS], Section 7): each approver signs an individual Authorization Context sharing the same action hash, and commitment occurs only when k valid, distinct signoffs exist before expiry. This document makes that sketch normative and testable. It adds: ordered approval trails (Section 4); an explicit role roster and admission semantics (Section 3); a bounded approval window with a monotonic-time constraint for ordered mode (Section 4); an incremental server-side admission rule that keeps a non-conforming signer out of the trail in the first place (Section 6); the consolidated fail-closed quorum predicate (Section 5); and an adversarial conformance suite (Section 9).¶

2. Terminology

In addition to the terminology of [EP-RECEIPTS]:¶

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHOULD", "MAY" are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals.¶

Quorum.

The set of distinct approver signoffs required to authorize one action under a Quorum Policy.¶

Quorum Policy.

A named, versioned rule set declaring the approval mode, the required count, the roster of eligible (role, approver) slots, the distinct-humans rule, and the approval window. Carried in the policy that governs the action; see Section 3.¶

Member.

One element of a candidate quorum: a (role, approver public key, signoff) triple, where the signoff is an unmodified EP signoff ([EP-RECEIPTS], Section 5.3) over the Authorization Context the approver signed.¶

Trail.

The ordered sequence of members admitted so far for one action -- the partial quorum under construction.¶

Quorum Gate.

The fail-closed predicate (Section 5) that decides whether a trail is a satisfied quorum. The Verifying Executor ([EP-RECEIPTS], Section 9) MUST consult it before performing the action.¶

3. The Quorum Policy

A Quorum Policy is a JSON object:¶

{
  "mode": "ordered",
  "required": 3,
  "approvers": [
    {
      "role": "program_officer",
      "approver": "ep:approver:po_rivera"
    },
    {
      "role": "authorizing_official",
      "approver": "ep:approver:ao_chen"
    },
    {
      "role": "inspector_general",
      "approver": "ep:approver:ig_okafor"
    }
  ],
  "distinct_humans": true,
  "window_sec": 900
}

Members:¶

Rules:¶

• An approver slot identifies an eligible approver in the Approver Directory ([EP-RECEIPTS], Section 5.2). The role is the organizational role under which that approver is admitted to this quorum; it is the unit of role eligibility (Section 5, check 4).¶
• A member is admitted only if its (role, approver) pair is present in the roster. A correct signature by a real, enrolled approver who is not on the roster for this action MUST be rejected (wrong_role).¶
• required MUST NOT exceed the number of distinct human approvers the roster can supply under distinct_humans. A policy that cannot be satisfied is a misconfiguration; verifiers treat an unsatisfiable trail as not satisfied, as always.¶
• The Quorum Policy is part of the action's governing policy and is therefore committed by the policy_hash of every member's Authorization Context ([EP-RECEIPTS], Section 4). A signature collected under one Quorum Policy version MUST NOT satisfy a requirement evaluated under another.¶

4. Approval Modes

Threshold mode ("threshold"). Any required distinct approvers drawn from the roster, in any order, satisfy the quorum. This is the classic M-of-N rule.¶

Ordered mode ("ordered"). Approvals MUST occur in the roster's listed order: the i-th admitted member MUST match approvers[i-1] in both role and approver. In addition, signature times MUST be strictly increasing -- each member's issued_at MUST be later than the previous admitted member's issued_at (non_increasing_time). Ordered mode expresses escalation chains in which a later authority signs after, and in knowledge of, an earlier one (e.g., Program Officer, then Authorizing Official, then Inspector General).¶

In both modes, every admitted member's issued_at MUST fall within window_sec of the first admitted member's issued_at (window_exceeded). The window bounds the lifetime of a partial quorum so that a long-dormant partial trail cannot be completed much later by an attacker who has since compromised a remaining approver.¶

Strong ordered mode ("ordered_chain": true). Ordering by issued_at alone is asserted by whoever stamps the time. To prove order by the signatures themselves, a policy MAY require a cryptographic chain: each signoff after the first carries, inside its signed Authorization Context, a prev_context_hash equal to the SHA-256 of the canonical context of its predecessor in the trail; the first signoff carries none. Because each approver's device signs over its own context, the i-th approver thereby attests to having signed after the exact (i-1)-th approval, and no party -- including the operator -- can reorder, insert, or backdate a signoff without invalidating a signature (broken_chain). This is RECOMMENDED for the highest-consequence ordered actions. Implementations that do not set ordered_chain retain timestamp ordering and remain conformant.¶

