Agent-to-Agent Trust, Identity, and Verifiable Provenance

5.1. On-Behalf-Of (OBO)

Microsoft Entra ID OBO provides user-to-service delegation:¶

   User --> Agent A (token A) -->
   Agent A requests token for Agent B on behalf of user -->
   Entra validates the chain -->
   Issues scoped delegated token

Key insight: a trusted third party validates the delegation chain. Agents do not self-assert trust.¶

OBO breaks down for agent-to-agent because agents are ephemeral, agent spawning is dynamic, and no registry exists for agent templates.¶

5.2. OAuth 2.0 Token Exchange (RFC 8693)

[RFC8693] defines scope constraint across delegation hops -- downstream tokens MUST be a subset of upstream grants. This document adopts that principle for agent scope inheritance.¶

6.1. Certificate Profile

Certificates used for agent identity MUST conform to [RFC5280]. In addition, a relying party validating an agent certificate MUST enforce all of the following, and MUST refuse the certificate if any is not satisfied:¶

• An agent certificate MUST carry the basicConstraints extension asserting cA = FALSE. A certificate that omits basicConstraints MUST be refused; the extension's absence MUST NOT be treated as equivalent to cA = FALSE.¶
• A Template Registry CA certificate MUST carry basicConstraints asserting cA = TRUE, marked critical, and MUST assert keyCertSign in its keyUsage extension.¶
• Certificates MUST be signed using SHA-256 or a stronger digest. Certificates signed with SHA-1 or MD5 MUST be refused irrespective of key size.¶
• Public keys MUST provide at least a 112-bit security level as defined in [SP800-57] -- RSA 2048 or NIST P-256 or stronger. A relying party MUST refuse a certificate whose key falls below this level.¶

An agent certificate asserting cA = TRUE is a certificate entitled to issue further certificates. Such an agent can create children without presenting a request to the Template Registry CA, which removes both checks required by Section 8.1 from the spawn path entirely. No signature is invalid in that scenario and no chain fails to verify; the containment described in Section 7 is simply never consulted. This is the reason the constraint is stated normatively here rather than left to the general RFC 5280 profile.¶

These requirements are necessary and not sufficient. A certificate satisfying all of them establishes identity only; it carries no authorization. See Section 16.6.¶

6.2. Certificate Signing Request Flow

   Template Author
       | defines template (scopes, spawn rules, TTL, owner)
       | generates CSR
       v
   Template Registry CA
       | validates template conformance
       | signs the template certificate
       v
   Signed Agent Template (registered)
       |
       v
   Orchestrator spawns agent from signed template
       | agent receives certificate derived from template certificate
       | CA chain proves provenance
       v
   Resource validates certificate chain:
       - certificate signed by trusted template?
       - template signed by CA?
       - scope within bounds?
       - ALLOW or DENY

6.3. Certificate Chain Structure

   Template Registry CA (root of trust)
       |
       v
   Orchestrator Agent Certificate
       Subject:             orchestrator-v1
       Issuer:              Template Registry CA
       Owner:               owner@example.com
       OrgID:               org-123
       PermittedOperations: spawn, delegate
       AllowedScopes:       read:data, write:data
       CanSpawn:            [reader-template-v1], MaxChildren: 5
       PolicyRef:           policy-store/orchestrator-v1/current
       TTL:                 1h
            |
            v
       Child Agent Certificate
           Subject:             reader-agent-v1
           Issuer:              orchestrator-v1
           CA Chain:            orchestrator-v1 -> Template Registry CA
           PermittedOperations: read only
           AllowedScopes:       read:data (MUST be subset of parent)
           TTL:                 15min (SHOULD be shorter than parent)

7.1. Static Fields

The following fields MUST be present in every agent template certificate and MUST NOT be modified without full re-certification:¶

7.2. Dynamic Policy Bounds

Dynamic policies MUST be bounded by the static template fields. A dynamic policy MUST NOT grant scopes beyond AllowedScopes. A dynamic policy MUST NOT add spawn targets beyond CanSpawn.¶

A relying party MUST evaluate these bounds itself, on every validation, against the static fields of the template certificate naming the subject the policy governs. This evaluation MUST be independent of signature validity, and a policy exceeding the bounds MUST be refused before it is applied, however many valid signatures it carries.¶

The signatures required by Section 9.3 and the bounds required here answer different questions. The signatures establish who authorized a change. They do not, and cannot, establish that the change was within the ceiling the template set, because the ceiling is stated in a certificate that no signature over the policy covers. An implementation that treats a valid dual signature as sufficient authority to apply a policy has removed the only mechanism bounding what a policy may grant.¶

