Attenuating Authorization Tokens for Agentic Delegation Chains
draft-niyikiza-oauth-attenuating-agent-tokens-01
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Author | Niki Aimable | ||
| Last updated | 2026-06-15 | ||
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draft-niyikiza-oauth-attenuating-agent-tokens-01
Web Authorization Protocol (OAuth) N. A. Niyikiza
Internet-Draft Tenuo
Intended status: Standards Track 15 June 2026
Expires: 17 December 2026
Attenuating Authorization Tokens for Agentic Delegation Chains
draft-niyikiza-oauth-attenuating-agent-tokens-01
Abstract
This document defines Attenuating Authorization Tokens (AATs), a
signed credential format for task-scoped delegation in AI agent
systems. An AAT encodes the tools an agent may invoke and the
argument constraints that apply to those invocations. A token holder
authorized to delegate can derive a token offline with equal or
narrower authority, subject to the parent token's depth and lifetime
limits. The resulting delegation chain is verifiable offline by any
enforcement point that has the root issuer's trust anchor key.
This specification profiles the OAuth Rich Authorization Requests
format (RFC 9396) for tool-level capability claims, adds delegation-
chain claims, and defines a core constraint vocabulary for argument
restrictions. The chain verification algorithm authenticates each
delegation step and enforces monotonic attenuation without network
contact with the root issuer.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 17 December 2026.
Copyright Notice
Copyright (c) 2026 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Limitations of Existing OAuth Mechanisms for Agentic
Delegation . . . . . . . . . . . . . . . . . . . . . . . 6
1.2. Design Goals . . . . . . . . . . . . . . . . . . . . . . 6
1.3. Relationship to Prior Work . . . . . . . . . . . . . . . 7
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 8
3. Token Structure . . . . . . . . . . . . . . . . . . . . . . . 9
3.1. Chain Position and Invocation Semantics . . . . . . . . . 9
3.2. Common Claims . . . . . . . . . . . . . . . . . . . . . . 10
3.3. Capability Claims via authorization_details . . . . . . . 13
3.3.1. Tool Identifier Requirements . . . . . . . . . . . . 14
3.4. Argument Constraints . . . . . . . . . . . . . . . . . . 15
3.5. Extension Constraint Registry . . . . . . . . . . . . . . 17
3.5.1. Attenuation Compliance Requirement . . . . . . . . . 17
3.5.2. Enforcement Point Obligations . . . . . . . . . . . . 18
3.5.3. Example Registration: Path Containment . . . . . . . 19
3.6. Examples . . . . . . . . . . . . . . . . . . . . . . . . 19
3.6.1. Root Token . . . . . . . . . . . . . . . . . . . . . 19
3.6.2. Derived Token . . . . . . . . . . . . . . . . . . . . 20
3.7. Root Issuer Support and Root Token Issuance . . . . . . . 22
3.7.1. Root Issuer Discovery . . . . . . . . . . . . . . . . 22
3.7.2. Agent Token Request . . . . . . . . . . . . . . . . . 22
3.7.3. Root Token Issuance . . . . . . . . . . . . . . . . . 23
4. Attenuation Invariants . . . . . . . . . . . . . . . . . . . 24
4.1. Capability Lattice Model (Non-Normative) . . . . . . . . 24
4.2. I1: Delegation Authority . . . . . . . . . . . . . . . . 25
4.3. I2: Depth Monotonicity . . . . . . . . . . . . . . . . . 26
4.3.1. Implementation Resource Limits . . . . . . . . . . . 27
4.4. I3: TTL Monotonicity . . . . . . . . . . . . . . . . . . 27
4.5. I4: Capability Monotonicity . . . . . . . . . . . . . . . 28
4.6. I5: Cryptographic Linkage . . . . . . . . . . . . . . . . 31
4.7. I6: Proof of Possession . . . . . . . . . . . . . . . . . 31
5. Proof of Possession . . . . . . . . . . . . . . . . . . . . . 32
5.1. Rationale . . . . . . . . . . . . . . . . . . . . . . . . 32
5.2. PoP Token Structure . . . . . . . . . . . . . . . . . . . 32
5.3. Verification . . . . . . . . . . . . . . . . . . . . . . 34
6. Token Derivation . . . . . . . . . . . . . . . . . . . . . . 35
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7. Chain Verification Algorithm . . . . . . . . . . . . . . . . 36
8. Security Considerations . . . . . . . . . . . . . . . . . . . 42
8.1. Threat Model . . . . . . . . . . . . . . . . . . . . . . 42
8.1.1. Threats Mitigated . . . . . . . . . . . . . . . . . . 42
8.1.2. Threats Not Mitigated . . . . . . . . . . . . . . . . 43
8.2. Attenuation as the Security Invariant . . . . . . . . . . 44
8.3. Root Key Compromise . . . . . . . . . . . . . . . . . . . 45
8.4. Holder Key Compromise . . . . . . . . . . . . . . . . . . 45
8.5. Replay Attacks . . . . . . . . . . . . . . . . . . . . . 45
8.6. Constraint Evaluation . . . . . . . . . . . . . . . . . . 46
8.7. Depth Limit . . . . . . . . . . . . . . . . . . . . . . . 46
8.8. Unknown Constraint Types . . . . . . . . . . . . . . . . 47
8.9. Token Revocation . . . . . . . . . . . . . . . . . . . . 47
8.10. Approval Gates . . . . . . . . . . . . . . . . . . . . . 48
8.11. Clock Skew . . . . . . . . . . . . . . . . . . . . . . . 48
8.12. Role-Based Key Separation . . . . . . . . . . . . . . . . 49
8.13. Algorithm Confusion . . . . . . . . . . . . . . . . . . . 49
9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 50
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 50
10.1. JWT Claims Registry . . . . . . . . . . . . . . . . . . 50
10.2. OAuth Authorization Details Types Registry . . . . . . . 51
10.3. AAT Constraint Type Registry . . . . . . . . . . . . . . 51
10.3.1. Designated Expert Instructions . . . . . . . . . . . 51
10.3.2. Registration Template . . . . . . . . . . . . . . . 52
10.3.3. Initial Registry Entries . . . . . . . . . . . . . . 53
10.4. OAuth Authorization Server Metadata Registry . . . . . . 54
10.5. OAuth Token Type Registration . . . . . . . . . . . . . 55
10.6. OAuth Parameters Registry . . . . . . . . . . . . . . . 55
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 55
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 55
12.1. Normative References . . . . . . . . . . . . . . . . . . 55
12.2. Informative References . . . . . . . . . . . . . . . . . 57
Appendix A. Comparison with Related Authorization Mechanisms
(Non-Normative) . . . . . . . . . . . . . . . . . . . . . 59
A.1. Token Exchange (RFC 8693) . . . . . . . . . . . . . . . . 60
A.2. Rich Authorization Requests (RFC 9396) . . . . . . . . . 60
A.3. DPoP (RFC 9449) . . . . . . . . . . . . . . . . . . . . . 60
A.4. Biscuit . . . . . . . . . . . . . . . . . . . . . . . . . 61
Appendix B. Implementation Notes (Non-Normative) . . . . . . . . 61
B.1. Algorithm Recommendations . . . . . . . . . . . . . . . . 61
B.2. Recognizing Derived Token iss Values in Middleware . . . 62
B.3. Relationship to WIMSE . . . . . . . . . . . . . . . . . . 62
B.4. Delegation Depth Guidance . . . . . . . . . . . . . . . . 62
B.5. Implementation Size Limits . . . . . . . . . . . . . . . 63
B.6. Signed Passthrough Metadata . . . . . . . . . . . . . . . 64
B.7. TTL Guidance . . . . . . . . . . . . . . . . . . . . . . 64
Appendix C. Policy Languages with Decidable Containment
(Non-Normative) . . . . . . . . . . . . . . . . . . . . . 65
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Appendix D. CBOR/CWT Considerations (Non-Normative) . . . . . . 65
Appendix E. Implementation Status (Non-Normative) . . . . . . . 65
E.1. Reference Implementation . . . . . . . . . . . . . . . . 66
E.2. Formal Verification . . . . . . . . . . . . . . . . . . . 66
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 66
1. Introduction
AI agent systems increasingly decompose a user request into delegated
steps performed by multiple agents, services, or tools. Each step
may need authority derived from the user or an originating service,
but rarely needs the full authority available to the workflow as a
whole.
Existing OAuth mechanisms can scope tokens to principals, resources,
APIs, or authorization details, but they do not define an offline,
holder-derivable delegation chain in which each downstream holder can
attenuate authority and any enforcement point can verify that the
resulting token is no broader than its parent. In particular, OAuth
does not define a standard way for a token holder to derive a token
that cryptographically constrains a receiving agent to specific tools
and argument values for a specific task.
Without such attenuation, a token broad enough to support a multi-
step workflow can carry more authority than an intermediate agent
needs for its current step. Prompt injection, model hallucination,
or compromise can then exercise that excess authority. Attenuation
limits this exposure by letting each delegation step pass onward only
the authority needed for the next step.
A distinct problem is the confused deputy [HARDY88]: a deputy that
combines a caller-supplied resource designation with the deputy's own
standing authority can be induced to perform an action the caller
could not perform directly. Capability systems address this by
carrying designation and authority together in an unforgeable
artifact. AATs apply that pattern to agentic delegation: the invoker
can derive a token whose tool and argument constraints designate the
task's resource and authority, and the agent acts under that received
token rather than under ambient authority of its own. Section 8
describes the resulting guarantees and limits.
WIMSE [WIMSE-ARCH] provides mechanisms for establishing workload
identity and propagating it across service boundaries. OAuth 2.0
[RFC6749] provides token issuance and scoping. AATs complement these
mechanisms with delegation-aware attenuation semantics: a holder can
derive a narrower token and pass it downstream, while enforcement
points can verify the resulting chain offline. This avoids making
the authorization server a participant in every delegation hop, which
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is important for agentic workflows that execute tool invocations in
rapid succession, operate across trust boundaries, or run with
intermittent connectivity.
Capability-based systems [DENNIS66] provide the underlying model.
Authority is carried by unforgeable tokens scoped to specific
operations; a holder can attenuate a capability before passing it on,
but cannot amplify it [SALTZER75]. This document defines such a
mechanism for OAuth-based agent systems, complementing WIMSE's
identity layer with a delegation and attenuation layer. The
resulting chain lets enforcement points evaluate both the leaf token
and the delegation path that produced it.
The following diagram shows the delegation flow this specification
enables:
Root Issuer
|
| issues root AAT (Section 3.7)
v
Orchestrating Agent
|
| derives AAT (Section 6)
v
Planning Agent
|
| derives AAT (Section 6)
v
Tool-Invoking Agent
|
| presents AAT with PoP JWT (Section 5)
v
Enforcement Point
(verifies chain offline, Section 7)
At each derivation step, the derived token's authorized capabilities
are a subset of the parent's: authority can stay the same or narrow,
but never widen. The enforcement point verifies the complete chain
using only the root token's trust anchor key; no network calls are
required. How token chains are carried to enforcement points is
deployment-specific; this document does not define a transport
binding.
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1.1. Limitations of Existing OAuth Mechanisms for Agentic Delegation
OAuth 2.0 Token Exchange [RFC8693] enables a principal to obtain a
new token with reduced scope by contacting the authorization server.
The server enforces the scope reduction. This requires a synchronous
round-trip to the authorization server at each delegation hop. In
multi-agent chains, this makes the authorization server a participant
in every delegation decision, coupling the delegation topology to
authorization server (AS) availability. [RFC8693] supports
representing prior delegation actors via nested act claims, but those
claims are informational for access control decisions rather than a
cryptographically self-verifiable attenuation chain. The AS mediates
each grant independently, and RFC 8693 does not define a token-local
mechanism for proving that downstream delegation intent remains
consistent with the original authorization scope.
Rich Authorization Requests (RAR) [RFC9396] extend OAuth tokens with
structured authorization detail objects, enabling expressive
capability descriptions. RAR addresses the expressiveness problem.
It does not define how a token holder can produce a narrower token,
or how a chain of such derivations can be verified.
Proposals to extend the authorization code flow with explicit agent
consent, such as introducing a requested_actor parameter at the
authorization endpoint, address who the agent is and whether the user
approved the delegation. They do not constrain which tools the agent
may invoke or with what argument values. AATs are complementary:
they scope authority to specific tools and arguments after identity
and consent have been established.
To the author's knowledge, no existing OAuth standard defines a
delegation chain protocol with token-local chain authentication,
deterministic attenuation checks, and offline chain verification.
1.2. Design Goals
1. *Least privilege at the invocation boundary.* An agent's
authorization token encodes which tools it may call and with what
argument constraints, scoped to the task, not to the full
authority of the calling principal.
2. *Offline derivation.* A token holder can derive a more
restrictive token without contacting the root issuer.
3. *Independent chain verification.* Any enforcement point holding
the trust anchor can verify the complete delegation chain without
network calls.
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4. *Verifiable attenuation.* A derived token cannot grant broader
authority than its parent, and this property can be verified from
the signed chain.
5. *JWT/JWS interoperability.* The primary encoding specified in
this document represents AATs as signed JWTs [RFC7519] using JWS
[RFC7515], allowing deployments to verify chains using existing
JSON Object Signing and Encryption (JOSE) infrastructure without
new cryptographic dependencies.
1.3. Relationship to Prior Work
Macaroons [MACAROONS] introduced the concept of attenuating tokens
with contextual caveats. Macaroons use HMAC chaining, which provides
attenuation but not proof of possession, and express caveats as free-
form predicates evaluated at the target service at runtime. This
specification adds asymmetric proof of possession, structured tool-
level capability claims, and a typed constraint vocabulary. It
defines a normative subsumption relation, enabling any party holding
the chain to verify monotonicity structurally, without predicate
evaluation at a central service.
Biscuit [BISCUIT] extends the Macaroons model with public-key
signatures and offline attenuation. Biscuit expresses authorization
policies in a Datalog variant, requiring a logic engine at
verification time. This specification uses structured constraint
types decidable by structural analysis and defines an explicit
delegation-chain model with holder-bound invocation-time proof of
possession, chain-position claims, and attenuation invariants. A
detailed comparison appears in Appendix A.
Recent OAuth work on transaction tokens [OAUTH-TXN-TOKENS] and
identity and authorization chaining [OAUTH-ID-CHAINING] addresses the
propagation of identity, actor, transaction, and authorization
context across service and trust-domain boundaries. AATs are
complementary: they define token-local, holder-derivable attenuation
of concrete tool-and-argument authority within a delegation chain,
with offline verification by the enforcement point.
