Human Delegation Provenance Protocol (HDP): Cryptographic Chain-of-Custody for Agentic AI Systems

1.1. What HDP Is Not

HDP is not an authorization protocol, and an HDP token is not a capability, an access token, or a credential that entitles its holder to anything. Presenting a valid HDP token to a service does not authorize the presenter to perform the requested action. That decision belongs to the service's own access control mechanism, whether that is OAuth 2.0 (Section 13.2), a capability system such as UCAN or ZCAP-LD (Section 13.4, Section 13.5), or something else. HDP is designed to travel alongside such mechanisms, not to replace them, and not to be an input to them.¶

An HDP token MUST NOT be used as an input to any access decision. A service MUST NOT grant, refuse, or condition an action on the presence of an HDP token, on the outcome of verifying one, or on its contents. A missing token, or one that fails verification, is an audit finding and is recorded as one; it is not a reason to deny a request. A check that can deny is an access control mechanism whatever it is called, and running a second one beside the system's own is a well-known hazard: it adds an independent way to attack the system, and its denials can look to the base mechanism like failures that mechanism was not designed to handle.¶

Several parts of this document can be misread as authorization if this distinction is not kept in view. The scope object (Section 3.3) records what the human declared, in fields named authorized_tools and authorized_resources among others; it is a signed record of the authorization event, not a grant. Integrity verification (Section 5) establishes that a record is authentic and intact, not that anyone may act. The HTTP transport (Section 8) shows a token accompanying a request because the task travels in the request and the receiving side records it, not because the token authorizes it.¶

The reader of an HDP token is whoever examines the record afterwards: post-incident reconstruction of which agent did what, under whose authorization, and in what order; compliance evidence that a human authorized a class of action; and review of how each agent read the human's request. A token is carried with the task and read at audit.¶

Earlier revisions of this document also described a reader that relied on the token before an action: expiry checks, session binding, replay defense, and verifier-local revocation served that reader. This revision removes them. The fields that supported them, header.expires_at and header.session_id, remain in the v0.1 wire format and are described in Section 3.1 as record metadata. Whether they belong in a future token version is an open question.¶

1.2. Motivation

The need for agentic delegation provenance is not hypothetical. Production deployments of AI orchestration systems (LangChain, AutoGPT, CrewAI, and similar frameworks) today pass natural language task descriptions between agents with no cryptographic binding to the original human authorization. The operational risk compounds as models become more capable and agents are granted access to higher- consequence tools.¶

A provenance token that travels alongside the task (tamper-evident, offline-verifiable, and scoped to what the human actually approved) provides the foundation for auditable, accountable agentic systems.¶

1.3. Design Goals

HDP is designed with the following goals in order of priority:¶

1. Offline verifiability. Verifying a record's integrity MUST require only the issuer's public key. No network call, registry lookup, or third-party endpoint is required.¶
2. Self-sovereignty. Any organization MUST be able to issue and verify HDP tokens without registering with a central authority or anchoring to a third-party key.¶
3. Tamper evidence. Any modification to a token's recorded content (its header, principal, scope, or any recorded hop) MUST be detectable by integrity verification. Completeness of the chain (that no trailing hop has been omitted) is a separate property; see Section 10.4.¶
4. Minimal footprint. The protocol MUST be implementable in any language with Ed25519 and JSON support. No mandatory infrastructure beyond key management is required.¶
5. Privacy by design. Principal identity fields MUST be separable from the audit-relevant parts of the token, so tokens can be transmitted to agents without exposing PII.¶

1.4. Relationship to IPP (draft-haberkamp-ipp-01)

The Intent Provenance Protocol [I-D.haberkamp-ipp] addresses the same problem space. HDP and IPP share the use of Ed25519 signatures and append-only provenance chains but make different architectural trade-offs, which are detailed in Section 13. The two protocols are not interoperable. HDP is offered as a distinct design point, not a revision of IPP.¶

The full HDP protocol specification is available at [HDP-SPEC]. A TypeScript reference implementation is available at [HDP-IMPL]. At the time of writing it implements -02 of this document, including the checks before an action that this revision removes (Section 1.1), and is being updated to match.¶

1.5. Generality of the Chain-of-Custody Mechanism

The core of HDP is an append-only, cryptographically chained record: each hop extends a signed entry that covers all prior state, gaps in the hop sequence are tamper-evident, and any party can verify the entire chain offline using only a public key. This chain-of-custody mechanism is independent of what the chain carries.¶