Refusal under this section is a distinct condition from a signature failure, and implementations SHOULD report it distinctly: the document is authentic and its content is not permitted. Reporting it as a signature failure misdirects the operator to the wrong lane of Section 9.1.¶

8.1. Two-Check Spawn Rule

Every spawn MUST pass two independent checks. Either check failing MUST result in spawn denial with no fallback.¶

Check 1 -- Static (CanSpawn in certificate): The requested child template MUST appear in the spawning agent's CanSpawn list.¶

Check 2 -- Dynamic (Registry live lookup): The requested child template MUST be currently registered, CA-signed, not self-signed, owned by an authorized party, and not present in the current CRL.¶

Rationale: CanSpawn alone is insufficient because the certificate may be stale and a template may have been revoked since issuance. Registry lookup alone is insufficient because any party could forge a request claiming any template. Both checks MUST pass.¶

8.2. Spawn Validation Sequence

   1.  CanSpawn check -- child template in CanSpawn list?
       NO  --> DENY, audit log
       YES --> continue

   2.  Registry check -- registered, CA-signed, not revoked?
       NO  --> DENY, audit log
       YES --> continue

   3.  Scope check -- requested scope subset of parent scopes?
       NO  --> DENY, audit log
       YES --> continue

   4.  MaxChildren check -- current children < MaxChildren?
       NO  --> DENY, audit log
       YES --> spawn approved

   5.  Child certificate issued with:
       - parent identity embedded (delegation chain)
       - scope equal to requested scope (not exceeding parent)
       - spawn timestamp and nonce (replay prevention)
       - audit log entry written

8.3. Scope Constraint

A child agent's AllowedScopes MUST be a subset of the parent agent's AllowedScopes: every scope granted to the child MUST also be held by the parent. A child MUST NOT be granted any scope the parent does not itself hold; scope escalation across agent hops is explicitly prohibited. Equality is permitted -- a child MAY be granted the same scope set as its parent, or fewer.¶

Example:¶

   Parent has:  read:data, write:data
   Child gets:  read:data              -- valid
   Child gets:  read:data, write:data  -- valid (same as parent)
   Child gets:  admin:data             -- MUST be rejected

8.4. Audit Requirements

Every spawn event MUST be logged with: spawning agent identity, child template identity, requested scope, granted scope, timestamp, outcome (ALLOWED or DENIED), and reason if denied.¶

9.1. Two-Lane Model

Template certificate (static): WHO the agent is and WHO it can spawn. Changes require full re-certification.¶

Dynamic policy (fast lane): WHAT the agent can do within the bounds of the template certificate. Changes require dual signature (Section 9.3).¶

Dynamic policies MUST NOT exceed the bounds defined in the static template certificate.¶

9.2. Ownership

Template ownership MUST be established at certificate signing time and embedded in the Owner and OrgID fields. Only the verified owner of the organization that signed the template MAY submit policy changes.¶

9.3. Dual Signature Requirement

Every policy change MUST be signed by two independent parties:¶

1. Owner -- proves the right to change the policy.¶
2. Policy Authority -- proves the policy passed automated validation gates.¶

Neither signature alone is sufficient. Both MUST be present and valid for a policy to be accepted.¶

   Owner key  +  Policy Authority key  =  policy accepted
       ^                  ^
   who owns it        passed the gates

9.4. Dynamic Policy Document Structure

A dynamic policy document MUST contain the following fields. This is the complete set; a document containing any other field MUST be refused, since an unrecognized field is either meaningless or an attempt to convey authority the profile does not define.¶

Version is scoped to the Subject, not global. A relying party MUST refuse a policy whose Version is not strictly greater than the Version of the policy currently in force for that Subject.¶

9.5. Canonicalization

Signatures and hashes defined by this document are computed over bytes, so the mapping from a policy document to those bytes MUST be deterministic and identical across implementations.¶

Wherever this document requires a canonical form -- of a policy document, of any field subset thereof, or of an audit log entry -- that canonical form MUST be the JSON Canonicalization Scheme (JCS) serialization defined in [RFC8785]. A single canonical form is specified for all of them deliberately: an implementation carrying two serializations will eventually apply the wrong one, and the resulting failure presents as a signature or integrity error with no indication that serialization was the cause.¶