The capability-based security model underlying AATs draws on
[DENNIS66], which introduced capabilities as unforgeable tokens of
authority, and [MILLER06], which formalized the principle of least
authority (POLA) and the attenuation property in object-capability
systems. AATs apply these principles at the protocol layer: each
token is a capability scoped to specific tools and arguments, and
derivation can only attenuate, never amplify, the authority it
carries.
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[DEEPMIND26] argues that safe multi-agent delegation requires
explicit transfer of authority, responsibility, and trust at each
delegation step, with bounded operational scope. [CAMEL25] shows
that capability-based controls enforced at the tool boundary can
provide provable security properties in an agentic framework. These
results motivate a protocol-layer mechanism that encodes delegation
scope in verifiable credential artifacts enforced independently of
model behavior. AATs realize one protocol-layer approach to that
goal.
2. Terminology
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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
*Attenuating Authorization Token (AAT):* A signed credential as
defined in this document. The fully specified encoding in this
document is a signed JWT. An AAT encodes tool-level capability
claims and supports offline derivation of derived tokens with
authority equal to or narrower than the parent's.
*Root Token:* An AAT with no parent token, del_depth: 0, and par_hash
absent. A root token is signed by the private key corresponding to a
trust anchor and establishes the authority ceiling for all derived
tokens. A root token is a chain position, not a distinct token type.
*Root Issuer:* The entity that mints root tokens. The root issuer
holds the private key corresponding to a trust anchor and is
responsible for verifying agent identity and requested authority
before issuance.
*Token Holder:* The entity that possesses an AAT and the private key
corresponding to its cnf.jwk claim. The token holder is the party
authorized to derive further tokens from it, subject to the chain's
depth limits. The holder of the leaf token is also the party
authorized to present the chain for tool invocation by signing the
PoP JWT.
*Derived Token:* An AAT produced by a token holder from a parent AAT,
also referred to as a child token. A derived token's authority is a
subset of its parent's authority (equal or narrower). Derivation
does not require a round-trip to the root issuer.
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*Tool:* An addressable function or API operation that an agent may
invoke. A tool is identified by a string identifier. Tool
identifiers are compared as exact strings; see Section 3.3.1 for
requirements.
*Argument Constraint:* A predicate over a tool argument value that
the argument MUST satisfy for the invocation to be authorized.
Constraints are evaluated at the enforcement point before invocation.
*Capability Claim:* The set of (tool, argument constraints) pairs
encoded in an AAT's authorization_details claim.
*Attenuation:* The process of deriving a token with a capability
claim that is a subset of the parent token's capability claim.
Attenuation is the only permitted direction of derivation.
*Chain:* An ordered sequence of AATs from root to leaf, where each
token was derived from its predecessor.
*Leaf Token:* The last token in a chain. The leaf token is the one
presented to the enforcement point for authorization. The PoP JWT is
signed by the private key corresponding to the leaf token's cnf.jwk.
*Enforcement Point:* The component that receives a tool invocation
request, verifies the presented token chain, evaluates argument
constraints, and permits or denies execution.
*Trust Anchor:* A public key that enforcement points are configured
to trust as the root of a delegation chain. Root tokens are signed
by the private key corresponding to a trust anchor.
*Proof of Possession (PoP):* A cryptographic demonstration that the
presenter of a token controls the private key corresponding to the
public key bound in the token's cnf claim. In this specification,
the token holder presents the chain and signs the PoP JWT using the
same private key.
3. Token Structure
3.1. Chain Position and Invocation Semantics
This specification does not define separate token types for
delegation and execution. An AAT's role is determined by its
position in the presented chain.
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The root token establishes the authority ceiling. Intermediate
tokens record attenuations made by holders along the delegation path.
The leaf token is the token whose holder presents a PoP JWT and whose
capability claims are evaluated against the requested tool
invocation.
A holder of any AAT MAY derive a child token when del_depth is
strictly less than del_max_depth. The derived token MUST carry
authority equal to or narrower than the parent token, as enforced by
the capability monotonicity invariant (I4, Section 4.5). A token
MUST NOT be accepted for a tool invocation except as the leaf of a
successfully verified chain.
3.2. Common Claims
The following claims appear in all AATs. All claims listed as
REQUIRED MUST be present. Claims listed as OPTIONAL MAY be omitted;
their absence carries the semantics described in the table.
+=======================+===========+=========+====================+
| Claim |Type |Required |Description |
+=======================+===========+=========+====================+
| jti |string |REQUIRED |Unique token |
| | | |identifier. SHOULD |
| | | |be a UUIDv7 value. |
| | | |When a UUID is used,|
| | | |it MUST be encoded |
| | | |as a lowercase |
| | | |hyphenated string in|
| | | |the form xxxxxxxx- |
| | | |xxxx-xxxx-xxxx- |
| | | |xxxxxxxxxxxx per |
| | | |[RFC9562]. |
+-----------------------+-----------+---------+--------------------+
| iss |string |REQUIRED |Identifier of the |
| | | |entity that signed |
| | | |this token. For |
| | | |root tokens, MUST be|
| | | |a URI identifying |
| | | |the root issuer. |
| | | |For derived tokens, |
| | | |MUST be a JWK |
| | | |Thumbprint URI |
| | | |([RFC9278]) over the|
| | | |signing key using |
| | | |SHA-256; the exact |
| | | |URI form is given |
| | | |after this table. |
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+-----------------------+-----------+---------+--------------------+
| iat |NumericDate|REQUIRED |Time at which the |
| | | |token was issued. |
| | | |MUST NOT be more |
| | | |than MAX_IAT_SKEW in|
| | | |the future relative |
| | | |to the enforcement |
| | | |point's clock (see |
| | | |Section 4.4). In a |
| | | |chain, a derived |
| | | |token's iat MUST NOT|
| | | |be earlier than its |
| | | |parent's iat. |
+-----------------------+-----------+---------+--------------------+
| exp |NumericDate|REQUIRED |Time at which the |
| | | |token expires. MUST|
| | | |be greater than iat.|
| | | |MUST NOT exceed iat |
| | | |plus |
| | | |MAX_TOKEN_LIFETIME |
| | | |(see Section 4.4). |
+-----------------------+-----------+---------+--------------------+
| cnf |object |REQUIRED |Confirmation claim |
| | | |[RFC7800]. MUST |
| | | |contain jwk with the|
| | | |holder's public key.|
| | | |The jwk value MUST |
| | | |be a public key; |
| | | |private key material|
| | | |MUST NOT appear in |
| | | |this field. |
+-----------------------+-----------+---------+--------------------+
| del_depth |integer |REQUIRED |Delegation depth. 0 |
| | | |for root tokens. |
| | | |Incremented by |
| | | |exactly 1 at each |
| | | |derivation step (see|
| | | |Section 4.3). |
+-----------------------+-----------+---------+--------------------+
| del_max_depth |integer |REQUIRED |Maximum delegation |
| | | |depth permitted in |
| | | |this chain. MUST be|
| | | |a non-negative |
| | | |integer not |
| | | |exceeding the |
| | | |implementation's |
| | | |MAX_DELEGATION_DEPTH|
| | | |(Section 4.3). |
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+-----------------------+-----------+---------+--------------------+
| par_hash |string |MUST |Base64url-encoded |
| | |(derived)|SHA-256 digest of |
| | |/ MUST |the parent token |
| | |NOT |signing input, using|
| | |(root) |base64url encoding |
| | | |without padding as |
| | | |defined in [RFC7515]|
| | | |Appendix C. For |
| | | |JWT/JWS AATs, the |
| | | |parent token signing|
| | | |input is the JWS |
| | | |Signing Input. MUST|
| | | |be absent in root |
| | | |tokens. MUST be |
| | | |present in all |
| | | |derived tokens. |
+-----------------------+-----------+---------+--------------------+
| authorization_details |array |REQUIRED |Tool capability |
| | | |claims. Format |
| | | |defined in |
| | | |Section 3.3. |
+-----------------------+-----------+---------+--------------------+
Table 1
Implementations MUST support Ed25519 [RFC8032] for token signing and
verification. Implementations MAY support additional algorithms.
In both root and derived tokens, iss is a URI. For root tokens, iss
is a URI identifying the root issuer, consistent with conventional
OAuth usage. For derived tokens, iss is a JWK Thumbprint URI
[RFC9278] of the form urn:ietf:params:oauth:jwk-thumbprint:sha-
256:<thumbprint>, where <thumbprint> is the SHA-256 JWK thumbprint
([RFC7638]) of the signing key. This makes I1 verifiable offline:
the enforcement point can confirm that the thumbprint embedded in
derived.iss matches parent.cnf.jwk without any external lookup.
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This specification intentionally omits the sub claim. In
conventional OAuth tokens, sub identifies the resource owner or
principal on whose behalf the token is issued. In an AAT chain, the
holder's identity is fully determined by cnf.jwk: the entity
presenting the token proves possession of the private key
corresponding to cnf.jwk. Including a sub claim would introduce an
additional identity binding that is not cryptographically enforced by
this specification and could be set arbitrarily by any delegating
party. Implementations that require a human-readable subject
identifier MAY convey one in additional JWT claims outside this
specification (see Appendix B.6).
3.3. Capability Claims via authorization_details
This specification profiles [RFC9396] for tool-level capability
claims. An AAT capability entry is an authorization_details entry
whose type is set to "attenuating_agent_token". Such an entry MUST
include a tools member that maps tool names to argument constraint
sets.
{
"authorization_details": [
{
"type": "attenuating_agent_token",
"tools": {
"read_file": {
"path": {
"constraint_type": "one_of",
"values": ["/data/q3-report.pdf", "/data/q4-report.pdf"]
}
},
"search_index": {
"query": {
"constraint_type": "one_of",
"values": ["public filings", "public releases"]
},
"limit": { "constraint_type": "range", "max": 100 }
}
}
}
]
}
A tool entry with an empty constraint map {} is valid and indicates
that the tool is authorized without argument restrictions.
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When a tool entry contains one or more argument constraints, the
enforcement point operates in closed-world mode for that tool
invocation: any argument not named in the constraint map MUST be
rejected. A constrained argument that is absent from the invocation
MUST also be rejected. The presence of a constraint asserts that the
issuer has reasoned about that argument. An invocation that omits it
has not been validated against that reasoning. This is a security
property, not a configuration option.
Issuers who wish to permit an argument to be omitted MUST NOT include
a constraint for it in the constraint map. There is no "optional
constraint" mechanism; the constraint map is a closed specification
of the required invocation shape. To authorize an argument without
restricting its value, use a wildcard constraint (see below).
Optional constrained arguments are outside the core constraint model;
profiles or extension constraint types that require such behavior
must define it explicitly.
A token issuer that wishes to allow unconstrained arguments alongside
constrained ones MUST explicitly include a wildcard constraint for
each argument that should be unrestricted. A wildcard constraint
satisfies closed-world mode while permitting any value for that
argument (see Section 3.4). Enforcement points MUST enforce closed-
world mode and MUST NOT permit unconstrained arguments when any
constraint is present for the tool (see Section 7, step 6b).
The authorization_details array MAY contain entries of other types
alongside attenuating_agent_token entries, consistent with the
extensibility model of [RFC9396]. Enforcement points implementing
this specification process only entries with type set to
attenuating_agent_token and MUST ignore entries of other types. An
authorization_details array containing multiple entries with type:
"attenuating_agent_token" is invalid; the tools map in a single entry
provides sufficient structure for all tool-level capability claims.
Root tokens and leaf tokens MUST contain exactly one entry with type:
"attenuating_agent_token". Non-leaf derived tokens MAY contain zero
entries of this type, in which case they represent the empty
capability set and can only derive further empty-capability tokens.
Such a non-leaf derived token MAY carry an empty
authorization_details array.
3.3.1. Tool Identifier Requirements
Tool identifiers are the keys of the tools map in an
authorization_details entry. The following requirements apply.
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Tool identifiers MUST be unique within the tools map of a single
token. An authorization_details entry containing duplicate tool
identifier keys is malformed and MUST be rejected.
Tool identifiers are compared as exact strings. Enforcement points
MUST NOT apply Unicode normalization, URI normalization, case
folding, percent decoding, or alias resolution when matching tool
identifiers in the token, the PoP JWT, and the requested invocation.
This rule applies only to tool identifier matching. Argument values
are evaluated according to the semantics of their constraint type; a
registered constraint type MAY define normalization as part of its
check or subsumes procedure.
Tool identifiers SHOULD be URIs ([RFC3986]). URI-format identifiers
provide namespace isolation across agents and reduce semantic
collision when multiple agents expose tools with identical local
names. Deployments spanning multiple agents or trust domains SHOULD
use URI-format identifiers; single-agent deployments MAY use local
identifiers where collision is not a concern.
A tool identifier carries no inherent authorization semantics beyond
naming a capability. The root issuer is responsible for verifying
that requested tool identifiers are meaningful in the deployment and
that the requester is authorized to receive authority for those tools
before minting a root token (Section 3.7.3).
3.4. Argument Constraints
Each argument constraint is an object with a constraint_type member
and type-specific members. The following constraint types are
defined normatively. The check predicate and subsumes relation for
each type are normative: two independent implementations MUST produce
identical results when evaluating either predicate against the same
inputs.
The core constraint set is intentionally limited to constraint types
with simple, deterministic, format-independent check and subsumes
rules. Domain-specific matchers and policy-language constraints,
such as resource-identifier matchers, URI or path normalization
rules, or authorization policy expressions, are not core constraint
types. To be used interoperably in AAT authorization_details, they
MUST be defined as registered extension constraint types
(Section 3.5). The registration process confirms that the extension
defines an unambiguous runtime check predicate and a decidable,
sound, and deterministic subsumes procedure. Deployments requiring
richer policy expressiveness SHOULD use a registered extension
constraint type (see Appendix C).
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+=================+=====================+===========================+
| constraint_type | Additional Members | Semantics |
+=================+=====================+===========================+
| exact | value (any scalar) | Argument MUST equal |
| | | value exactly. |
+-----------------+---------------------+---------------------------+
| range | min (number, | Argument MUST be a |
| | optional), max | number satisfying the |
| | (number, optional), | specified bounds. Both |
| | min_inclusive | bounds are optional. |
| | (boolean, optional, | min_inclusive and |
| | default true), | max_inclusive control |
| | max_inclusive | whether the respective |
| | (boolean, optional, | bound is included in |
| | default true) | the valid range; both |
| | | default to true (closed |
| | | interval). |
+-----------------+---------------------+---------------------------+
| one_of | values (array) | Argument MUST be a |
| | | member of values. |
+-----------------+---------------------+---------------------------+
| not_one_of | excluded (array) | Argument MUST NOT be a |
| | | member of excluded. |
+-----------------+---------------------+---------------------------+
| contains | required (array) | Argument, which MUST be |
| | | an array, MUST contain |
| | | every element listed in |
| | | required. |
+-----------------+---------------------+---------------------------+
| subset | allowed (array) | Argument, which MUST be |
| | | an array, MUST be a |
| | | subset of allowed. |
+-----------------+---------------------+---------------------------+
| wildcard | (none) | Any value is accepted. |
+-----------------+---------------------+---------------------------+
| all | constraints (array) | Logical AND of nested |
| | | constraints. See |
| | | Section 4.5 for |
| | | subsumption rules. |
+-----------------+---------------------+---------------------------+
| any | constraints (array) | Logical OR of nested |
| | | constraints. See |
| | | Section 4.5 for |
| | | subsumption rules. |
+-----------------+---------------------+---------------------------+
Table 2
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Enforcement points MUST reject invocations where any argument
violates its associated constraint. Enforcement points MUST deny
authorization if they encounter a constraint_type they do not
recognize (fail-closed behavior). This fail-closed rule applies only
to constraint types within authorization_details. Enforcement points
MUST ignore unrecognized top-level JWT claims; a token MUST NOT be
rejected solely because it contains claims outside those defined in
this specification.