This document profiles that mechanism for one application: human-authorized agentic delegation. In this profile the carried payload is the scope object (Section 3.3) and each hop record describes an agent delegation action. The same mechanism could carry other payloads, for example data provenance, consent delegation, or physical-world command chains, each as a distinct profile. Such profiles are out of scope for this document; HDP v0.1 defines only the agentic-delegation profile. Where practical, the signing (Section 4.1, Section 4.2) and verification (Section 5) procedures are described in a payload-agnostic way so that future profiles can reuse them unchanged.¶

3.1. Header

The header object carries the record's identifier, its timing, and the session in which the human authorized the task.¶

{
  "token_id"        : "550e8400-e29b-41d4-a716-446655440000",
  "issued_at"       : 1711483200000,
  "expires_at"      : 1711569600000,
  "session_id"      : "sess-20260326-abc123",
  "version"         : "0.1",
  "parent_token_id" : "..."
}

token_id:

REQUIRED. A version 4 UUID [RFC9562]. Unique identifier for this token.¶

issued_at:

REQUIRED. Unix milliseconds. Time of issuance.¶

expires_at:

REQUIRED. Unix milliseconds. MUST be greater than issued_at. Records the end of the period for which the principal authorized the task. It has no effect on integrity verification (Section 5), and a service treats a token received after this time no differently from any other (Section 1.1). A hop whose timestamp is at or after expires_at is recorded like any other, and an auditor reports it as recorded after the authorization period ended (Section 5.1). HDP defines no default lifetime.¶

session_id:

REQUIRED. Opaque string identifying the session in which the principal authorized the task, established out of band between issuer and agent framework before issuance. It lets an auditor associate the record with the session's other records, including superseding and jointly approved records (Section 6, Section 7).¶

version:

REQUIRED. MUST equal the value of the top-level hdp field.¶

parent_token_id:

OPTIONAL. If present, identifies an earlier record this record is linked to: one it supersedes (Section 6), or one recording another principal's approval of the same task (Section 7).¶

3.2. Principal

The principal object identifies the authorizing human. It MUST contain id and id_type. All other fields are OPTIONAL.¶

{
  "id"             : "usr_alice_opaque",
  "id_type"        : "opaque",
  "display_name"   : "Alice Chen",
  "poh_credential" : "...",
  "metadata"       : {}
}

The id_type field MUST be one of the following defined values, or a custom string prefixed with x-:¶

• opaque: Application-defined identifier. No resolution semantics are implied.¶
• email: An email address as defined in [RFC5321].¶
• uuid: A UUID as defined in [RFC9562].¶
• did: W3C Decentralized Identifier [W3C.DID]. DID resolution is application- defined and not required by this protocol.¶
• poh: A Proof-of-Humanity credential identifier. Verification semantics are application-defined; see Section 9.3.¶

HDP does not mandate any specific identity model. The did id_type is available for deployments with existing DID infrastructure; it is not required.¶

3.3. Scope

The scope object records what the human authorized. It is signed as part of the root signature and MUST NOT be modified after issuance.¶

The scope object is a record, not a grant. Its fields describe the authorization the human gave at issuance so that the record can later be compared with what agents declared they did. Nothing in this object confers authority on an agent that holds the token (Section 1.1).¶

{
  "intent"               : "Analyze Q1 sales data and report.",
  "authorized_tools"     : ["database_read", "file_write"],
  "authorized_resources" : ["db://sales/q1-2026", "file://reports/"],
  "data_classification"  : "confidential",
  "network_egress"       : false,
  "persistence"          : true,
  "max_hops"             : 3
}

The values above are illustrative. In particular, the max_hops value shown is an issuer choice for this example, not a protocol limit.¶

intent:

REQUIRED. The principal's request, in natural language. Free-form string. It is what an auditor compares each hop's action_summary against, and SHOULD be written to be both human- and agent-readable. Where practical it SHOULD be the principal's own words.¶

authorized_tools:

OPTIONAL. Array of tool identifiers the principal declared as authorized. The field records the declaration; it does not grant access to the tools named.¶

authorized_resources:

OPTIONAL. Array of resource identifiers (URIs, paths, etc.) the principal declared as authorized. As with authorized_tools, this records the declaration and grants nothing.¶

data_classification:

REQUIRED. One of: public, internal, confidential, restricted. Records the sensitivity level of data the principal declared the agent may access.¶

network_egress:

REQUIRED. Boolean. Records whether the principal declared that the agent may make outbound network requests.¶

persistence:

REQUIRED. Boolean. Records whether the principal declared that the agent may write persistent state.¶

max_hops:

OPTIONAL. Positive integer, chosen by the issuer: the number of hops this record holds. Once the chain holds max_hops hops it is not extended (Rule 4 of Section 4.3). Delegation beyond that depth is not recorded in this token, and the agent that appended the final hop is accountable for everything delegated beyond it (Section 10.5). An issuer MAY choose any value within the representation bounds in Section 3, Paragraph 3. If max_hops is absent, HDP places no limit on the depth recorded.¶

HDP does not mandate a central taxonomy for intent, authorized_tools, or authorized_resources. These are self-described by the issuer. Comparing what agents declared with the declared scope is an audit concern.¶

The authorized_tools and authorized_resources arrays are independent lists. HDP v0.1 defines no binding between a tool and the resources it may be used on: the example above lists two tools and two resources, and nothing in it states which tool the principal authorized against which resource. If both resources were databases and one tool wrote while the other read, the record could not say which tool went with which database. Applications MUST NOT infer a per-tool resource binding from a v0.1 scope. Issuers that need the binding recorded SHOULD state it in intent, and SHOULD list a resource for every tool that acts on one, as Appendix A does.¶

The fixed values are also too coarse. The four data_classification levels are not the ones every issuer uses, and a single persistence flag cannot record that a principal let an agent write log records but nothing else.¶

The next token version is planned to replace authorized_tools, authorized_resources, data_classification, network_egress, and persistence with one nested structure: a map from each resource to what the principal declared for it, with values chosen by the issuer. The map will be required but may be empty, so that a record declaring nothing can be told apart from one that says nothing. The following is illustrative only; member names are not yet fixed:¶

{
  "intent"      : "Analyze Q1 sales data and report.",
  "permissions" : {
    "db://sales/q1-2026" : { "actions": ["read"],
                             "classification": "confidential" },
    "file://reports/"    : { "actions": ["write"] },
    "log://agent-audit"  : { "actions": ["append"] }
  }
}

These are wire-format changes and are not part of v0.1.¶

3.4. Chain

The chain array is append-only. Each element records a single delegation event (hop). The array is empty at issuance and grows as the token passes through agents. Agents MUST NOT remove or modify existing entries.¶

{
  "seq"               : 1,
  "agent_id"          : "orchestrator-v2",
  "agent_type"        : "orchestrator",
  "agent_fingerprint" : "sha256:abc123...",
  "timestamp"         : 1711483260000,
  "action_summary"    : "Decompose task; delegate to sub-agents.",
  "parent_hop"        : 0,
  "hop_signature"     : "<base64url-encoded Ed25519 signature>"
}

seq:

REQUIRED. Positive integer. Sequential index, starting at 1. MUST be exactly one greater than the previous hop's seq. Gaps in sequence are a protocol violation.¶

agent_id:

REQUIRED. Identifier of the agent adding this hop. The identifier need not be globally meaningful; it is sufficient that the delegator which assigned it can interpret it (Section 9.1).¶

agent_type:

REQUIRED. String. A descriptive label for the agent's role, such as orchestrator, sub-agent, or tool-executor. HDP attaches no normative meaning to any value, and the delegator MAY use any string. Earlier revisions allowed only four values, the fourth being the literal string custom; tokens using those values remain valid.¶

agent_fingerprint:

OPTIONAL. Model or binary fingerprint for the acting agent.¶

timestamp:

REQUIRED. Unix milliseconds. Time of hop extension, as declared by the party extending the chain. Hop timestamps are declared values: the hop signature proves who attested the value, not that it is accurate. MUST be greater than or equal to the previous hop's timestamp (Rule 5 of Section 4.3).¶

action_summary:

REQUIRED. Description of the hop's action, recording how the agent read the task it was given, written to be both human- and agent-readable. The description MUST distinguish an intended action from an attempted, blocked, or observed action whenever that distinction affects its interpretation. Out-of-scope attempts and observed violations MAY be recorded, with the deviation stated explicitly; their inclusion does not imply principal approval. This remains a signed declaration, not proof that an action occurred (Section 10.1).¶

parent_hop:

REQUIRED. Non-negative integer. Index of the hop that triggered this delegation, where 0 indicates the root (human) authorization.¶

hop_signature:

REQUIRED. Base64url-encoded (no padding) Ed25519 signature. See Section 4.2. Absence is a protocol violation per Rule 6 of Section 4.3.¶