5. The Quorum Gate (fail-closed predicate)

A trail is a satisfied quorum for an action with hash H under policy P if and only if ALL of the following hold. A verifier MUST return "satisfied" only when every check passes, and MUST return "not satisfied" on the first failure, on a malformed policy or member, or on any unrecognized condition (Q2):¶

1. Well-formed policy. P has a recognized mode, an integer required >= 1, and a non-empty approvers roster. Otherwise: not satisfied.¶
2. All signatures valid. For every member, the EP signoff verifies under [EP-RECEIPTS], Section 5.3 / 6.3 -- the WebAuthn [WEBAUTHN] assertion (Class A) verifies against the member's approver_public_key, with the assertion challenge equal to the member's context hash and user verification asserted. One invalid signature (one_bad_signature) fails the whole quorum.¶
3. Action binding. Every member's Authorization Context carries action_hash == H (action_mismatch). A member bound to any other action does not count.¶
4. Role admission. Every member's (role, approver) pair is present in the roster (wrong_role).¶
5. Distinct humans. When distinct_humans is true (the default), approvers are pairwise distinct and (per the base draft's SelfApprovalImpossible) distinct from the initiator (duplicate_human).¶
6. Distinct keys (check 5a). When distinct_humans is true, the admitted members' approver_public_key values are pairwise distinct (duplicate_key). This defends against a single device key enrolled under two approver identifiers, which would otherwise pass the distinct-humans check by name while being one signer.¶
7. Threshold. At least required admitted members exist (under_threshold).¶
8. Order (ordered mode only). The i-th admitted member matches approvers[i-1]; signature times are strictly increasing (out_of_order, non_increasing_time).¶
9. Ordering chain (strong ordered mode only, check 7a). When ordered_chain is true, the first admitted member carries no prev_context_hash and every later member's prev_context_hash equals the SHA-256 of the canonical context of its predecessor (broken_chain). When ordered_chain is false this check is not applicable.¶
10. Window. Every admitted member's issued_at is within window_sec of the first member's issued_at (window_exceeded).¶

The predicate is the same whether computed by the orchestrating operator before consumption or by an independent Verifying Executor or auditor offline (Q5): it is a pure function of (P, H, members). Because each member is an unmodified EP signoff, check 2 is exactly the base draft's verifier invoked per member; EP-QUORUM adds the set-level checks 1, 3-10 on top.¶

6. Incremental Admission (canAccept)

To keep a non-conforming signer out of the trail rather than discovering it only at consume time (Q4), an orchestrator MUST evaluate an incremental admission rule before recording each new signoff. Given the policy P, the action hash H, the already-admitted trail, and one incoming candidate member, the rule ADMITS the candidate only if all of the following hold, and otherwise REJECTS it with the named reason:¶

1. P is well-formed and its roster is non-empty (else no_policy / no_eligible_approvers).¶
2. The candidate's context carries action_hash == H (else action_mismatch).¶
3. The candidate's (role, approver) is on the roster (else ineligible_role).¶
4. When distinct_humans is true, no already-admitted member shares the candidate's approver (else duplicate_human).¶
5. In ordered mode, the candidate matches the next unfilled roster slot (approvers[len(trail)]) in both role and approver (else out_of_order).¶
6. If the trail is non-empty, the candidate's issued_at is within window_sec of the first member's issued_at (else window_exceeded); and in ordered mode it is strictly greater than the last admitted member's issued_at (else non_increasing_time).¶
7. The candidate's signature verifies (else invalid_signature).¶

A rejected candidate MUST NOT be written into the trail. Incremental admission is an enforcement convenience and an early-rejection UX; it is not a substitute for the Quorum Gate. A conforming Verifying Executor MUST re-evaluate the full Quorum Gate (Section 5) over the assembled trail before performing the action, regardless of incremental admission, because the executor does not trust the orchestrator to have applied admission honestly (this mirrors the base draft's execution-side enforcement, [EP-RECEIPTS], Section 9).¶

7. Multi-Handshake Composition

This section is informative. It describes, end to end, the process by which the normative machinery of Section 3 through Section 6 composes individual signing ceremonies into one multi-party decision. Every property named here is required elsewhere in this document or in the base draft; this section introduces no new requirements.¶

The handshake is the unit of approval. Each quorum member is one complete run of the base draft's single-approver signing ceremony ([EP-RECEIPTS], Section 5.3): a fresh, one-time challenge is issued for that approver against a verified handshake, bound to the exact action bytes through the action hash and binding hash it commits to, and bounded by an explicit validity window; the approver answers it with a user-verified, device-held signature whose WebAuthn assertion challenge is the member's context hash (the per-member requirement is check 2 of the Quorum Gate, Section 5). Nothing about the ceremony is weakened or amortized when the action needs several approvers: there is no batch signature and no shared signing session; k approvals are k separate ceremonies, each independently verifiable and each independently refusable.¶