Composite constraint types (all, any) are recursive.
MAX_CONSTRAINT_DEPTH is an implementation-defined finite integer
specifying the maximum nesting depth of a constraint tree.
Implementations MUST enforce a finite MAX_CONSTRAINT_DEPTH to prevent
resource exhaustion from pathologically deep constraint trees. A
value of 32 is RECOMMENDED. Enforcement points MUST reject any
constraint tree whose nesting depth exceeds MAX_CONSTRAINT_DEPTH.
3.5. Extension Constraint Registry
Implementations MAY define extension constraint types beyond those
listed in Section 3.4. Extension constraint types MUST be registered
in the IANA AAT Constraint Type Registry defined in Section 10.3.
The registry exists to preserve security and interoperability in the
presence of domain-specific constraints; it is not a requirement that
all implementations support arbitrary extensions. An enforcement
point that does not recognize a registered extension type MUST deny
authorization (Section 3.5.2), but it is not required to implement
that type.
3.5.1. Attenuation Compliance Requirement
The capability monotonicity invariant (I4, Section 4.5) applies to
extension constraint types without exception. An extension
constraint type MUST NOT be registered unless its registration
defines all of the following.
*A subsumption verification procedure.* The registration MUST provide
a complete, formal definition of what it means for one instance of
the constraint to be at least as restrictive as another instance of
the same constraint type. This procedure MUST satisfy three
properties:
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1. *Decidable.* The procedure MUST terminate in finite time for all
inputs. It MUST NOT require solving problems that are
undecidable or computationally intractable in the general case.
If the constraint language used by the type is not closed under
decidable containment analysis, the registration MUST prescribe a
conservative syntactic strategy and MUST formally justify that
the strategy is sound (never accepts a non-subsuming pair).
2. *Sound.* The procedure MUST NOT return true unless the semantic
subsumption relation holds. That is, if the procedure returns
true for (C_parent, C_child), then for all argument values v:
C_child.check(v) implies C_parent.check(v). The procedure MAY be
conservative: it MAY return false for semantically subsuming
pairs that it cannot verify, but it MUST NOT return true for non-
subsuming pairs.
3. *Deterministic.* Two independent implementations of the procedure
MUST produce identical results for the same inputs. The
procedure MUST be specified precisely enough to ensure this.
Ambiguity in the specification of the procedure is grounds for
rejection of the registration.
This specification does not prescribe the internal mechanism of the
subsumption verification procedure. Registrations MAY use structural
comparison of token claims, formal type-checking, proof-carrying
tokens, or any other mechanism that satisfies the three properties
above. See Appendix C for non-normative guidance on policy languages
with decidable containment algorithms.
*Cross-type subsumption rules.* For each core constraint type defined
in Section 3.4, the registration MUST specify whether a derived token
may substitute an extension type instance for a parent constraint of
that core type (or vice versa). If substitution is permitted, the
registration MUST state the conditions. Any (parent type, child
type) pair not explicitly declared valid MUST be treated as invalid
by enforcement points.
3.5.2. Enforcement Point Obligations
When an enforcement point encounters an extension constraint type
during chain verification, it MUST:
1. Locate the registered subsumption verification procedure for that
type. If no registration exists, the enforcement point MUST
reject the chain (fail-closed).
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2. Evaluate the subsumption relation at every chain link where the
constraint appears, as part of the I4 check. A chain link where
the derived constraint does not subsume the parent constraint
MUST be rejected.
3. Evaluate the constraint's check predicate against the presented
argument value during authorization. If the predicate returns
false, the invocation MUST be denied.
An enforcement point that does not implement a registered extension
constraint type MUST deny authorization rather than skip the
constraint. The presence of an unrecognized constraint type in a
token represents a restriction the issuer intended to enforce.
Silently omitting that check would violate the attenuation guarantee.
3.5.3. Example Registration: Path Containment
The following is an illustrative example of a conforming extension
constraint registration. It is not defined normatively in this
document.
*Type name:* path_containment
*Additional members:* root (string, required). An absolute path
root.
*check predicate:* The argument, after resolving all . and ..
components and removing redundant separators, must be equal to root
or lie beneath root after path-segment normalization. The
normalization step is part of the predicate; implementations that
compare raw argument strings without normalization do not conform to
this registration.
*subsumes relation:* subsumes(C_parent, C_child) is true if and only
if C_child.root is C_parent.root or lies beneath C_parent.root under
the normalized path-segment ordering.
*Cross-type subsumption:* A derived exact constraint subsumes a
parent path_containment constraint if and only if the exact value,
after normalization, is equal to the parent's root or lies beneath
it. All other cross-type pairs involving path_containment are
invalid.
3.6. Examples
3.6.1. Root Token
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{
"jti": "01957a3f-4e23-7b01-a9d1-0050569c2e4f",
"iss": "https://auth.example.com",
"iat": 1741600000,
"exp": 1741603600,
"del_depth": 0,
"del_max_depth": 3,
"cnf": {
"jwk": {
"kty": "OKP",
"crv": "Ed25519",
"x": "11qYAYKxCrfVS_7TyWQHOg7hcvPapiMlrwIaaPcHURo"
}
},
"authorization_details": [
{
"type": "attenuating_agent_token",
"tools": {
"read_file": {
"path": {
"constraint_type": "one_of",
"values": ["/data/q3-report.pdf", "/data/q4-report.pdf"]
}
},
"search_index": {}
}
}
]
}
3.6.2. Derived Token
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{
"jti": "01957a41-0081-7c20-bf3a-00a0c91e1234",
"iss": "urn:ietf:params:oauth:jwk-thumbprint:sha-256:KAKn...",
"iat": 1741600120,
"exp": 1741601920,
"del_depth": 1,
"del_max_depth": 3,
"par_hash": "sha256_base64url_of_parent_token_signing_input",
"cnf": {
"jwk": {
"kty": "OKP",
"crv": "Ed25519",
"x": "rAl9xvTDAeUADPnIWlGpFHtGg4Y8OqcQE5N4XYNdLPs"
}
},
"authorization_details": [
{
"type": "attenuating_agent_token",
"tools": {
"read_file": {
"path": {
"constraint_type": "exact",
"value": "/data/q3-report.pdf"
}
}
}
}
]
}
Note that the derived token:
* Carries a par_hash linking it to its parent.
* Has del_depth incremented to 1.
* Restricts read_file to a single file rather than either file
authorized by the parent.
* Omits search_index, which the parent permitted. Tool omission is
valid attenuation.
* Expires 1800s after its own issuance, versus the parent's 3600s
window.
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3.7. Root Issuer Support and Root Token Issuance
The token endpoint is used only for root AAT issuance. Derived
tokens are created locally by token holders as described in Section 6
and do not require token endpoint interaction. Enforcement points
verify presented chains offline as described in Section 7.
3.7.1. Root Issuer Discovery
A root issuer that supports AAT issuance SHOULD advertise this
capability using the following metadata parameter in its
authorization server metadata document [RFC8414], if supported.
+====================+============================+
| Metadata Parameter | Value |
+====================+============================+
| aat_issuer | Boolean. true if the AS |
| | can issue AAT root tokens. |
+--------------------+----------------------------+
Table 3
This document requests registration of aat_issuer in the IANA OAuth
Authorization Server Metadata registry (Section 10.4).
3.7.2. Agent Token Request
An agent requesting a root AAT MUST include a req_cnf parameter in
its token endpoint request (in the OAuth 2.0 sense, the agent acts as
the client for this request). This specification profiles the
req_cnf token request parameter defined by [RFC9201] for AAT root
token issuance. The parameter carries a key confirmation object
whose JSON syntax and semantics follow [RFC7800] Section 3.1. This
document does not define a new OAuth token endpoint key-confirmation
parameter. The value MUST be a JSON object containing a jwk member
with the agent's public key in JWK format [RFC7517]. This is the key
that the root issuer will bind into the root token's cnf.jwk claim.
The key submitted in req_cnf is the AAT holder key that will be
embedded in the root token's cnf.jwk. This key is distinct from any
credential the client uses to authenticate to the token endpoint.
Client authentication establishes which OAuth client is requesting
issuance; req_cnf establishes which key will hold the issued AAT and
derive or present downstream tokens. Deployments MAY require these
credentials to be controlled by the same workload or agent.
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POST /token HTTP/1.1
Host: as.example.com
Content-Type: application/x-www-form-urlencoded
grant_type=client_credentials
&authorization_details=%5B%7B%22type%22%3A%22attenuating_agent_
token%22%2C...%7D%5D
&req_cnf=%7B%22jwk%22%3A%7B%22kty%22%3A%22OKP%22%2C...%7D%7D
The request MUST also include authorization_details in RAR format
[RFC9396] with type set to attenuating_agent_token, enumerating the
tools and argument constraints the agent is requesting authority to
invoke or delegate.
3.7.3. Root Token Issuance
Upon a valid request, the AS constructs and returns a root AAT. The
AS:
1. Sets iss to the AS's own URI.
2. Sets jti to a unique token identifier, RECOMMENDED to be a UUIDv7
value per [RFC9562].
3. Sets iat to the current time and exp to the token's expiry time,
subject to the constraints in Section 4.4.
4. Sets del_depth to 0, del_max_depth to the maximum delegation
depth permitted for this grant, and par_hash to absent.
5. Sets cnf.jwk to the public key submitted in the agent's req_cnf
request parameter. The root issuer MUST validate that the
submitted key is well-formed and is a public key. The root
issuer SHOULD require the agent to demonstrate possession of the
corresponding private key, for example via a signed proof-of-
possession assertion in the token request.
6. Sets authorization_details to the capability claims granted,
which MAY be a subset of what the agent requested.
7. For each tool identifier in the requested authorization_details,
the root issuer SHOULD verify that the identifier is meaningful
in the deployment and that the requester is authorized to receive
authority for that tool. If this verification fails, the root
issuer MUST reject the request. The mechanism for mapping
requester identity to tool authority is deployment-specific and
outside the scope of this specification.
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8. Signs the token with the AS's own private key.
The AS returns the token in a standard OAuth 2.0 token endpoint
response ([RFC6749] Section 5.1) with the following field values:
{
"access_token": "<compact-serialized AAT JWT>",
"token_type": "aat",
"expires_in": <seconds until exp>
}
The token_type value "aat" is registered in Section 10.5. Clients
MUST NOT treat the returned token as a bearer token for use with
arbitrary resource servers. Its only valid use is as the root of an
AAT delegation chain presented to an enforcement point per Section 7.
Note: this specification defines token endpoint issuance for
interoperability with existing OAuth 2.0 deployments. Unlike bearer
tokens, an AAT carries its own holder key binding and is not usable
as a credential for HTTP resource access. Alternative issuance
profiles are outside the scope of this document.
The AS does not need to store or track derived tokens issued
downstream by the initial token holder. Chain verification is
performed by enforcement points using only the root token's public
key as a trust anchor.
4. Attenuation Invariants
Every derived token in a chain MUST satisfy all of the following
invariants. The verification algorithm in Section 7 enforces these
invariants; enforcement points MUST reject any chain that violates
any invariant.
4.1. Capability Lattice Model (Non-Normative)
The attenuation invariants in this section are instances of a single
abstract structure: a capability lattice. This subsection states
that structure informally to give readers a mental model for
interpreting the normative rules that follow.
For a token T, define its capability set C(T) as the set of (tool,
args) pairs that T authorizes (that is, the pairs for which T would
permit invocation). The core security property of this protocol is:
C(child) ⊆ C(parent)
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Every delegation step moves down or stays at the same position in
this partial order. A derived token can only authorize a subset of
what its parent authorized. It cannot add tools, loosen argument
constraints, or extend the chain's authority in any dimension.
The ⊆ relation is not defined by enumerating (tool, args) pairs
(argument spaces are typically infinite) but by the structural
subsumption rules in Section 4.5. At the tool level, the derived
token's tool set must be a subset of the parent's. At the argument
level, when the parent's constraint map is non-empty, the derived
token must preserve the parent's key set exactly (Section 4.5
explains why closed-world semantics require this).
When the parent's map is empty, the derived token may introduce keys,
transitioning from open-world to closed-world. No per-key parent
constraint exists in this case; the derived closed-world invocation
set is a subset of the parent's unrestricted invocation set. When a
parent constraint exists, a derived constraint c_child subsumes a
parent constraint c_parent (written c_child ⊑ c_parent) if every
argument value that satisfies c_child also satisfies c_parent.
Two boundary cases complete the structure. The empty capability set
∅ is the bottom element: a token with no tools authorized is a valid
token that cannot authorize tool invocations. If it is not the leaf,
it can only derive further empty-capability tokens. The root token's
capability set is the ceiling for the entire chain: no derived token
at any depth can exceed what the root authorized.
Token lifetime (I3) is a mandatory attenuation dimension orthogonal
to the capability lattice. A derived token with C(child) ==
C(parent) is still strictly more constrained if its exp is earlier
than its parent's. Time-to-live (TTL) bounds are enforced
independently of capability monotonicity. Both must hold for a chain
to be valid.
Invariants I1 through I6 are the normative enforcement mechanism for
this property. I4 (Section 4.5) directly enforces C(child) ⊆
C(parent). The remaining invariants enforce the conditions under
which that comparison is meaningful: that the chain is
cryptographically linked (I1, I5), that depth and time bounds are
respected (I2, I3), and that the presenter holds the key (I6).
4.2. I1: Delegation Authority
derived.iss == jwk_thumbprint_uri(parent.cnf.jwk)
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where jwk_thumbprint_uri constructs the [RFC9278] URI from the key's
SHA-256 thumbprint. The entity that signed the derived token MUST be
the holder of the parent token. Authority flows from parent holder
to derived token issuer. This invariant establishes a cryptographic
holder-key trail: each link in the chain was signed by the party that
held the preceding token. Attributing that key to a human, service,
organization, or control plane is a deployment responsibility.