The action_summary is supplied by whoever submits the hop, and in v0.1 the issuer signs what it is given (Section 4.2). When the agent writes its own summary, the record holds the agent's account of what it did and is only as reliable as that agent. Where a component outside the agent's control observes the agent's actions, such as a virtual machine sidecar, a tool gateway, or an MCP proxy, that component SHOULD record the hop in place of the agent, and the record SHOULD identify which component recorded it. HDP v0.1 has no field for the recording component; until one is added, the component SHOULD name itself in action_summary.¶

3.5. Signature

The signature object carries the root signature computed by the issuer.¶

{
  "kid"   : "alice-signing-key-v1",
  "alg"   : "Ed25519",
  "value" : "<base64url Ed25519 signature over canonical JSON>"
}

The alg field MUST be Ed25519 for HDP v0.1. The kid field SHOULD be used by verifiers to identify the correct public key when multiple keys are in circulation.¶

4.1. Root Signature

The root signature is computed by the issuer at token creation time. It covers the token's header, principal, and scope, the fields that constitute the human authorization event.¶

The signing procedure is:¶

1. Construct the unsigned token object containing the hdp, header, principal, scope, and chain (empty array at issuance) fields.¶
2. Serialize the object to canonical JSON using RFC 8785 [RFC8785] (JSON Canonicalization Scheme). This ensures deterministic byte representation across implementations and platforms.¶
3. Compute the Ed25519 [RFC8032] signature over the canonical JSON bytes using the issuer's secret key.¶
4. Encode the signature bytes as base64url [RFC4648] (no padding).¶
5. Attach the signature object (kid, alg, value) to the token.¶

The signature field itself MUST NOT be included in the canonical JSON payload before signing. Because the root signature is computed while chain is empty, the signed payload is deterministically recoverable from a populated token by removing the signature field, resetting chain to an empty array, and re-serializing with RFC 8785. The root signature therefore covers hdp, header, principal, and scope; the chain is protected by the hop signatures (Section 4.2) rather than by the root signature.¶

4.2. Hop Signature

Each hop MUST carry a hop_signature. This signature binds the new hop record to the entire accumulated delegation history and to the root signature, making retroactive chain modification detectable.¶

The hop signing procedure is:¶

1. Construct the new hop record (all fields except hop_signature).¶
2. Build the signing payload as a JSON array: [hop_1, hop_2, ..., hop_(n-1), new_hop_unsigned] where hop_1 through hop_(n-1) are the previously signed hops (WITH their hop_signature fields) and new_hop_unsigned is the new hop record WITHOUT its hop_signature.¶
3. Prepend the root signature value (base64url string) to the array as its first element: [root_sig_value, hop_1, ..., new_hop_unsigned]. This chains the hop signature to the root.¶
4. Serialize the array to canonical JSON per RFC 8785.¶
5. Compute the Ed25519 signature over the canonical JSON bytes using the issuer's secret key.¶
6. Encode as base64url and attach as the hop_signature field on the new hop record.¶
7. Append the signed hop to the token's chain array.¶

The asymmetry between previously-signed hops (WITH hop_signature) and the new hop (WITHOUT hop_signature) in step 2 is intentional and critical. The verifier MUST reconstruct this exact payload structure when verifying each hop. See Section 5.¶

In HDP v0.1, all signatures (the root signature and every hop signature) are produced by the issuer using a single key. An extending agent that is not the issuer submits its hop to the issuer, which signs it and returns the extended token. Two consequences follow, and implementers should weigh both.¶

First, a v0.1 hop signature attests that the issuer recorded a delegation claim naming the agent in agent_id. It does not attest that the named agent consented to, or knew of, the hop, because the agent signed nothing. The chain is a record of what the issuer recorded, not of what each agent agreed to. Deployments in which that distinction matters need per-agent signing.¶

Second, the single-key design is practical only where the issuer is reachable whenever any agent wishes to extend the chain. This adds a round trip to every delegation, and it makes delegation across trust domains awkward, since an issuer in one domain must sign on behalf of agents in another. Only verification is offline; extension is not.¶

The single-key design does not, however, gain anything for offline verification that per-agent signing would lose. If each hop carried the public key of the agent appending it, signed into the chain by that agent's delegator, a verifier would authenticate every key after the first from the chain itself and would still resolve exactly one key out of band: the issuer's.¶