JCS is specified rather than described here because a bespoke serialization would require this document to define property ordering, string escaping, and number formatting correctly and completely, and any error in that definition surfaces to implementers as a signature failure that appears to be a cryptographic fault. Serializations that differ only in whitespace, key order, or the escaping of non-ASCII characters produce different bytes and therefore different signatures; implementers MUST NOT assume a language's default JSON encoder produces the canonical form, as most do not.¶

9.6. Signature and Hash Coverage

Both signatures required by Section 9.3 MUST be computed over the canonical form (Section 9.5) of the complete set of fields defined in Section 9.4. No field in that table is excluded from either signature.¶

In particular, Version MUST be inside the signed preimage. Version exists to prevent replay, and it cannot serve that purpose while it remains modifiable without invalidating a signature: an attacker holding no key can otherwise take a superseded but validly signed policy, increment its Version, and present it as current. Both signatures verify, the content hash matches, and the version reads as current. The same reasoning applies to Subject: a signature that does not bind the policy to the agent it governs permits a policy issued for one agent to be presented for another.¶

The content hash required by Section 9.7 MUST be SHA-256, or a stronger digest, computed over the same canonical form and the same field set as the signatures. The hash field itself is stored alongside the policy document and MUST NOT be included in its own preimage.¶

The signatures and the content hash are stored with the policy document but are not part of it. A relying party MUST NOT accept a document in which they appear as policy fields.¶

9.7. Policy Change Sequence

   1.  Identity and ownership verification
       - requester matches Owner in template certificate?
       - requester OrgID matches certificate OrgID?
       NO  --> rejected, audit logged

   2.  Automated gate (OPA):
       - scope within AllowedScopes?
       - spawn targets within CanSpawn?
       - no conflicts with active policies?
       ANY FAIL --> rejected

   3.  Dual signature applied:
       Owner signs, Policy Authority countersigns

   4.  Policy stored with dual signature, version, timestamp,
       and content hash

   5.  Agent validates at runtime:
       - both signatures valid?
       - version current? (replay prevention)
       - hash matches? (tamper detection)
       - policy within template certificate bounds?
       ANY FAIL --> DENY, audit logged

9.8. Threat Coverage

10.1. Full Re-Verification Required

A new template version MUST undergo full re-verification from scratch. Trust MUST NOT be inherited from a previous version.¶

Rationale: inheriting trust from v1 would allow a compromised v1 certificate to bootstrap trust for v2.¶

10.2. Versioning Principle

Everything on the old template chain continues to work unchanged as long as the certificate is valid. New connections and functionality are opt-in -- only available after the new template chain is fully verified and explicitly adopted.¶

10.3. Non-Inheritance Rules

The following MUST NOT be inherited from a previous version:¶

• Trust chain¶
• Cross-organizational grants¶
• CanSpawn list¶
• Dynamic policies¶

10.4. Template Lifecycle

ACTIVE:

Template is trusted. New spawns accepted.¶

DISABLED:

No new spawns. Existing agents run to TTL expiry. Reversible.¶

DELETED:

Certificate revoked, CRL updated, registry entry removed. Irreversible. Audit log preserved.¶

Disable SHOULD precede delete. Implementations SHOULD enforce a mandatory waiting period between DISABLED and DELETED.¶

10.5. Cross-Organizational Grant Re-Issuance

Cross-organizational grants MUST be explicitly re-issued for each new template version. Grants MUST NOT automatically roll over on version upgrade.¶

11.1. Explicit Grant Requirement

Cross-organizational agent spawning MUST be explicitly authorized by the resource-owning organization. No implicit trust exists between organizations.¶

11.2. Grant Structure

A cross-organizational grant MUST contain:¶

11.3. Trust Anchor Options

Federated CA:

Both organizations trust a shared root CA.¶

Explicit CA trust:

Org-B explicitly trusts Org-A's CA.¶

Third-party CA:

Both organizations use a public CA.¶

11.4. Unilateral Revocation

The granting organization MAY revoke a cross-organizational grant without the cooperation of the receiving organization. Upon revocation, all agents spawned under the affected grant MUST be treated as untrusted on next validation.¶

11.5. Federated Audit

Each organization MUST maintain its own independent audit trail. Audit records MUST NOT depend on the other organization's systems.¶

12.1. Template Revocation

Template revocation MUST be performed by recording the template certificate as revoked in the CA's revocation state -- an X.509 CRL, an OCSP responder, or an equivalent revocation registry consulted on every validation. All agent certificates derived from a revoked template MUST be treated as untrusted on the next revocation check.¶

12.2. Individual Agent Revocation

Individual agents MAY be revoked by explicit certificate revocation or by removal of authorization relationships at the resource layer.¶