4.3. I2: Depth Monotonicity
derived.del_depth == parent.del_depth + 1
derived.del_depth <= parent.del_max_depth
derived.del_depth <= derived.del_max_depth
derived.del_depth <= MAX_DELEGATION_DEPTH
derived.del_max_depth <= parent.del_max_depth
Delegation depth increments exactly by one at each link. A presented
chain is a single linear path: it cannot skip depths or contain the
same token instance more than once. Broader delegation activity may
form a graph across multiple derived tokens and chains, but each
invocation is verified against one ordered root-to-leaf path.
del_max_depth is an absolute ceiling, not a remaining count. A token
is terminal (its holder cannot derive further tokens) when del_depth
== del_max_depth. A root token with del_max_depth: 0 is therefore
immediately terminal and cannot produce any derived tokens.
The del_max_depth claim is an issuer-imposed bound on chain growth.
It limits resource exhaustion and bounds the number of offline trust
extensions that can occur under one root grant. Issuers use this
value to express the maximum delegation depth they are willing to
authorize for the grant. Intermediate token holders can only lower
del_max_depth, never raise it (I2), so the root issuer's depth bound
is enforced by chain verification across the entire chain.
Issuers SHOULD set del_max_depth to accommodate the expected
delegation topology, including subprocess delegation, operational
handoffs, and holder-key handoff. A value that is too low can
prevent downstream holders from expressing legitimate attenuation,
increasing pressure to reuse broader tokens directly. Once a chain
reaches del_max_depth, no descendant can extend it further; this
specification defines no in-chain mechanism for increasing that
ceiling.
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MAX_DELEGATION_DEPTH is an implementation-defined finite integer
specifying the maximum permitted delegation chain depth.
Implementations MUST enforce a finite maximum delegation depth to
prevent resource exhaustion from pathologically deep chains. The
appropriate value depends on the deployment topology; swarm
architectures with deep fan-out may require significantly larger
values than linear delegation chains. See Appendix B.4 for guidance.
The del_max_depth claim in any token in the chain MUST NOT exceed the
implementation's MAX_DELEGATION_DEPTH.
4.3.1. Implementation Resource Limits
MAX_TOKEN_SIZE is an implementation-defined finite integer specifying
the maximum encoded size of a single token in bytes. Implementations
MUST enforce this limit to prevent memory exhaustion from
pathologically large tokens. A value of 65536 bytes (64 KiB) is
RECOMMENDED.
MAX_STACK_SIZE is an implementation-defined finite integer specifying
the maximum total encoded size of a chain in bytes. Implementations
MUST enforce this limit. A value of 262144 bytes (256 KiB) is
RECOMMENDED.
4.4. I3: TTL Monotonicity
derived.exp <= parent.exp
derived.exp > now
derived.exp > derived.iat
derived.iat >= parent.iat
derived.iat <= now + MAX_IAT_SKEW
derived.exp <= derived.iat + MAX_TOKEN_LIFETIME
MAX_IAT_SKEW is an implementation-defined finite integer specifying
the maximum number of seconds a token's iat may be in the future
relative to the enforcement point's clock. Implementations MUST
enforce a finite MAX_IAT_SKEW. A value of 30 seconds is RECOMMENDED.
MAX_TOKEN_LIFETIME is an implementation-defined finite integer
specifying the maximum permitted duration in seconds between a
token's iat and exp. Implementations MUST enforce a finite
MAX_TOKEN_LIFETIME. A value of 90 days is RECOMMENDED as an upper
bound; deployments SHOULD use significantly shorter lifetimes in
practice (see Appendix B.7).
A derived token cannot outlive its parent. Authority cannot extend
beyond the lifetime of the token that granted it. A derived token's
issuance time MUST NOT precede its parent's issuance time. A token
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with an earlier iat indicates clock manipulation or chain forgery.
Tokens with iat more than MAX_IAT_SKEW in the future relative to the
enforcement point's clock MUST be rejected. A token's lifetime MUST
NOT exceed MAX_TOKEN_LIFETIME.
4.5. I4: Capability Monotonicity
tools(derived) ⊆ tools(parent)
∀ tool ∈ tools(derived):
constraints(derived, tool) ⊑ constraints(parent, tool)
A derived token MUST NOT authorize tools that the parent did not
authorize. For each tool that appears in both parent and derived
token:
* If the parent's constraint map for that tool is non-empty, the
derived token's constraint map MUST contain exactly the same set
of argument keys. Under closed-world semantics (Section 3.3), the
constraint map keys define the required invocation shape: any
argument not named is forbidden, and any named argument must be
present. Adding a key would produce invocations that the parent's
closed-world check rejects (the extra argument is unknown).
Dropping a key would produce invocations that omit a parent-
required argument. In both cases the derived invocation set is
disjoint from the parent's, not a subset.
* If the parent's constraint map is empty (open-world), the derived
token MAY introduce constraint keys, transitioning to closed-
world. Any closed-world constraint set is a subset of the
unrestricted open-world set.
For each argument constraint key present in both parent and derived
token, the derived constraint MUST be at least as restrictive as the
parent's constraint.
Constraint subsumption is defined per constraint type. The normative
rules are:
* *exact:* A derived exact constraint subsumes a parent constraint
of the same or different type as follows: it subsumes a parent
exact if the values are identical; it subsumes a parent range if
the exact value is a number that falls within the parent range; it
subsumes a parent one_of if the exact value is a member of the
parent set; it subsumes a parent wildcard unconditionally. All
other parent types are invalid cross-type targets for a derived
exact constraint.
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* *range:* A derived range constraint is valid only if its bounds
are at least as restrictive as the parent's (derived min >= parent
min, derived max <= parent max). A missing bound on the parent is
treated as unbounded; a missing bound on the derived constraint is
only valid if the parent bound is also missing. A derived bound's
inclusivity may only become more restrictive: a derived
min_inclusive: false is valid when the parent has min_inclusive:
true at the same min value (exclusive is strictly tighter), but
the reverse is not. The same applies to max_inclusive.
* *one_of:* A derived one_of constraint is valid only if its value
set is a subset of the parent's value set. Cross-type pairs
involving a derived not_one_of against a parent one_of are
invalid: a not_one_of constraint accepts values outside the
parent's permitted set and cannot be verified as subsuming a
one_of without domain knowledge. Enforcement points MUST reject
this cross-type pair.
* *not_one_of:* A derived not_one_of constraint is valid only if its
excluded set is a superset of the parent's excluded set (can only
add exclusions, never remove them).
* *wildcard:* A derived wildcard is valid only if the parent is also
wildcard. Any other constraint type subsumes a parent wildcard.
* *all:* A derived all constraint is valid attenuation of a parent
all if the derived constraint contains all clauses present in the
parent (none may be dropped) and each corresponding clause
satisfies the subsumption relation. The derived constraint MAY
add additional clauses at any position, which only further
restrict the accepted value set. Dropping any parent clause from
the derived all would expand authority and MUST be rejected.
Clause matching for all is subsumption-based: for each clause C_p
in the parent array, the enforcement point MUST find at least one
clause C_d in the derived array such that C_d subsumes C_p per
this section. Each parent clause MUST be matched to a distinct
derived clause (one-to-one assignment); a single derived clause
MUST NOT be used to satisfy more than one parent clause. If any
parent clause cannot be matched, the check MUST fail. Unmatched
additional clauses in the derived array are permitted.
The following pseudocode describes the matching algorithm.
Because a greedy match can lead to a dead end, the algorithm
backtracks until it finds a one-to-one assignment or exhausts the
search space.
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function check_all_subsumption(parent_clauses, derived_clauses):
used = set()
return match(parent_clauses, 0, derived_clauses, used)
function match(parents, idx, derived, used):
if idx == len(parents):
return PASS
C_p = parents[idx]
for i, C_d in enumerate(derived):
if i not in used and subsumes(C_d, C_p):
used.add(i)
if match(parents, idx + 1, derived, used) == PASS:
return PASS
used.remove(i) // backtrack
return FAIL
The search space is bounded by the number of parent and derived
clauses. Implementations MAY employ Hopcroft-Karp or similar
maximum matching algorithms for the general case.
* *any:* A derived any constraint subsumes a parent any constraint
if every clause in the derived constraint is subsumed by at least
one clause in the parent constraint, using the per-type
subsumption rules defined in this section. Formally: for each
clause_d in derived.any.constraints, there MUST exist a clause_p
in parent.any.constraints such that clause_d ⊑ clause_p. Removing
clauses is valid (it narrows the accepted set). Adding clauses is
invalid (it widens it). The derived any MUST contain at least one
clause. Cross-type subsumption between clauses is permitted: for
example, a derived clause of exact("pdf") is subsumed by a parent
clause of one_of(["pdf", "csv"]) under the cross-type rules in
this section.
Example: a parent token carries any([exact("pdf"), exact("csv"),
exact("xlsx")]). A derived token MAY carry any([exact("pdf"),
exact("csv")]) because each derived clause is subsumed by a parent
clause. A derived token MUST NOT carry any([exact("pdf"),
exact("docx")]) because exact("docx") is not subsumed by any
parent clause.
* *contains:* A derived contains constraint is valid attenuation of
a parent contains if the derived required set is a superset of the
parent's required set. Requiring additional elements is a
restriction; removing required elements would expand the set of
accepted argument arrays and MUST be rejected.
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* *subset:* A derived subset constraint is valid attenuation of a
parent subset if the derived allowed set is a subset of the
parent's allowed set. Shrinking the allowed set is a restriction;
adding allowed elements would expand the set of accepted argument
arrays and MUST be rejected.
Any (parent constraint type, derived constraint type) pair not
explicitly permitted by the above rules, or by a registered extension
constraint's cross-type subsumption declaration (Section 3.5.1), MUST
be rejected.
4.6. I5: Cryptographic Linkage
derived.par_hash ==
base64url-nopad(SHA-256(parent token signing input))
Token signatures and par_hash serve distinct security roles.
Signature verification authenticates each token under the
verification key selected for that token: a trust anchor for a root
token, or the parent token's cnf.jwk for a derived token. Delegation
authority (I1) then checks that the child issuer corresponds to the
parent holder key. However, these checks do not by themselves bind
the child to a unique parent token instance when the same holder key
has multiple compatible parent tokens. The par_hash claim provides
that token-instance binding by committing the child to exactly one
parent token's signing input.
Each derived token is cryptographically bound to its parent by
including the SHA-256 digest of the parent token's signing input in
the par_hash claim. For JWT/JWS AATs, the parent token signing input
is the JWS Signing Input: the ASCII string BASE64URL(JWS Protected
Header) || '.' || BASE64URL(JWS Payload) as defined in [RFC7515]
Section 5.1.
This binding prevents grant-context substitution: a child token
signed by a key that holds multiple compatible parent tokens cannot
be re-associated with a different parent task grant. The capability
set may still be attenuated, but the task/session lineage, revocation
ancestry, approval context, or policy snapshot would change.
4.7. I6: Proof of Possession
pop_signature verifies under leaf.cnf.jwk
The presenter of a token chain MUST demonstrate control of the
private key corresponding to the leaf token's cnf.jwk. Proof of
Possession is defined in Section 5.
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5. Proof of Possession
5.1. Rationale
A token without proof of possession can be replayed by any party that
obtains a copy of the token. In agent systems, tokens flow through
model context, tool invocation results, and inter-agent message
channels, all of which are observable by other components. PoP binds
a specific invocation to the private key of the leaf token's holder.
5.2. PoP Token Structure
The holder of the leaf token produces a PoP JWT for each tool
invocation. The PoP JWT is a compact serialization signed with the
holder's private key. It MUST contain the required claims listed
below.
+==========+=============+==========+===============================+
| Claim | Type | Required | Description |
+==========+=============+==========+===============================+
| jti | string | REQUIRED | Fresh random identifier. |
| | | | The holder MUST NOT reuse a |
| | | | jti value across PoP JWTs |
| | | | it produces. When a UUID |
| | | | is used, it MUST be encoded |
| | | | as a lowercase hyphenated |
| | | | string per [RFC9562]. |
| | | | Whether an enforcement |
| | | | point can detect reuse |
| | | | depends on whether stateful |
| | | | jti tracking is deployed |
| | | | (see Section 8.5). |
+----------+-------------+----------+-------------------------------+
| iat | NumericDate | REQUIRED | Time of PoP creation. MUST |
| | | | reflect the actual time of |
| | | | creation. Enforcement |
| | | | points validate this |
| | | | against a clock tolerance |
| | | | window (see Section 5.3). |
+----------+-------------+----------+-------------------------------+
| aat_id | string | REQUIRED | The jti of the leaf token |
| | | | being presented. |
+----------+-------------+----------+-------------------------------+
| aat_tool | string | REQUIRED | The tool identifier being |
| | | | invoked. MUST exactly |
| | | | match a key in the tools |
| | | | map of the leaf token's |
| | | | authorization_details. |
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| | | | Tool identifier matching |
| | | | follows the exact-string |
| | | | comparison rules in |
| | | | Section 3.3.1. |
+----------+-------------+----------+-------------------------------+
| aat_aud | string | OPTIONAL | Audience identifier for the |
| | | | enforcement point or |
| | | | resource accepting the PoP |
| | | | JWT. Deployments or |
| | | | profiles that require |
| | | | audience binding MUST |
| | | | require this claim and |
| | | | enforce audience match at |
| | | | verification time. |
+----------+-------------+----------+-------------------------------+
| hta | object | REQUIRED | The tool arguments for this |
| | | | invocation. Keys are |
| | | | argument names; values are |
| | | | argument values. |
+----------+-------------+----------+-------------------------------+
Table 4
The PoP JWT payload MUST be serialized as JCS-canonical JSON
([RFC8785]) before JWS signing. This is a whole-payload requirement,
not specific to the hta member. The JWS signing input is therefore
BASE64URL(JWS Protected Header) || '.' || BASE64URL(JCS(PoP claims)).
Whole-payload JCS canonicalization ensures a deterministic byte
representation; in particular, it gives hta stable equality semantics
so that argument map comparison is unambiguous across implementations
and languages regardless of JSON serialization choices.
The PoP JWT MUST be signed using the private key corresponding to the
leaf token's cnf.jwk. The enforcement point verifies the PoP JWT
signature against the leaf token's cnf.jwk.
{
"jti": "c980f2a1-4a37-4e88-bb3c-9defd37c1a45",
"iat": 1741600300,
"aat_id": "01957a41-0081-7c20-bf3a-00a0c91e1234",
"aat_tool": "read_file",
"aat_aud": "https://tools.example.com",
"hta": { "path": "/data/q3-report.pdf" }
}
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5.3. Verification
PoP verification is only meaningful against a leaf token whose chain
has been fully verified per Section 7. An enforcement point MUST
complete chain verification (Section 7, steps 1-6) before evaluating
the PoP JWT. A valid PoP JWT against an unverified or invalid chain
MUST NOT result in authorization.