Issuer signing remains the baseline. Per-agent hop signing on that pattern is planned as an option for a future version, for deployments that can manage agent keys; it is not planned as a replacement. The baseline is kept because it confines signing to one party. OAuth 1.0 [RFC5849] required every client to sign its requests, which developers found hard to get right, and OAuth 2.0 [RFC6749] dropped the requirement and relies on TLS instead. Under the planned option an agent uses a fresh key for each delegation, signed into the chain by its delegator, so an agent key lasts no longer than its task and key rotation comes down to rotating the issuer's key. That design does not yet handle a key compromised during a task, and the extension will have to say how. The option is not part of v0.1, and v0.1 tokens carry no per-agent keys.¶

4.3. Chain Integrity Rules

The following rules govern chain construction and MUST be enforced by both extenders and verifiers:¶

1. Hop seq values MUST start at 1 and increment by exactly 1. No gaps are permitted.¶
2. Existing hop records MUST NOT be modified or removed.¶
3. A hop's parent_hop MUST reference a valid prior hop index (0 for the root human authorization, or the seq value of a prior hop).¶
4. If scope.max_hops is set, the chain length MUST NOT exceed it. A token with a full chain MUST NOT be extended; further delegation is not recorded in it (Section 10.5).¶
5. Each hop's timestamp MUST be greater than or equal to the timestamp of the hop before it. A chain in which a hop's timestamp is less than its predecessor's fails integrity verification (Step 3 of Section 5). Because the issuer signs every hop in v0.1, all hop timestamps pass through one clock domain, which is what makes this rule enforceable.¶
6. The hop_signature field MUST be present on every hop. A hop without a hop_signature is a protocol violation and is a verification failure.¶

5.1. Audit Results

Audit tooling MUST report the following results separately. They are results of examining a record, not new fields in an HDP token:¶

Record integrity:

Whether the record conforms to the format and passes the steps above, using a trusted archived issuer key. If the key or a supported verification algorithm is unavailable, integrity is unverified rather than valid.¶

Recording period:

Whether any hop's timestamp is at or after header.expires_at. Such hops are reported as recorded after the authorization period ended, not discarded. Hop timestamps are declared values (Section 3.4).¶

Session:

Where the audit concerns a particular session, whether header.session_id matches it. A mismatch means the record belongs to another session; it does not make the record invalid.¶

Linked records:

For a record with parent_token_id, its relationship to the parent: supersession, joint approval, or unknown, determined as described in Section 7.¶

A valid record establishes what the issuer recorded. It does not establish that an action occurred, that the recorded chain is complete (Section 10.4), or that the human authorized a particular action. Applications requiring audit SHOULD retain the tokens, the trusted public keys, and any receipts (Section 10.4) for their audit retention period. Key-compromise information MUST qualify any conclusion drawn from a mathematically valid signature (Section 10.8).¶

8.1. HTTP Header: HDP-Token

HDP tokens MAY be transmitted in HTTP requests and responses using the HDP-Token header. The header value is the base64url encoding (RFC 4648, no padding) of the UTF-8 JSON serialization of the complete token object.¶

POST /api/task HTTP/1.1
Host: agent.example.com
HDP-Token: eyJoZHAiOiIwLjEiLCJoZWFkZXIiOnsi...
Content-Type: application/json

Implementations MUST NOT include tokens in URL query parameters. The reason is privacy, not secrecy: a token is not a bearer secret, but it carries the principal object and the principal's request, and URLs are written to server logs and browser histories.¶

A token accompanies a request because the task it records travels in that request, and so that the receiving side, or a recording component in front of it (Section 3.4), can record the next hop and keep the record where an auditor can find it. Its presence does not authorize the request, and the receiving service MUST NOT use it as an input to its decision to act (Section 1.1).¶

HTTP header values are routinely written to access logs, proxy logs, and error reports, and the HDP-Token value contains the principal object and the full chain. Deployments SHOULD configure logging to treat HDP-Token as sensitive, as they would an Authorization header.¶

8.2. Token by Reference: HDP-Token-Ref

When token size is a concern (e.g., long chains), the token MAY be stored server-side and referenced using the HDP-Token-Ref header. The reference is either the token's token_id, or a content-addressed reference: the string sha256: followed by the base64url encoding (no padding) of the SHA-256 digest [RFC6234] of the token's canonical JSON serialization per RFC 8785.¶