A malformed trail never exists. Composition is incremental and server-side. Each candidate member is evaluated against the admission rule of Section 6 before it is recorded, so a wrong-action, off-roster, duplicate, out-of-order, stale, or invalidly signed member handshake is refused at the moment it is presented and never enters the trail. The stored trail is therefore, at every instant, a prefix of a potentially satisfiable quorum; there is no later cleaning pass that removes bad members, because bad members are never written. The Verifying Executor still re-evaluates the full Quorum Gate over the assembled trail, as Section 6 requires, precisely because it does not trust the orchestrator to have admitted honestly.¶

Order is a signed commitment, not a claim. In strong ordered mode (Section 4), each member's signed context after the first commits to the hash of its predecessor's context. Because that commitment sits inside the bytes the approver's device signed, and each predecessor in turn committed to its own predecessor, the i-th signature transitively covers the entire prior trail: the order of approvals is non-repudiable, and no party -- including the operator -- can reorder, insert, or backdate a member without invalidating a signature (check 7a, Section 5). Plain ordered mode retains the weaker, timestamp-based ordering described in Section 4.¶

Denial is a first-class terminal outcome. Any approver in the trail can refuse, and a refusal is not the mere absence of an approval. Under the base draft a denial is signed over the same context hash with a decision of "denied" and is terminal: a denied authorization can never later become approved ([EP-RECEIPTS], Sections 5.3 and 6.1). At the composition level, one dissenting approver leaves durable, attributable evidence of the dissent, which a relying party can weigh as adverse evidence rather than as a gap in the trail.¶

The composed decision is consumed once. A satisfied quorum authorizes exactly one execution of exactly one action. Each member's signoff individually retains the base draft's one-time-consumption rule (Section 10.4), and consumption of the composed decision is refused until the Quorum Gate is satisfied; a partial trail confers no partial authority (Section 10.3).¶

The whole composition is verifiable offline. Because the satisfied/not-satisfied judgment is a pure function of the policy, the action hash, and the members (Section 5, Q5), an auditor holding only the quorum receipt can recompute every check above -- per-member signatures, action binding, distinctness of humans and keys, roster admission, threshold, order and ordering chain, and window -- without contacting the operator. As in the base draft, such offline verification establishes the integrity and authenticity of the recorded composition as of signing time; it does not by itself establish current validity, such as the present enrollment or revocation status of the approvers involved.¶

8. Member Representation in the Receipt

A quorum receipt is an ordinary EP Trust Receipt ([EP-RECEIPTS], Section 6.2) whose contexts and signoffs arrays carry one entry per admitted member, plus the Quorum Policy (by reference via policy_hash, and optionally inline for convenience). For the offline quorum computation, each member is the triple:¶

{
  "role": "program_officer",
  "approver_public_key": "<SPKI of the approver's enrolled key>",
  "signoff": {
    "@type": "ep.signoff",
    "context": {
      "context_type": "ep.signoff.v1",
      "action_hash": "...",
      "approver": "...",
      "issued_at": "...",
      "...": "..."
    },
    "webauthn": {
      "authenticator_data": "...",
      "client_data_json": "...",
      "signature": "..."
    }
  }
}

The context and webauthn members are exactly as defined by the base draft; EP-QUORUM does not alter their canonicalization, hashing, or signature verification. The role and approver_public_key are the join keys against the Quorum Policy roster and the Approver Directory.¶

9. Conformance

An implementation conforms to EP-QUORUM if, for the published adversarial conformance vectors, it returns the expected satisfied/not-satisfied verdict for every vector and rejects every non-conforming candidate at incremental admission with the expected reason. The reference suite (EP-QUORUM-v1) comprises the following vectors, each carrying real Class-A WebAuthn assertions:¶

The reference suite is maintained such that three cross-language reference verifiers (JavaScript, Python, Go), which share one repository and are therefore a consistency check rather than clean-room independent implementations, MUST agree on every vector; divergence is a conformance defect in at least one verifier. Independent implementations remain future interoperability evidence. The "accept" vectors guard against a verifier that is too strict (denying valid quorums); the "reject" vectors guard against a verifier that is too lenient (the security-critical direction).¶

10. Security Considerations

EP-QUORUM inherits all Security Considerations of [EP-RECEIPTS] and adds the following. Several restate, honestly, what a quorum does not buy.¶

11. IANA Considerations

This document has no IANA actions.¶

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