The enforcement point MUST reject a PoP JWT that:
1. Has a signature that does not verify under the leaf token's
cnf.jwk.
2. References an aat_id that does not match the jti of the presented
leaf token.
3. When deployment policy requires PoP audience binding, omits
aat_aud or contains an aat_aud claim that does not identify the
enforcement point or resource accepting the invocation.
4. Names a tool in aat_tool that is not authorized by the leaf
token.
5. Presents arguments in hta that violate constraints in the leaf
token, per the verification algorithm in Section 7 (step 6b).
6. Has iat that is outside the enforcement point's accepted clock
tolerance window (RECOMMENDED: ±30 seconds).
The PoP JWT iat timestamp and clock tolerance window bound the replay
surface to a short interval. Implementations that wish to avoid
shared state MAY use fixed-width time buckets (for example, accepting
PoP JWTs whose iat falls within the current or immediately preceding
30-second bucket) to simplify enforcement point implementation.
Note: The time bucket approach is stateless but probabilistic: a PoP
JWT captured early in a bucket remains usable until the end of the
following bucket. This approach MUST NOT be used for tool
invocations that have side effects or are not idempotent. For any
tool invocation where duplicate execution causes unintended side
effects, stateful jti tracking MUST be used.
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Full replay prevention, which guarantees that a given PoP JWT is
accepted at most once, requires stateful tracking of presented jti
values across all enforcement points in a deployment. The mechanism
for that state (shared cache, database, token-binding infrastructure)
is deployment-specific and outside the scope of this specification.
Deployments with strong replay prevention requirements SHOULD consult
the security considerations in Section 8.5.
6. Token Derivation
A holder of any AAT whose del_depth is strictly less than
del_max_depth MAY derive a child token as follows.
1. Set jti to a fresh unique token identifier, RECOMMENDED to be a
UUIDv7 value per [RFC9562].
2. Set iat to the current time, but not earlier than parent.iat.
Set exp to any value <= parent.exp, subject to the constraints in
Section 4.4. Token lifetime is a mandatory attenuation
dimension. Every derived token is temporally bounded by its
parent regardless of capability scope. Expiration is the base
specification's built-in limit on token lifetime; see
Appendix B.7 for deployment guidance.
3. Select the set of tools to authorize. This set MUST be a subset
of the tools authorized by the parent token.
4. For each tool, construct a constraint map with the same argument
keys as the parent's constraint map for that tool (Section 4.5).
For each key, select a constraint that is at least as restrictive
as the parent's, per the subsumption rules in Section 4.5. If
the parent's constraint map is empty, the derived token MAY
introduce constraint keys.
5. Set del_depth to parent.del_depth + 1.
6. Set del_max_depth to any integer value greater than or equal to
child.del_depth and less than or equal to parent.del_max_depth.
Setting del_max_depth equal to child.del_depth produces a
terminal token that cannot be further delegated; higher values
permit further delegation up to the parent's ceiling. Both
bounds are inclusive; the upper bound enforces I2.
7. Set par_hash to base64url(SHA-256(parent token signing input)),
using base64url encoding without padding ([RFC7515] Appendix C).
For JWT/JWS AATs, the parent token signing input is the JWS
Signing Input.
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8. Set cnf.jwk to the intended holder's public key. The value MUST
be a public key; private key material MUST NOT appear in this
field.
9. Sign the token with the private key corresponding to the parent
token's cnf.jwk. The iss claim MUST be set to the JWK Thumbprint
URI [RFC9278] of that signing key, using the SHA-256 hash
algorithm.
Derivation is performed locally by the token holder. No
authorization server communication is required.
A derivation in which none of the authority dimensions is strictly
narrowed (the tool set is identical, all constraints are unchanged,
del_max_depth is unchanged, and exp is unchanged) is technically
valid by the invariants. Such a child has the same capability and
lifetime authority as its parent while consuming one delegation
depth. It does not improve least privilege, but deployments may use
it for holder-key handoff or subprocess delegation. Enforcement
points MAY log same-scope derivations as anomalous according to
deployment policy.
7. Chain Verification Algorithm
The enforcement point receives a chain of tokens ordered from root to
leaf and MUST execute the following algorithm. Any failure MUST
result in denial.
Verification requires only the token chain and the trust anchor
public key. No network calls or authorization server availability
are required. Chain verification itself is fully offline. Strong
replay protection for side-effecting tool invocations may
additionally require stateful jti tracking as described in
Section 8.5; that state is outside the inputs of this algorithm.
Inputs:
chain: ordered array of signed JWTs, [root, ..., leaf]
trust_anchors: set of public keys trusted as root issuers
tool: the tool being invoked
args: the arguments being passed to the tool
pop_jwt: the PoP JWT presented by the agent
Algorithm:
1. If chain is empty, DENY.
2. Verify chain size limits:
a. Verify the encoded size of each token does not exceed
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MAX_TOKEN_SIZE. If any token exceeds this limit, DENY.
b. Verify the total encoded size of the chain does not exceed
MAX_STACK_SIZE. If the chain exceeds this limit, DENY.
c. For each token, decode the base64url payload segment and
extract only the `jti` field using minimal JSON parsing.
If the payload is not valid JSON or does not contain a
string-valued `jti` field, DENY. Collect all extracted
`jti` values; if any value appears more than once in the
presented chain, DENY (token-instance cycle detection). This check
does not forbid the same actor, holder key, or organizational
component from appearing more than once in a delegation graph,
provided each occurrence is represented by a distinct token and
every adjacent link in the presented chain satisfies this
algorithm. This limited extraction
prior to signature verification is permitted and required
for this structural check; it does not constitute the
application-layer claim deserialization prohibited by the
post-algorithm note. The extracted `jti` values MUST be
treated as untrusted until each token's signature is
verified. Full claim parsing MUST still be deferred until
after signature verification succeeds for each token.
3. Verify root token:
a. Verify the root token's JWS alg header is on the
implementation's permitted algorithm allowlist and is
consistent with the verifying trust anchor key's kty and
crv parameters. If alg is "none", not on the allowlist,
or inconsistent with the key type, DENY. (Sec 8.13)
b. Verify the root token signature against a key in
trust_anchors. After signature verification succeeds,
parse the root token's claims. All subsequent root
checks (3c through 3n) operate on parsed claims.
c. Verify root.del_depth == 0.
d. Verify root.par_hash is absent.
e. Verify root.exp > now.
f. Verify root.iat <= now + MAX_IAT_SKEW.
g. Verify root.exp > root.iat.
h. Verify root.exp <= root.iat + MAX_TOKEN_LIFETIME.
i. Verify root.del_max_depth is a non-negative integer not
exceeding MAX_DELEGATION_DEPTH. If absent or invalid, DENY.
j. Verify root.jti is present and is a non-empty string.
If absent or not a string, DENY.
k. Verify root.iss is present and is a URI. If absent or
not a URI-formatted string, DENY.
l. Verify root.cnf is present, contains a `jwk` member, and
that the `jwk` encodes a public key (MUST NOT contain a
private key parameter such as `d` for EC/OKP keys or
`p`, `q` for RSA keys). If absent or invalid, DENY.
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m. Verify root.authorization_details is present and is a
non-empty array containing exactly one entry with type
"attenuating_agent_token". If absent, empty, or if the
number of such entries is not exactly one, DENY.
Note: for a single-token chain (root = leaf), step 4 has
no adjacent parent-child pair to evaluate. Validation is
therefore performed by step 3 (root checks), step 5
(chain-length consistency), step 6 (leaf
capability/constraint checks), and step 7 (PoP), before
permit in step 8.
Steps 3j through 3m ensure that required claims are
present before step 6 depends on them, closing the
bypass window that exists when step 4 does not run.
n. For each constraint in each constraint map in the root
token's attenuating_agent_token entry, verify the
constraint tree depth does not exceed MAX_CONSTRAINT_DEPTH.
If any constraint tree exceeds this limit, DENY.
4. For each adjacent pair (parent, child) in chain:
a. Verify child token's JWS alg header is on the
implementation's permitted algorithm allowlist and is
consistent with parent.cnf.jwk's kty and crv parameters.
If alg is "none", not on the allowlist, or inconsistent
with the key type, DENY. (Sec 8.13)
b. Verify child signature under the key in parent.cnf.jwk. (I1)
After signature verification, verify required claims are
present:
b1. Verify child.jti is present and is a non-empty
string. If absent or not a string, DENY.
b2. Verify child.cnf is present, contains a `jwk`
member, and that the `jwk` encodes a public key
(MUST NOT contain a private key parameter such as
`d` for EC/OKP keys or `p`, `q` for RSA keys). If
absent or invalid, DENY.
b3. Verify child.authorization_details is present and
is an array. If absent or not an array, DENY.
b4. Verify child.del_depth and child.del_max_depth are
both present and are non-negative integers. If
absent or not integers, DENY.
b5. Verify child.iss, child.iat, child.exp, and
child.par_hash are all present. If any is absent, DENY.
c. Verify child.iss equals jwk_thumbprint_uri(parent.cnf.jwk). (I1)
d. Verify child.del_depth == parent.del_depth + 1. (I2)
e. Verify child.del_depth <= parent.del_max_depth. (I2)
f. Verify child.del_depth <= MAX_DELEGATION_DEPTH. (I2)
g. Verify child.del_max_depth <= parent.del_max_depth.(I2)
Note: the requirement that every token's
del_max_depth <= MAX_DELEGATION_DEPTH is transitively
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satisfied: step 3i verifies this for the root, and
step 4g at each link ensures the value can only
decrease. Implementations MAY add this check
explicitly as defense in depth.
h. Verify child.exp <= parent.exp. (I3)
i. Verify child.exp > now. (I3)
j. Verify child.iat >= parent.iat. (I3)
k. Verify child.iat <= now + MAX_IAT_SKEW. (I3)
l. Verify child.exp > child.iat. (I3)
Note: the requirement child.exp <= child.iat +
MAX_TOKEN_LIFETIME is transitively satisfied: by
induction, child.exp <= root.exp (step 4h at each
link), root.exp <= root.iat + MAX_TOKEN_LIFETIME
(step 3h), and child.iat >= root.iat (step 4j at
each link), therefore child.exp <= root.iat +
MAX_TOKEN_LIFETIME <= child.iat + MAX_TOKEN_LIFETIME.
Implementations MAY add this check explicitly as
defense in depth.
m. Verify child.del_depth <= child.del_max_depth. (I2)
n. Verify child.authorization_details contains at most
one entry with type "attenuating_agent_token". If
more than one such entry is present, DENY. Note: zero
entries of this type are permitted at this step and
represent an empty capability set. Step 4p will verify
this is a valid attenuation of the parent (an empty tool
set is always a subset). If the child is the leaf token,
step 6a will reject zero entries.
For the remaining checks in this adjacent-pair step, define
child_aat as the child entry with type
"attenuating_agent_token" if present, or as an empty capability
entry with an empty `tools` map if absent. Define parent_aat
the same way for the parent token: the parent entry with type
"attenuating_agent_token" if present, or an empty capability
entry with an empty `tools` map if absent. Root validation
(step 3m) ensures the root parent has such an entry; non-root
parents with zero entries represent the empty capability set.
Entries of other types in `authorization_details` are ignored
by this algorithm.
o. For each constraint in each constraint map in child_aat.tools,
verify the constraint tree depth does not exceed
MAX_CONSTRAINT_DEPTH. If any constraint tree exceeds
this limit, DENY.
p. Verify capability monotonicity (Section 4.5): (I4)
p1. Verify every tool in child_aat.tools
is also present in parent_aat.tools.
If any child tool is absent from the parent, DENY.
p2. For each tool present in both parent_aat.tools and
child_aat.tools: if the parent's constraint map is
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non-empty, verify the child's constraint map contains
exactly the same set of argument keys. If any key is
added or removed, DENY.
p3. For each tool present in both parent_aat.tools and
child_aat.tools: if the parent's constraint map is empty,
the child's constraint map MAY contain any set of keys.
p4. For each argument key present in both constraint maps
for a matched tool, verify the child's constraint subsumes
the parent's per the per-type rules in Section 4.5. If
any constraint fails subsumption, DENY.
q. Verify child.par_hash equals base64url-nopad( (I5)
SHA-256(parent token signing input)), where
base64url-nopad denotes base64url encoding without
padding as described in JWS Appendix C. For JWT/JWS AATs,
the parent token signing input is the JWS Signing Input.
5. (Defense in depth) Verify len(chain) equals
leaf.del_depth + 1. A mismatch indicates a malformed
or incorrectly assembled chain.
6. Verify leaf token:
a. Verify leaf.authorization_details contains exactly one
entry with type "attenuating_agent_token". If zero or
more than one such entry is present, DENY.
Define leaf_aat as that entry. Entries of other types in
`authorization_details` are ignored by this algorithm.
b. Verify tool is present in leaf_aat.tools. Then, for each argument
in args: if the tool's constraint map is non-empty and
the argument name is not present in the constraint map,
DENY (closed-world mode). For each argument name present
in the constraint map, if that argument is absent from
args, DENY (constrained argument MUST be present). For
each argument name present in both the constraint map
and args, verify the argument value satisfies the
constraint. If any constraint check fails, DENY.
7. Verify PoP JWT:
a. Verify the PoP JWT's JWS alg header is on the
implementation's permitted algorithm allowlist and is
consistent with leaf.cnf.jwk's kty and crv parameters.
If alg is "none", not on the allowlist, or inconsistent
with the key type, DENY. (Sec 8.13)
b. Verify pop_jwt signature under leaf.cnf.jwk. After
signature verification succeeds, parse the PoP JWT claims. (I6)
c. Verify pop_jwt.aat_id == leaf.jti.
d. If deployment policy requires PoP audience binding, verify
pop_jwt.aat_aud identifies this enforcement point or resource
context. If absent or mismatched, DENY.
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e. Verify pop_jwt.aat_tool equals tool using the exact-string
matching rules in Section 3.3.1.
f. Verify pop_jwt.hta, when JCS-canonicalized, equals the
JCS-canonical form of the args map for this invocation. If the
canonical byte sequences differ, DENY.
g. Verify pop_jwt.iat is within the clock tolerance
window. If outside the window, DENY.
8. PERMIT.
Enforcement points MUST verify the JWS signature of each token before
deserializing its payload claims into application-layer data
structures. Signature verification operates on the raw encoded
header and payload bytes (the JWS Signing Input) and does not require
claim parsing. Full claim parsing MUST NOT occur until after
signature verification succeeds for that token. This ordering
prevents parser-based denial-of-service attacks on maliciously
crafted payloads. The sole exception is step 2c: extracting only the
jti string field for cycle detection prior to signature verification
is permitted, provided the implementation treats the extracted value
as untrusted until the corresponding signature is verified.