A recipient resolving a content-addressed reference MUST validate the resolved token's input representation, serialize the complete token (including signature and all hop signatures) with RFC 8785, encode the result as UTF-8, compute SHA-256, and compare the digest with the reference. The digest in the reference MUST be exactly 32 bytes encoded as canonical unpadded base64url; malformed encodings or a digest mismatch MUST cause rejection. The comparison commits to canonical JSON, not to whitespace or member ordering in the stored serialization. For a UUID reference, the resolved header.token_id MUST identify the same UUID; equality is determined by the UUID's 128-bit value, not hexadecimal letter case. The recipient MUST reject a mismatch. Successful reference resolution MUST be followed by integrity verification (Section 5); a valid signature alone does not establish that the requested reference was resolved correctly.¶

POST /api/task HTTP/1.1
Host: agent.example.com
HDP-Token-Ref: 550e8400-e29b-41d4-a716-446655440000

Implementations using token-by-reference MUST secure the token store and use transport-layer security (TLS) for all reference resolution. The store MUST be write-once per reference: once a reference resolves to a token, it MUST NOT later resolve to a different one. Here, sameness means an identical complete canonical JSON token, not an identical token_id alone. A UUID reference therefore identifies one immutable snapshot, optionally the final record. It MUST NOT be used as a mutable pointer to the latest chain. Because extension preserves token_id, subsequent snapshots MUST use new content-addressed references when transported by reference; the previous UUID mapping remains unchanged.¶

The write-once requirement exists because a reference by token_id is a substitution point. A different but validly signed token stored under the same token_id passes integrity verification and presents the wrong provenance. Session binding narrows the set of tokens that could be substituted but does not eliminate it. A content-addressed reference removes the substitution point, when the recipient performs the required digest check, and SHOULD be preferred where the resolving party does not control the store. A content-addressed reference changes each time the chain is extended, which is the intended behaviour: each extension is a different record.¶

8.3. Hash-Linked Snapshots

A store that holds every snapshot of a token in full repeats the chain prefix on each extension, so keeping all the snapshots of an n-hop chain costs storage on the order of n squared hops. A store MAY instead hold a token as hash-linked entries:¶

• The base entry is the token as issued, with an empty chain.¶
• Each later entry is a JSON object with two members: prev, the content-addressed reference (Section 8.2) of the snapshot it extends, and hop, the one hop appended.¶

{
  "prev" : "sha256:<digest of the snapshot this entry extends>",
  "hop"  : { "seq": 3, "agent_id": "...", "hop_signature": "..." }
}

The store keeps each entry under the content-addressed reference of the complete snapshot it produces: the token that results from appending hop to the snapshot named by prev. References keep their meaning. A content-addressed reference always identifies a complete token, and resolving one from a hash-linked store means reconstructing that token by following prev back to the base entry, appending the hops in order, and checking the digest as Section 8.2 requires. A store MAY return the entries in place of the complete token; the recipient then reconstructs the token and performs the same check. The layout of a store's entries is a storage matter and is not part of the token wire format.¶

Hash-linking saves storage, not verification. A hop signature covers every hop before it (Section 4.2), so an auditor still has to fetch and check every hop up to the snapshot being verified.¶

Records also grow in a second direction. An action may take as an argument something delegated under another task, and a full audit of the action then involves that task's record as well: with chains of five hops, an action with k such arguments brings on the order of 5 * (1 + k) hops into its audit. Where a hop's action takes such an argument, its action_summary SHOULD refer to the other record by content-addressed reference rather than embedding it.¶

8.4. Key Distribution: Well-Known Endpoint

Issuers that wish to publish their Ed25519 public keys for automated discovery SHOULD serve a JSON document at /.well-known/hdp-keys.json with the following structure:¶

{
  "keys": [
    {
      "kid" : "alice-signing-key-v1",
      "alg" : "Ed25519",
      "pub" : "<base64url-encoded 32-byte Ed25519 public key>"
    }
  ]
}

This endpoint is a discovery convenience only. It is not part of verification, which takes the issuer's public key as an input and remains fully offline (Section 10.9). A verifier that has obtained the key by other means has no reason to consult it.¶

This format is intentionally minimal. Implementations MAY extend it with additional metadata. The alg field MUST be "Ed25519" for HDP v0.1 keys. Consumers MUST reject entries with unrecognized alg values. Consumers MUST validate that the decoded public key is exactly 32 bytes.¶

9.1. Minimum-Disclosure Principal Fields

The principal object may contain PII (email address, display name). Issuers SHOULD apply the principle of minimum disclosure when constructing tokens that will traverse multiple agents. Specifically:¶