12.3. Automation Requirement

Routine revocation (TTL expiry, task completion) MUST be fully automated. Human involvement SHOULD be reserved for incident response and high-impact decisions.¶

13.1. Fail Closed

Any verification step that cannot be completed MUST result in DENY. This includes:¶

• CA unreachable¶
• Registry unreachable¶
• CRL unreachable¶
• Certificate expired or revoked¶
• Scope escalation attempt¶
• Policy invalid or unsigned¶
• Dual signature missing or invalid¶

Implementations MUST NOT provide a degraded mode that allows partial spawning or execution when verification infrastructure is unavailable.¶

14.1. Field Placement

Implementations MUST place every field in the location specified by this document. Variable element placement is NOT permitted.¶

14.2. CSR Validation

Non-conforming CSRs MUST be rejected by the CA. A non-conforming template MUST NOT be signed.¶

14.3. Test Vectors

Implementations MUST provide test vectors -- concrete examples of valid and invalid template chains -- for conformance validation.¶

14.4. Conformance Certification

Implementations claiming conformance with this document MUST pass conformance certification before shipping.¶

14.5. Reference Implementation

A reference implementation SHOULD be made available including:¶

• Template Registry CA¶
• Spawn chain validation¶
• SDK with certificate chain validation, registry lookup, and CRL check handling¶
• Template linter for pre-submission CSR validation¶

Three independent implementations exist at the time of writing. Each states the revision it was written and verified against, and continues to state that revision until someone re-reads the newer text and re-runs the vectors. An implementation pinned to an earlier revision remains a correct implementation of that revision; a version string is a conformance claim rather than a label, and advancing one without re-verifying asserts conformance nobody has tested.¶

The changes in this revision were surfaced by building the first of these against the -01 text and attacking it, rather than by review of the text alone. Implementers are encouraged to report what the document does not say clearly enough, which is a different and more useful signal than agreement with it.¶

16.1. Scope Escalation

Implementations MUST enforce at every hop that child agent scopes are a subset of parent scopes: child scopes are contained within parent scopes, and no scope may be introduced that the parent does not already hold. Failure to enforce this allows privilege escalation across the agent chain.¶

16.2. Replay Attacks

Spawn requests MUST include a timestamp and nonce. Implementations MUST reject spawn requests where the timestamp exceeds a defined freshness window or the nonce has been seen previously.¶

16.3. Compromised Templates

A compromised template certificate MUST be revoked immediately. A single CRL update invalidates all downstream agent certificates derived from that template.¶

16.4. Single Point of Compromise

The dual signature requirement (Section 9.3) ensures no single compromised party can push unauthorized policy changes. CA compromise requires full ecosystem re-issuance.¶

16.5. Cross-Organizational Trust

Cross-organizational grants MUST be explicitly issued. Implicit trust between organizations is prohibited.¶

16.6. PKI Does Not Enforce Authorization

Because agent identity is carried in X.509 certificates and the chain is verified on every validation, it is natural to assume the certificate chain also constrains what an agent may do. It does not, and an implementation built on that assumption is unprotected while appearing rigorous.¶

X.509 carries no representation of the scopes defined by this document. AllowedScopes, CanSpawn and MaxChildren are conveyed in the template, and no certificate path validation algorithm inspects them. A conformant X.509 validator presented with an agent certificate whose associated template claims scopes far beyond those its parent holds will report success, because the certificate is genuinely valid and correctly issued. The over-scoping is not a defect the validator is looking for.¶

The nameConstraints extension does not close this gap. It constrains the namespace within which a CA may issue, not the authority a subject may claim once issued. Likewise certificatePolicies expresses which policies a chain supports, not whether a child exceeded its parent, and policy processing is disabled by default in common implementations.¶

Consequently Section 7.2 and Section 8.3 are load-bearing implementation logic. They are not restatements of a property the certificate chain already guarantees, and an implementation that omits them because "the chain verifies" has no containment at all. Certificate validity establishes identity. Authorization is evaluated separately, every time.¶

16.7. Audit Integrity

Audit logs MUST be tamper-evident. Logs MUST NOT be deletable by agents or orchestrators. Audit log infrastructure SHOULD be outside the control of the agent ecosystem itself.¶

Each entry's hash MUST be computed as SHA-256, or a stronger digest, over the canonical form (Section 9.5) of the entry's fields including the previous entry's hash, and excluding the entry's own hash field. As with policy documents, an unspecified preimage is not a detail: two implementations hashing different field sets each read the other's chain as broken, and a chain that cannot be verified across implementations provides no evidence to anyone but its author.¶