Enforcement points MUST reject any token whose JWS alg header is
"none". The "none" algorithm provides no cryptographic protection
and MUST NOT be used in any AAT or PoP JWT.
The hta comparison in step 7f requires both the enforcement point and
the holder producing the PoP JWT to use JCS canonicalization
([RFC8785]). The enforcement point MUST canonicalize the args map
independently and compare the resulting byte sequence against the
canonical form committed to by the PoP JWT signature.
Implementations MUST NOT compare raw JSON strings; surface
differences such as key ordering or numeric representation (e.g., 1.0
vs 1) are resolved by canonicalization before comparison.
The JWS alg header value MUST be consistent with the key type of the
key used to verify the signature: the trust anchor public key for
root tokens, and the cnf.jwk of the parent token for derived tokens.
Enforcement points MUST reject any token where the declared alg is
not compatible with the verifying key's kty and crv parameters. For
example, a token whose alg is "EdDSA" MUST be verified against an OKP
key with "crv": "Ed25519" or "crv": "Ed448". A mismatch between the
declared algorithm and the verifying key type MUST result in denial,
regardless of whether the signature bytes would verify under an
alternate interpretation.
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8. Security Considerations
8.1. Threat Model
This section characterizes the threats that AATs mitigate and the
threats that are outside the scope of this mechanism.
Implementations SHOULD use this characterization to identify required
complementary controls for their threat environment.
8.1.1. Threats Mitigated
*Prompt injection leading to unauthorized tool invocation.* An
attacker who injects instructions into an agent's input cannot cause
the agent to invoke tools outside the scope encoded in its token.
The enforcement point rejects any invocation of an unauthorized tool
regardless of the agent's stated rationale.
*Hallucinated tool invocations with out-of-scope arguments.* Even
when an agent invokes an authorized tool, argument constraints in the
leaf token restrict the argument values the enforcement point will
accept. An agent that hallucinates an argument value outside the
authorized range is denied at the enforcement point before the tool
executes.
*Confused deputy attacks.* In the classic form, a deputy is induced
to use its own authority on a resource designated by another party.
In agentic systems, that designation can come from an invoking
principal, prompt injection, tool output, or model error. AATs avoid
relying on the standing authority the classic form exploits: an agent
acts under a token presented for the current invocation. When the
invoker derives that token to designate the task's resource,
designation and authority travel together, and the agent cannot be
steered outside the authority carried by the token. A token
authorizing more than one resource can still be steered within its
scope, so issuers SHOULD scope leaf tokens as narrowly as the task
permits. The delegation chain verifies provenance and attenuation,
and the enforcement point checks the presented invocation against the
leaf token's constraints. How a constraint value maps to the
resource the tool ultimately acts upon is defined by the tool
contract and implemented by the tool: the protocol authorizes the
presented invocation, and the tool remains responsible for resolving
that invocation to the correct resource.
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*Privilege escalation across delegation hops.* The capability
monotonicity invariant (I4) ensures that authority can only narrow at
each delegation step. A derived token cannot authorize tools or
argument values absent from its parent token. An agent that attempts
to mint a derived token with broader scope will produce a token that
fails chain verification at the enforcement point.
*Compromised sub-agents.* If a sub-agent is compromised, the blast
radius is bounded by the scope of the token it holds. The attacker
cannot use the compromised agent to escalate to broader authority,
invoke tools outside the token's scope, or derive tokens with wider
permissions than the compromised token encodes.
*Grant-context substitution.* The par_hash claim (I5) binds each
derived token to the specific bytes of its parent token. Suppose a
delegator key holds two parent tokens, A and B, issued for different
tasks but authorizing compatible capabilities. The holder derives
child token C from A. Without par_hash, a presenter could assemble
the chain (B, C). The link may satisfy delegation authority, depth,
lifetime, and capability monotonicity: C is signed by the key named
in B.cnf.jwk, has the expected depth, does not outlive B, and
authorizes no capability outside B. However, the chain has been re-
associated with task B rather than task A. The par_hash check
rejects this because C commits to the signing input of A, not B.
*Token replay for irreversible operations.* For irreversible or side-
effecting tool invocations, stateful jti tracking at the enforcement
point enables prevention of PoP JWT replay. See Section 8.5 for the
distinction between stateful and probabilistic replay controls and
the deployment requirements for each.
8.1.2. Threats Not Mitigated
*Malicious or compromised root issuer.* The security of all chains
depends on the integrity of the trust anchor key. A root issuer that
mints tokens with overly broad scopes, or whose signing key is
compromised, undermines the authorization guarantees of every chain
it anchors. AATs provide no mechanism to detect or constrain a
malicious root issuer. Key management, rotation procedures, and root
issuer accountability are deployment concerns outside the scope of
this specification.
*Compromised enforcement point.* An enforcement point that skips
chain verification, ignores constraint evaluation, or accepts forged
tokens provides no security guarantee regardless of the token format.
AATs assume enforcement points are honest and implement the
verification algorithm in Section 7 correctly. Enforcement point
integrity is a deployment concern.
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*Actions within authorized argument constraints.* AATs restrict which
tools an agent may invoke and what argument values are permitted.
They do not restrict which authorized invocations an agent chooses to
make, in what order, or how many times. An agent that makes
excessive or unintended use of its authorized tools within the bounds
of its token is not detectable at the enforcement point. Rate
limiting, audit logging, and behavioral monitoring are complementary
controls for this threat.
*Compromised holder key.* If an agent's private key is stolen, the
attacker can exercise the full authority encoded in that agent's
token until the token expires. The blast radius is bounded by the
token scope, but within that scope the attacker has full
authorization. Short token lifetimes (Appendix B.7) limit the
exposure window.
*Model exfiltration and side-channel attacks.* An attacker who
extracts an agent's model weights, system prompt, or in-context state
may be able to predict or manipulate the agent's behavior
independently of its token constraints. AATs operate at the
authorization layer and have no visibility into the model layer.
8.2. Attenuation as the Security Invariant
The capability-containment guarantee of this specification rests on
the enforcement of the capability monotonicity invariant (I4). An
enforcement point that fails to check I4, or that checks it
incorrectly, provides no blast radius containment. The broader chain
security properties also depend on the remaining invariants:
delegation authority (I1), depth bounds (I2), lifetime bounds (I3),
parent-token linkage (I5), and proof of possession (I6).
Implementers MUST test I4 enforcement against the full constraint
attenuation matrix in Section 4.5, including all (parent type, child
type) pairs, and MUST reject all pairs not explicitly permitted.
Those other invariants rely on well-established cryptographic
primitives and validation patterns with substantial prior art in
deployed systems. I4 is novel. Formal verification of the I4
subsumption rules is in progress, using bounded model checking
([ALLOY]) for set-theoretic constraint types and SMT solving ([Z3])
for numeric and structural constraint types. Implementers are
encouraged to publish independent analyses of both the core
subsumption rules and any extension constraint types they deploy.
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The Tenuo reference implementation includes a test suite covering
monotonicity of the attenuation invariants under arbitrary sequences,
normalization idempotence across encode/decode round-trips, and
enforcement agreement between in-memory and deserialized constraint
evaluation. See Appendix E for implementation status.
8.3. Root Key Compromise
A compromised trust anchor key allows an attacker to issue arbitrary
root tokens. This breaks the security guarantees of all chains
anchored to that key.
In the base chain verification algorithm, configured trust anchors
are used to verify root tokens. Establishing, rotating, or revoking
those trust anchors is outside the scope of this specification.
Remote attestation mechanisms, such as the RATS architecture
[RFC9334], can complement AAT deployments by providing evidence about
root issuer or enforcement point environments.
Deployments SHOULD implement key rotation procedures and revocation
mechanisms appropriate to their risk model. The specific mechanism
for root key revocation, including revocation list formats,
distribution protocols, and enforcement point update procedures, is
outside the scope of this specification.
8.4. Holder Key Compromise
A compromised holder key allows an attacker to present existing
tokens issued to that holder. The attacker cannot derive tokens with
broader scope than the compromised token grants. Mitigation is
revocation of tokens bound to the compromised key, or expiry-based
recovery for short-lived tokens.
8.5. Replay Attacks
The PoP JWT binds a specific invocation to a fresh PoP jti, a
timestamp, the target tool, the presented arguments, and, when
required by deployment policy, the enforcement point or resource
audience. The timestamp window limits the interval during which a
captured PoP JWT remains usable to approximately twice the clock
tolerance (RECOMMENDED: ±30 seconds, giving a window of roughly 60
seconds). This provides probabilistic replay resistance and is
appropriate only for idempotent, read-only tool invocations where
duplicate execution is harmless.
For tool invocations that are irreversible or have significant side
effects, including financial transactions, data deletion, writes to
external systems, and any operation that cannot be undone:
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enforcement points MUST implement stateful jti tracking for PoP JWTs
and MUST NOT rely solely on the timestamp window for replay
protection.
PoP JWTs are scoped to the invocation data they contain. Deployments
with multiple enforcement points, resource servers, tenants, or
resource contexts that could accept the same AAT chain SHOULD require
the aat_aud claim and reject PoP JWTs whose audience does not
identify the accepting enforcement point or resource. Without
audience binding, a PoP JWT captured at one enforcement point may be
replayable at another enforcement point that accepts the same chain,
tool name, and argument map within the timestamp window, unless
stateful jti tracking is shared across those contexts.
This specification requires stateful jti tracking for irreversible
operations but does not define the storage backend, consistency
model, or distribution protocol for that state. The required
consistency properties depend on the deployment topology and the risk
tolerance of the application. Deployments SHOULD treat the time-
windowed PoP as a probabilistic control and layer additional
idempotency mechanisms at the application level for high-value
operations.
8.6. Constraint Evaluation
The core constraint types are intended to have predictable evaluation
cost. Extension constraint types can introduce parser complexity,
algorithmic cost, normalization requirements, or external policy-
engine dependencies. Extension constraint types defined in
Section 3.5 and registered in the IANA AAT Constraint Type Registry
(Section 10.3) MUST document their computational complexity and any
resource limits implementations SHOULD enforce. Enforcement points
SHOULD impose evaluation timeouts on any extension constraint type
whose check predicate is not O(n) in the length of the argument
value.
8.7. Depth Limit
Enforcement points MUST enforce a finite MAX_DELEGATION_DEPTH to
prevent resource exhaustion from artificially deep chains. The
appropriate value is deployment-specific: linear orchestration chains
require far fewer hops than swarm architectures with deep fan-out
delegation. Implementations SHOULD choose a value that reflects the
maximum chain depth their deployment topology requires, without
imposing an artificial ceiling on legitimate use cases. See
Appendix B.4 for guidance on selecting an appropriate value.
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The security rationale for depth limiting goes beyond resource
exhaustion. Each delegation hop introduces an additional agent into
the trust chain: the enforcement point necessarily trusts not only
that the leaf token holder is honest, but that every intermediate
holder made sound attenuation decisions. A compromised or
misdirected intermediate agent can narrow constraints in ways that
serve an attacker's goals while remaining within the invariants. The
depth limit bounds the number of such trust extensions that a single
root grant can produce.
The del_max_depth claim in the root token is the root issuer's
explicit policy on chain topology. An implementation that ignores
del_max_depth or enforces only a global implementation limit without
checking per-token values violates this policy. Enforcement points
MUST check the per-token depth ceilings (child.del_depth <=
parent.del_max_depth in step 4e, child.del_max_depth <=
parent.del_max_depth in step 4g, and child.del_depth <=
child.del_max_depth in step 4m of Section 7) and the global
MAX_DELEGATION_DEPTH limit (step 4f of Section 7). Neither the per-
token policy checks nor the global implementation limit is sufficient
alone.
8.8. Unknown Constraint Types
Enforcement points MUST deny authorization when they encounter an
unknown constraint type. Permitting invocation in the presence of an
unrecognized constraint would silently remove a restriction the
issuer intended to enforce.
8.9. Token Revocation
Revocation of individual AATs, including derived tokens, is outside
the scope of this specification. The offline delegation model trades
per-token revocation granularity for verifiability without
authorization server availability. This tradeoff is inherent in the
verification model.
Deployments SHOULD use short token lifetimes to bound exposure after
key compromise, token theft, or scope misconfiguration. A short-
lived leaf token provides a bounded damage window even when no
revocation mechanism is deployed. Root tokens SHOULD be issued with
the shortest lifetime compatible with the intended delegation chain
depth.
Root trust anchor rotation (replacing the trust anchor signing key
and re-issuing root tokens) is the appropriate response to a root key
compromise. Enforcement points SHOULD support configurable trust
anchor sets to enable rotation without downtime.
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A companion document may define lineage-scoped cascading revocation.
In such a model, revocation is enforced by the enforcement point that
accepts the affected chain, not by requiring the root AS to track
derived tokens. Revoking a token invalidates that token and its
descendants in the same lineage, but does not invalidate unrelated
tokens or independent delegations held by the same agent, subject, or
holder key. Revocation transport, storage, distribution,
consistency, token-status, and introspection mechanisms are
deployment and control-plane concerns outside the scope of this
document.
8.10. Approval Gates
Deployments may require signed approvals before accepting particular
tool invocations. Such approvals are outside the base chain
verification algorithm unless defined by a profile or extension. A
profile that defines approval gates should specify how approval
requirements are encoded, how they are preserved or attenuated during
derivation, what request data an approval signs, how approval
freshness is checked, and which approval identities or keys are
trusted, including any threshold or quorum requirements.
8.11. Clock Skew
This specification uses clock-based checks in two distinct contexts
with different semantics. MAX_IAT_SKEW (Section 4.4, RECOMMENDED: 30
seconds) is a one-sided future-dating tolerance applied to token iat
values: it prevents a token issued slightly in the future from being
rejected due to minor clock drift between issuer and enforcement
point. The PoP JWT timestamp window (Section 5.3, RECOMMENDED: ±30
seconds) is a bilateral replay window applied to PoP JWT iat values:
it bounds how long a captured PoP JWT remains usable. These are
independent parameters enforced at different points in the
verification algorithm and SHOULD be configured separately.
PoP JWT timestamp verification requires synchronized clocks. The
RECOMMENDED tolerance window is ±30 seconds, which accommodates
typical Network Time Protocol (NTP) synchronized deployments with
generous margin. Deployments running on cloud infrastructure with
guaranteed NTP synchronization SHOULD target ±5 to ±10 seconds.
Deployments with stricter security requirements MAY reduce this
window further.
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Implementations MUST enforce a finite maximum tolerance window.