• Use id_type: "opaque" with an application-internal identifier rather than embedding the user's email address in tokens that will be sent to third-party agents.¶
• Omit display_name when the receiving agent does not require a human-readable identity.¶

The token structure separates the identity fields (principal) from the audit-relevant fields (header, scope, chain). Implementations MAY strip the principal object when forwarding tokens to agents that do not require principal identity. A stripped token is no longer verifiable: its root signature can no longer be checked, so nothing binds its header, scope, or chain to the principal's authorization. Stripped tokens MUST be marked as no longer verifiable and MUST NOT be reported as having valid integrity.¶

The same principle applies to the agent_id field in hop records (Section 3.4). An agent_id need not be meaningful to anyone but the delegator that assigned it. This is sufficient because accountability in a delegation chain is recursive: a delegator is responsible for how its direct delegate uses the delegation, even when the use occurred further down the chain, and that delegate is in turn responsible for its own direct delegate. A verifier or auditor therefore never needs to resolve an agent_id globally. It needs the delegator at each step to be able to identify the party it delegated to, and the agent_id and its action_summary are bound into the signed chain for exactly that purpose.¶

Which identifier to use is context dependent, and HDP does not prescribe one. Non-exhaustively: a widely known identifier, such as an enterprise employee or service number, suits deployments where correlation is not a concern; an identifier meaningful only to the delegator suits deployments where an observer must be prevented from correlating requests across chains; an identifier meaningful only to the audit system suits deployments that must hide the organisation's structure from everyone else who sees the token; a DID ([W3C.DID]) suits deployments where a trusted authority exists to assert claims about it. An issuer or extending agent MAY use a fresh identifier for every delegation. Opaque identifiers hide who is in a chain but not its length: a chain of them still shows how many hops a task took (Section 10.5).¶

HDP v0.1 offers no field-level confidentiality: a token is either conveyed whole or stripped, as above. Earlier revisions gave two reasons for not specifying encryption of principal fields: the keys in circulation are Ed25519 signing keys rather than encryption keys, and the set of verifiers was open, so there was no defined party to encrypt to. The second reason no longer holds. The reader of a token is the audit system (Section 1.1), so fields the agents do not need, such as principal fields, could be encrypted to it, much as SAML 2.0 allows individual attributes in an assertion to be encrypted. The cost is key management: the audit system needs an encryption key pair, distinct from any signing key, whose public key issuers can obtain and whose secret key is retained for as long as the records are. Field-level encryption to the audit system is therefore a candidate for a future version. Selective disclosure of principal fields and of individual hops, which would allow a token to be verified with parts withheld, is also planned for a future version.¶

9.2. Data Retention and the Right to Erasure

HDP tokens may constitute personal data under applicable privacy regulations (e.g., GDPR Article 4(1)) when the principal.id or principal.display_name fields contain directly or indirectly identifying information.¶

Implementations SHOULD:¶

• Store tokens with explicit retention periods set by the audit purpose they serve.¶
• Provide deletion mechanisms that remove stored tokens upon erasure requests.¶
• Use opaque identifiers in principal.id where possible, maintaining a separate mapping that can be destroyed independently of the token audit log.¶

Encrypting stored tokens does not by itself discharge an erasure obligation, since the ciphertext remains personal data for as long as the key exists. Destroying the key (crypto-shredding) is a recognised technique for rendering retained tokens unreadable and MAY be used together with the mapping-destruction approach above.¶

9.3. Proof of Humanity

The optional principal.poh_credential field MAY carry a credential attesting that the principal is a human (e.g., a Worldcoin World ID proof, a CAPTCHA session token, or a biometric attestation identifier). The HDP protocol does not define the semantics of this field; verification is entirely application-defined.¶

An auditor that checks the credential reports the result separately from record integrity (Section 5). A check made at audit shows the credential's status at that time, not when the token was issued; evidence of the earlier status has to be retained when that earlier check is made. The verifier callback SHOULD be idempotent and SHOULD NOT have side effects.¶

10.1. Threat Model

HDP is designed to provide provenance and tamper evidence, not runtime enforcement. An agent that exceeds its declared scope is still a bad actor; HDP creates an evidence trail, not a capability boundary. Applications requiring runtime enforcement MUST implement it in their own access control mechanism and MUST NOT use the HDP token as an input to it. HDP is not an authorization protocol (Section 1.1); the properties discussed below are properties of the record: who could have produced it, whether it has been altered, and what it does and does not establish.¶