Values beyond ±60 seconds provide negligible additional clock skew
tolerance while meaningfully expanding the PoP replay window and are
NOT RECOMMENDED. A value of ±30 seconds is the conservative
baseline; the ±60 second ceiling is intended only for heterogeneous
environments such as embedded systems or degraded connectivity
scenarios.
8.12. Role-Based Key Separation
Deployments that distinguish planning agents from tool-invoking
agents SHOULD use distinct holder keys for those runtime roles and
SHOULD derive across that boundary with a fresh cnf.jwk. This limits
the blast radius of a compromised planning component and preserves
operational accountability between components that decide what work
should be done and components that invoke tools.
Role-based key separation is deployment guidance, not a base protocol
invariant. Enforcement points implementing this specification verify
the holder-key chain, attenuation invariants, parent-token linkage,
and leaf PoP proof; they do not infer agent runtime roles from token
claims unless a deployment-specific profile defines such claims and
verification rules.
8.13. Algorithm Confusion
JWT/JWS AATs are signed JWTs. Implementations are subject to the
full class of JWT algorithm confusion attacks, including alg: "none"
acceptance, symmetric/asymmetric key confusion (RS256/HS256 key
reuse), and algorithm substitution across tokens in the same chain.
Enforcement points MUST maintain an explicit allowlist of permitted
signature algorithms and MUST reject any token whose alg header value
is not on that list. Implementations MUST reject tokens with alg:
"none" unconditionally and MUST NOT treat the absence of an alg
header as equivalent to any permitted algorithm.
Implementations MUST apply the algorithm allowlist independently to
each AAT in the chain and to the PoP JWT. Accepting a weaker
algorithm on an intermediate token because the leaf token used a
strong algorithm is a verification failure.
The RECOMMENDED algorithm set is the same as for DPoP [RFC9449]:
ES256, ES384, ES512, RS256, RS384, RS512, PS256, PS384, PS512, EdDSA.
Symmetric algorithms (HS256, HS384, HS512) MUST NOT be used for AAT
signatures; symmetric keys cannot provide the per-holder key binding
that PoP requires.
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9. Privacy Considerations
AAT payloads are integrity-protected but not encrypted. In cross-
domain deployments, an AAT chain can reveal delegation topology, task
context, tool identifiers, argument constraints, and holder-key
correlation information. Deployments SHOULD minimize disclosure of
AAT chains to parties that do not perform chain verification or
invocation authorization. Deployments SHOULD transmit AAT chains
over encrypted transport (e.g., TLS) and SHOULD protect stored tokens
as sensitive authorization metadata. A stored AAT is not usable
without the corresponding holder private key, but it can disclose
authorization scope and delegation structure. Token encryption is
outside the scope of this specification.
10. IANA Considerations
10.1. JWT Claims Registry
This document requests registration of the following claims in the
IANA JSON Web Token Claims Registry [RFC7519].
*AAT claims:*
+===============+====================+============+===========+
| Claim Name | Claim Description | Change | Reference |
| | | Controller | |
+===============+====================+============+===========+
| del_depth | Delegation chain | IETF | This |
| | depth | | document |
+---------------+--------------------+------------+-----------+
| del_max_depth | Maximum delegation | IETF | This |
| | chain depth | | document |
+---------------+--------------------+------------+-----------+
| par_hash | Parent token | IETF | This |
| | signing input hash | | document |
+---------------+--------------------+------------+-----------+
Table 5
The tools map is not a top-level JWT claim; it is a member nested
inside the authorization_details array entry with type:
"attenuating_agent_token", as defined in Section 3.3. Its structure
and semantics are governed by the AAT Constraint Type Registry
(Section 10.3) and the RAR profile defined in this document, not by
the JWT Claims Registry.
*PoP JWT claims:*
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+==========+===============================+============+===========+
| Claim | Claim Description | Change | Reference |
| Name | | Controller | |
+==========+===============================+============+===========+
| aat_id | AAT jti being | IETF | This |
| | presented | | document |
+----------+-------------------------------+------------+-----------+
| aat_tool | Tool identifier for | IETF | This |
| | PoP binding | | document |
+----------+-------------------------------+------------+-----------+
| aat_aud | Enforcement point or | IETF | This |
| | resource audience | | document |
| | for PoP binding | | |
+----------+-------------------------------+------------+-----------+
| hta | Tool arguments for | IETF | This |
| | PoP binding | | document |
+----------+-------------------------------+------------+-----------+
Table 6
10.2. OAuth Authorization Details Types Registry
This document requests registration of the following type in the IANA
OAuth Authorization Details Types Registry established by [RFC9396].
+=========================+===============+
| Type Name | Reference |
+=========================+===============+
| attenuating_agent_token | This document |
+-------------------------+---------------+
Table 7
10.3. AAT Constraint Type Registry
This document requests IANA create the "Attenuating Authorization
Token Constraint Types" registry. The registration policy for this
registry is Specification Required [RFC8126].
10.3.1. Designated Expert Instructions
Designated experts MUST verify that each submitted registration
satisfies all of the following criteria before approving it:
1. The type name is a lowercase string containing only letters,
digits, and underscores, and does not conflict with an existing
registered type name.
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2. The check predicate is fully specified: given any argument value,
an independent implementer can determine without ambiguity
whether the predicate returns true or false.
3. The subsumes verification procedure satisfies the decidable,
sound, and deterministic properties defined in Section 3.5.1. If
the constraint language does not support a general containment
algorithm, the registration prescribes a conservative syntactic
strategy and formally justifies its soundness.
4. The cross-type subsumption rules enumerate every (parent type,
child type) pair involving both the new type and all existing
core types that the registration declares valid, with explicit
conditions. Unlisted pairs are implicitly invalid; the
registration MUST NOT rely on the catch-all rejection rule to
handle pairs that deserve explicit treatment.
5. The reference is a stable, publicly accessible specification
suitable for interoperable implementation.
Designated experts SHOULD request clarification when cross-type rules
are incomplete, when the subsumption procedure's soundness is not
formally justified, or when the check predicate leaves ambiguous
cases unresolved.
10.3.2. Registration Template
Registration requests MUST use the following template:
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Type name:
(A lowercase string. Example: "path_containment")
Additional members:
(List each JSON member name, its JSON type, whether it is required
or optional, its default value if optional, and its semantics.
Example: "root (string, required): An absolute path root.")
check predicate:
(A complete, unambiguous specification of the boolean predicate
evaluated against an argument value at invocation time. Must
cover all edge cases including null, empty, and out-of-range
inputs.)
subsumes verification procedure:
(A complete formal definition of what it means for one instance
of this constraint type to be at least as restrictive as another.
Must state whether the procedure is conservative and, if so, which
semantically subsuming pairs it rejects. Must include a soundness
argument: if the procedure returns true for (C_parent, C_child),
then for all values v: C_child.check(v) implies C_parent.check(v).)
cross-type subsumption rules:
(An explicit enumeration of every (parent type, child type) pair
involving this type that is a valid attenuation, and the conditions
under which it is valid. List both directions: this type as parent
and this type as child. All unlisted pairs are implicitly invalid.
Example:
- (exact, this_type): valid if the exact value satisfies this
type's check predicate.
- (this_type, exact): invalid.
- (this_type, this_type): valid if [condition].)
security considerations:
(Any security properties, limitations, or attack surfaces specific
to this constraint type, including known cases where the check
predicate or subsumption procedure can be bypassed or confused.)
reference:
(A stable, publicly accessible document defining all of the above.)
10.3.3. Initial Registry Entries
The core constraint types defined in Section 3.4 of this document
constitute the initial registry entries. For each type, the check
predicate and additional members are defined in Section 3.4, and the
subsumption rules and cross-type pairs are defined in Section 4.5.
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+============+===================================+
| Type Name | Reference |
+============+===================================+
| exact | This document (Sections 3.4, 4.5) |
+------------+-----------------------------------+
| range | This document (Sections 3.4, 4.5) |
+------------+-----------------------------------+
| one_of | This document (Sections 3.4, 4.5) |
+------------+-----------------------------------+
| not_one_of | This document (Sections 3.4, 4.5) |
+------------+-----------------------------------+
| contains | This document (Sections 3.4, 4.5) |
+------------+-----------------------------------+
| subset | This document (Sections 3.4, 4.5) |
+------------+-----------------------------------+
| wildcard | This document (Sections 3.4, 4.5) |
+------------+-----------------------------------+
| all | This document (Sections 3.4, 4.5) |
+------------+-----------------------------------+
| any | This document (Sections 3.4, 4.5) |
+------------+-----------------------------------+
Table 8
10.4. OAuth Authorization Server Metadata Registry
This document requests registration of the following parameter in the
IANA OAuth Authorization Server Metadata registry established by
[RFC8414].
+====================+====================+============+===========+
| Metadata Parameter | Metadata | Change | Reference |
| | Description | Controller | |
+====================+====================+============+===========+
| aat_issuer | Indicates root AAT | IETF | This |
| | issuance support | | document |
+--------------------+--------------------+------------+-----------+
Table 9
aat_issuer is a boolean value. When present and true, it indicates
that the root issuer supports issuance of AAT root tokens as
described in Section 3.7. When absent, the AS is assumed not to
support AAT issuance.
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10.5. OAuth Token Type Registration
This document requests registration of the following token type in
the OAuth Token Type Registry ([RFC6749] Section 11.1):
* Type name: aat
* Additional Token Endpoint Response Parameters: (none)
* HTTP Authentication Scheme(s): (none; not a bearer token)
* Change controller: IETF
* Specification document(s): This document
10.6. OAuth Parameters Registry
This document makes no request to the OAuth Parameters Registry.
Root token issuance uses the existing req_cnf token request
parameter.
11. Acknowledgments
The author thanks Alan Karp for detailed review and discussion of
capability-system semantics, confused deputy framing, delegation
depth, revocation, and the relationship between AATs and prior
capability systems.
The author thanks Antoine Fressancourt for review and discussion of
cross-domain privacy, transport binding, remote attestation, and
constraint expressiveness.
12. References
12.1. Normative References
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
2015, <https://www.rfc-editor.org/info/rfc7515>.
[RFC7517] Jones, M., "JSON Web Key (JWK)", RFC 7517,
DOI 10.17487/RFC7517, May 2015,
<https://www.rfc-editor.org/info/rfc7517>.
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[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<https://www.rfc-editor.org/info/rfc7519>.
[RFC7638] Jones, M. and N. Sakimura, "JSON Web Key (JWK)
Thumbprint", RFC 7638, DOI 10.17487/RFC7638, September
2015, <https://www.rfc-editor.org/info/rfc7638>.
[RFC7800] Jones, M., Bradley, J., and H. Tschofenig, "Proof-of-
Possession Key Semantics for JSON Web Tokens (JWTs)",
RFC 7800, DOI 10.17487/RFC7800, April 2016,
<https://www.rfc-editor.org/info/rfc7800>.
[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032,
DOI 10.17487/RFC8032, January 2017,
<https://www.rfc-editor.org/info/rfc8032>.
[RFC8785] Rundgren, A., Jordan, B., and S. Erdtman, "JSON
Canonicalization Scheme (JCS)", RFC 8785,
DOI 10.17487/RFC8785, June 2020,
<https://www.rfc-editor.org/info/rfc8785>.
[RFC9278] Jones, M. and K. Yasuda, "JWK Thumbprint URI", RFC 9278,
DOI 10.17487/RFC9278, August 2022,
<https://www.rfc-editor.org/info/rfc9278>.
[RFC9396] Lodderstedt, T., Richer, J., and B. Campbell, "OAuth 2.0
Rich Authorization Requests", RFC 9396,
DOI 10.17487/RFC9396, May 2023,
<https://www.rfc-editor.org/info/rfc9396>.
[RFC9562] Davis, K., Peabody, B., and P. Leach, "Universally Unique
IDentifiers (UUIDs)", RFC 9562, DOI 10.17487/RFC9562, May
2024, <https://www.rfc-editor.org/info/rfc9562>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/info/rfc6749>.
[RFC9201] Seitz, L., "Additional OAuth Parameters for Authentication
and Authorization for Constrained Environments (ACE)",
RFC 9201, DOI 10.17487/RFC9201, August 2022,
<https://www.rfc-editor.org/info/rfc9201>.
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[RFC8414] Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
Authorization Server Metadata", RFC 8414,
DOI 10.17487/RFC8414, June 2018,
<https://www.rfc-editor.org/info/rfc8414>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[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/info/rfc2119>.
[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/info/rfc8174>.
12.2. Informative References
[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/info/rfc7942>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/info/rfc8949>.
[RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
"CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
May 2018, <https://www.rfc-editor.org/info/rfc8392>.
[RFC8693] Jones, M., Nadalin, A., Campbell, B., Ed., Bradley, J.,
and C. Mortimore, "OAuth 2.0 Token Exchange", RFC 8693,
DOI 10.17487/RFC8693, January 2020,
<https://www.rfc-editor.org/info/rfc8693>.
[RFC9052] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", STD 96, RFC 9052,
DOI 10.17487/RFC9052, August 2022,
<https://www.rfc-editor.org/info/rfc9052>.
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[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/info/rfc9334>.
[RFC9449] Fett, D., Campbell, B., Bradley, J., Lodderstedt, T.,
Jones, M., and D. Waite, "OAuth 2.0 Demonstrating Proof of
Possession (DPoP)", RFC 9449, DOI 10.17487/RFC9449,
September 2023, <https://www.rfc-editor.org/info/rfc9449>.
[OAUTH-TXN-TOKENS]
Tulshibagwale, A., Fletcher, G., and P. Kasselman,
"Transaction Tokens", March 2026,
<https://datatracker.ietf.org/doc/draft-ietf-oauth-
transaction-tokens/>.
[OAUTH-ID-CHAINING]
Schwenkschuster, A., Kasselman, P., Burgin, D., Jenkins,
M., Campbell, B., and A. Parecki, "OAuth Identity and
Authorization Chaining Across Domains", May 2026,
<https://datatracker.ietf.org/doc/draft-ietf-oauth-
identity-chaining/>.
[BISCUIT] Eclipse Foundation, "Biscuit: Distributed Authorization
Tokens", n.d., <https://doc.biscuitsec.org/reference/
specifications.html>.
[CEDAR] Cedar Policy, "Cedar Policy Language Reference Guide",
n.d., <https://docs.cedarpolicy.com/>.
[MACAROONS]
Birgisson, A., Politz, J. G., Erlingsson, U., Taly, A.,
Vrable, M., and M. Lentczner, "Macaroons: Cookies with
Contextual Caveats for Decentralized Authorization in the
Cloud", NDSS 2014, 2014,
<https://research.google/pubs/pub41892/>.
[SALTZER75]
Saltzer, J. H. and M. D. Schroeder, "The Protection of
Information in Computer Systems", Proceedings of the
IEEE Vol. 63, No. 9, 1975,
<https://doi.org/10.1109/PROC.1975.9939>.
[HARDY88] Hardy, N., "The Confused Deputy (or why capabilities might
have been invented)", ACM SIGOPS Operating Systems
Review Vol. 22, No. 4, 1988,
<https://dl.acm.org/doi/10.1145/54289.871709>.
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[CAMEL25] Debenedetti, E., Shumailov, I., Fan, T., Hayes, J.,
Carlini, N., Fabian, D., Kern, C., Shi, C., Terzis, A.,
and F. Tramèr, "Defeating Prompt Injections by Design",
2025, <https://arxiv.org/abs/2503.18813>.
[DEEPMIND26]
Tomašev, N., Franklin, M., and S. Osindero, "Intelligent
AI Delegation", 2026, <https://arxiv.org/abs/2602.11865>.
[WIMSE-ARCH]
Salowey, J., Rosomakho, Y., and H. Tschofenig, "Workload
Identity in a Multi System Environment (WIMSE)
Architecture", March 2026,
<https://datatracker.ietf.org/doc/draft-ietf-wimse-arch/>.
[WIMSE-S2S]
Campbell, B., Salowey, J. A., Schwenkschuster, A., and Y.
Sheffer, "WIMSE Workload-to-Workload Authentication",
October 2025, <https://datatracker.ietf.org/doc/draft-
ietf-wimse-s2s-protocol/>.
[DENNIS66] Dennis, J. B. and E. C. Van Horn, "Programming Semantics
for Multiprogrammed Computations", Communications of the
ACM Vol. 9, No. 3, 1966,
<https://doi.org/10.1145/365230.365252>.
[MILLER06] Miller, M. S., "Robust Composition: Towards a Unified
Approach to Access Control and Concurrency Control", PhD
Dissertation Johns Hopkins University, 2006,
<http://www.erights.org/talks/thesis/>.
[ALLOY] Jackson, D., "Alloy: A Lightweight Object Modelling
Notation", ACM Transactions on Software Engineering and
Methodology Vol. 11, No. 2, 2002,
<https://doi.org/10.1145/505145.505149>.
[Z3] de Moura, L. and N. Bjørner, "Z3: An Efficient SMT
Solver", TACAS 2008, LNCS 4963, 2008,
<https://github.com/Z3Prover/z3>.
Appendix A. Comparison with Related Authorization Mechanisms (Non-
Normative)
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A.1. Token Exchange (RFC 8693)
RFC 8693 allows a client to exchange one token for another,
potentially with reduced scope, by contacting the authorization
server. The server enforces scope reduction. This requires network
connectivity to the authorization server at each delegation hop and
cannot be performed offline.
This specification allows a token holder to derive a new token
locally, without contacting the authorization server. The
attenuation invariant is enforced by the chain verification
algorithm, not by a server-side policy check.
A.2. Rich Authorization Requests (RFC 9396)
RFC 9396 defines a structured format for expressing fine-grained
authorization details in OAuth tokens. This specification uses the
authorization_details claim format from RFC 9396 and extends it with:
(1) a delegation chain model that links tokens via cryptographic
hashes, (2) monotonic attenuation invariants that constrain what
derived tokens may express, and (3) proof-of-possession binding that
ties invocations to specific key holders.
A.3. DPoP (RFC 9449)
DPoP ([RFC9449]) is a token theft prevention mechanism that binds an
existing OAuth access token to a holder key, ensuring that a stolen
token cannot be presented without the corresponding private key.
DPoP does not change what the access token authorizes; the token's
authorization claims are unchanged. The resource server grants
whatever the access token permits; DPoP adds a cryptographic proof
that the presenter holds the bound key.
AATs encode the authorization itself. The token specifies which
tools may be invoked, with what argument constraints, and by which
key holder. Holders can derive tokens with authority equal to or
narrower than their own, without contacting the authorization server.
The PoP JWT in Section 5 serves a similar cryptographic role to a
DPoP proof, binding a specific invocation to the leaf token's holder
key, but operates in a different context. Everything else in this
specification (the chain model, the attenuation invariants, the
constraint type registry, the subsumption matrix) addresses questions
outside DPoP's scope.
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Structurally, DPoP is a two-party protocol between a client and a
resource server. There is no delegation model, no parent-child
chain, and no attenuation invariant. The chain model of this
specification (del_depth, par_hash, del_max_depth, and the six
attenuation invariants) is not defined by DPoP.
At the proof level, DPoP binds to an HTTP method (htm) and URI (htu).
AAT PoP JWTs bind to a tool name (aat_tool) and a structured argument
map (hta). Tool invocations are function calls, not HTTP requests,
and a URI alone carries insufficient information for argument-level
constraint evaluation. This is why aat_tool and hta differ
structurally from htm and htu: (1) hta carries the full argument map
required for constraint evaluation at the enforcement point; (2)
aat_id binds the proof to a specific leaf token jti and chain
position, which DPoP does not define.
The cryptographic mechanism is the same: an asymmetric key in
cnf.jwk, compact JWT serialization, verified against the leaf token's
bound key. DPoP could in principle be layered alongside AATs as a
transport-level binding for chain delivery, but that combination is
outside the scope of this specification.
A.4. Biscuit
Biscuit [BISCUIT] and AATs both support offline attenuation and
decentralized verification. Biscuit is a compact authorization token
with Datalog-based policy checks and is commonly used as a bearer
credential. AAT is an OAuth-shaped, holder-bound delegation-chain
protocol with tool-and-argument constraints, cryptographic chain
invariants, and invocation-time proof of possession.
Biscuit is a general-purpose authorization token format. It does not
natively encode OAuth-oriented delegation-chain claims such as depth
limits, parent-token linkage, or explicit chain position
declarations. This specification defines those properties in the
token model itself, making the chain independently verifiable as a
delegation protocol rather than as a sequence of policy blocks.
Appendix B. Implementation Notes (Non-Normative)
B.1. Algorithm Recommendations
* *Signing algorithm:* Ed25519 [RFC8032]. The normative requirement
is in Section 3.2. EdDSA provides compact 64-byte signatures
suitable for constrained agent environments. The JWS alg header
value for Ed25519 is "EdDSA".
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* *Key representation:* JWK [RFC7517] with "kty": "OKP" and "crv":
"Ed25519".
* *Token identifier:* UUIDv7 is recommended for jti values,
providing time-ordered identifiers without central coordination.
The algorithm allowlist requirement is normatively defined in
Section 7 (steps 3a, 4a, and 7a) and discussed in Section 8.13.
The cnf.jwk key type is not hardcoded to Ed25519. Implementations
should be designed to support algorithm migration without requiring
changes to token structure.
B.2. Recognizing Derived Token iss Values in Middleware
In both root and derived AATs, iss is a URI. For root tokens it is a
conventional issuer URI. For derived tokens it is a JWK Thumbprint
URI ([RFC9278]) with the urn:ietf:params:oauth:jwk-thumbprint:sha-
256: prefix. Middleware that routes or policy-evaluates based on iss
should recognize the JWK Thumbprint URI scheme and apply chain-aware
processing rather than attempting to resolve the URI as an issuer
endpoint. The verification key for derived tokens is parent.cnf.jwk,
resolved from the preceding chain link.
B.3. Relationship to WIMSE
The WIMSE architecture [WIMSE-ARCH] and service-to-service protocol
[WIMSE-S2S] address workload identity and authentication for entities
that hold and present AATs. A WIMSE workload credential identifies
an agent; the iss claim in a root AAT issued to that agent may
reference the agent's WIMSE workload identifier. The two
specifications are complementary: WIMSE establishes workload identity
and authentication; this specification defines a holder-derivable,
invocation-scoped delegation and attenuation mechanism that WIMSE
does not standardize.
B.4. Delegation Depth Guidance
The normative requirement is only that implementations enforce a
finite MAX_DELEGATION_DEPTH. This appendix provides non-normative
guidance for selecting an appropriate value.
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The appropriate MAX_DELEGATION_DEPTH depends on the deployment
topology. Linear orchestration chains (root issuer, one or two
planning layers, leaf executor) require few hops. Swarm
architectures with dynamic fan-out, sub-task delegation, or
hierarchical agent groups may require significantly deeper chains.
The implementation ceiling should reflect the maximum depth the
deployment actually needs, not an arbitrary conservative default.
Regardless of the implementation ceiling, issuers should set
del_max_depth to the depth required by the expected workflow, with
margin for subprocess delegation, operational handoffs, and holder-
key handoff. Lower values reduce the number of offline delegation
steps under a grant, but overly tight values can suppress attenuation
and encourage broader token reuse. The security value comes from
deliberate per-chain policy, not from an arbitrarily low ceiling.
B.5. Implementation Size Limits
The normative requirement is only that implementations enforce finite
limits on token size, chain size, constraint nesting depth, and tool
count to prevent resource exhaustion. This appendix provides non-
normative recommended defaults for implementations with no specific
deployment constraints:
+==================================+=====================+
| Parameter | Recommended Default |
+==================================+=====================+
| Maximum token size | 64 KB |
+----------------------------------+---------------------+
| Maximum chain size | 256 KB |
+----------------------------------+---------------------+
| Maximum tools per token | 256 |
+----------------------------------+---------------------+
| Maximum constraints per tool | 64 |
+----------------------------------+---------------------+
| Maximum constraint nesting depth | 32 |
+----------------------------------+---------------------+
| Maximum tool name length | 256 bytes |
+----------------------------------+---------------------+
| Maximum constraint value length | 4 KB |
+----------------------------------+---------------------+
Table 10
Deployments should document their enforced limits. Interoperating
parties should verify that their respective limits are compatible
before deployment.
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Implementations should prefer core structural constraints where the
policy permits, as these types produce compact tokens and simple
subsumption checks.
Implementations concerned about parser exposure on unverified
payloads in step 2c of the chain verification algorithm (Section 7)
may extract jti using a length-limited byte scan rather than a full
JSON parser, provided the extraction correctly handles JSON
whitespace and string escaping.
A single AAT is typically 1-4 KB when base64url-encoded. Chains of
two or more tokens will commonly exceed the 4-8 KB header size limits
enforced by common reverse proxies and load balancers, resulting in
431 errors. Deployments should transmit AAT chains in a request body
field rather than an HTTP header. For size-constrained environments,
Appendix D notes considerations for a future CBOR/CWT profile.
B.6. Signed Passthrough Metadata
Implementations may include additional JWT claims in AATs beyond
those defined in Section 3, using collision-resistant names for
passthrough metadata such as request trace identifiers or tenant
context. Such claims are integrity-protected within each token, but
the base chain verification algorithm does not preserve or interpret
them across derivation steps. Deployments that require chain-wide
preservation of passthrough metadata must define their own derivation
and verification rules, either through deployment-specific policy or
a companion profile.
B.7. TTL Guidance
The normative requirement is only that derived tokens cannot outlive
their parents and that token lifetime does not exceed
MAX_TOKEN_LIFETIME (Section 4.4). This appendix provides non-
normative guidance for selecting appropriate TTL values.
Expiration is the base specification's built-in limit on token
lifetime. A token that has expired cannot be used regardless of
whether a revocation mechanism is deployed. Short lifetimes reduce
the window of exposure from key compromise, token theft, or scope
misconfiguration. The operational cost of short TTLs is re-issuance
frequency; this cost is low when the root issuer is available and
derivation is offline.
The appropriate TTL depends on the token's position in the chain and
the deployment context. Root tokens should be long enough to cover
the full orchestration and execution window for the task, but no
longer. Leaf tokens should be scoped to the expected duration of a
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single tool invocation. Deployments with intermittent connectivity
(edge, embedded, or air-gapped) may need longer lifetimes, with the
awareness that longer lifetimes expand the compromise window.
Deployments should treat TTL as a policy expression rather than a
convenience parameter. A root token with a 24-hour TTL effectively
grants the holder 24 hours of authority regardless of how narrowly
the capability scope is defined.
Appendix C. Policy Languages with Decidable Containment (Non-Normative)
The core constraint set is intentionally limited to structural
constraint types with deterministic subsumption rules. Implementers
that need richer expressiveness can define extension constraint types
backed by analyzable authorization policy languages, such as Cedar
[CEDAR]. Such an extension must define the runtime check predicate,
the token encoding of the policy, and a sound, deterministic
subsumption procedure. The fact that a policy language can decide
whether an invocation is authorized is not, by itself, sufficient for
AAT attenuation; the extension must also define how an enforcement
point determines that a derived policy is no less restrictive than
its parent. This document does not recommend a specific policy
language. The normative requirement is that every extension
registration satisfy the decidable, sound, and deterministic
properties defined in Section 3.5.1.
Appendix D. CBOR/CWT Considerations (Non-Normative)
The claim semantics, attenuation invariants, constraint subsumption
rules, and chain verification algorithm defined in this document are
format-agnostic. They describe a protocol, not an encoding. JWT/JWS
is the only fully specified token encoding in this document.
A future CWT/COSE profile could represent the same semantic content
using CBOR Web Tokens [RFC8392] and COSE message signing [RFC9052].
Such a profile would need to define CWT claim-key assignments, COSE
algorithm requirements, deterministic CBOR serialization rules per
[RFC8949], the CWT parent token signing input used for par_hash, and
the deterministic encoding of PoP hta values. This appendix does not
define a CWT serialization, CWT claim-key mapping, COSE algorithm
profile, or CWT par_hash signing input. Those details are deferred
to a companion document.
Appendix E. Implementation Status (Non-Normative)
This appendix describes the implementation status of this
specification at the time of submission, per the practice described
in [RFC7942].
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E.1. Reference Implementation
Tenuo provides a reference implementation of this protocol. The
chain verification algorithm (Section 7) and token derivation
procedure (Section 6) are both implemented. Tenuo also includes an
implementation-specific CBOR/COSE wire representation, with Ed25519
signatures carried in COSE_Sign1 structures. That implementation
experience supports the format independence of the core protocol
model, but does not define a fully interoperable CWT profile; the CWT
profile is deferred as described in Appendix D.
RFC Editor Note: This section will be updated or removed before
publication.
E.2. Formal Verification
Formal verification of the attenuation algebra is in progress, using
three complementary techniques: bounded model checking ([ALLOY]) for
set-theoretic constraint types, SMT solving ([Z3]) for numeric and
structural constraint types, and property-based testing against the
Rust implementation for implemented constraint types. Bounded model
checking has found no counterexamples for scopes up to 8 constraints
and 8 values. The combination is intended to provide evidence toward
monotonicity of the I4 invariant across the full constraint
attenuation matrix.
RFC Editor Note: This section will be updated or removed before
publication.
Author's Address
Niki Aimable Niyikiza
Tenuo
Email: niki@tenuo.ai
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