10.2. Token Forgery

A forged token (one whose header, principal, or scope fields do not match the original issuance) will fail Step 2 of integrity verification (root signature check). The security of this step relies on the unforgeability of Ed25519 signatures and the collision resistance of SHA-512 (used internally by Ed25519). An attacker who does not possess the issuer's secret key cannot produce a valid root signature for a modified token.¶

10.3. Chain Tampering

Modification, reordering, or removal of any non-trailing hop is detectable: it either breaks the hop sequence check (Step 3) or invalidates the hop signatures of all subsequent hops (Step 4), because each hop signature covers all previous hops and the root signature. Insertion of a fabricated hop will similarly fail unless the attacker possesses the issuer's secret key. Removal of one or more trailing hops is a distinct case that these checks do not detect; see Section 10.4.¶

10.4. Chain Truncation and Completeness

Each hop signature covers only the hops that precede it and the root signature. Consequently, deleting one or more hops from the end of the chain, or presenting an earlier and shorter copy of a token, yields a token that still passes integrity verification. HDP therefore provides tamper evidence for the hops that are present, but does not by itself prove that the chain is complete.¶

Relatedly, a non-cooperating or compromised agent can decline to append a hop for an action it takes; HDP records declared delegation actions and cannot compel an agent to record one. HDP is an evidence trail, not an enforcement mechanism (Section 10.1).¶

Where the receiving side can authenticate the presenter (for example, because the transport identifies the calling agent), it SHOULD record the presenter's identity with the token it received, and an auditor SHOULD compare the final hop's agent_id with that recorded presenter. This is cheap and exposes one truncation case: a third party presenting a shorter, earlier copy of the token, whose final hop names someone else. Capability systems impose the same requirement at invocation: UCAN Invocation requires the delegation chain to end at the invoker, and a ZCAP-LD invocation proof is rooted in the invoker's key.¶

The presenter check does not make the chain complete. In a capability chain, truncation gains an attacker nothing beyond what the presenter check catches, because authority only narrows toward the tail: a truncated prefix is usable only by the delegatee of its last remaining hop, who holds that authority legitimately. HDP hops record actions, not grants, and there is no attenuation; a truncated chain carries the full original scope. The case HDP must consider is an intermediate agent that deletes the hops appended after its own, in order to hide what its sub-agents did. After truncation that agent genuinely is the presenter, and the presenter check passes.¶

Where the deployment uses a capability system such as UCAN or ZCAP-LD, the certificates presented at the resource are a better record of exercised authority than anything HDP carries. The resource sees every certificate in each chain presented to it, and no intermediate agent can remove one from the resource's log. HDP should not duplicate that record. What HDP adds on such a deployment is the part that never produces a certificate: what the principal asked for and how each hop read it, which can travel in the certificates' own metadata (Section 11).¶

Concurrent extensions from the same prefix can produce two valid branches with the same token_id, hop count, and final agent_id, but different recorded actions. A hop count or presenter check cannot distinguish them. Integrity verification checks the supplied branch; it does not discover other branches or select a uniquely final one.¶

Deployments that require one linear record per token MUST serialize extensions at the issuer. The issuer MUST atomically check that the submitted prefix matches its accepted chain head and advance that head when committing an extension. A stale prefix MUST be rejected for reconciliation against the current head. An already signed hop cannot simply be transplanted onto another branch; extending the reconciled prefix requires a new signature. Retries SHOULD return an already committed result when the application identifies the same extension request. Deployments that intentionally allow branching MUST retain and identify the branches separately and define how their audit process accounts for them. HDP v0.1 defines no branch merge operation. Issuer serialization is state used for construction, not a network dependency of verification.¶

Applications requiring evidence of a particular observed or final record SHOULD retain an authenticated receipt or settlement record binding the token_id, session_id, complete token digest as defined in Section 8.2, observation time, and observing party. It MUST distinguish an observed snapshot from a claimed final record. A verifier relying on that receipt MUST validate its authenticity and compare the token digest. A hop count MAY be included for diagnostics but MUST NOT be treated as a substitute for the digest. The receipt establishes which branch was observed or finalized under the application's policy; it does not prove that no unrecorded action or undisclosed branch exists. Off-record delegation remains a separate limitation (Section 10.5). These receipts are application-layer artifacts, not new token fields.¶

10.5. Recording Depth and Off-Record Delegation

Limiting delegation depth is an anti-pattern for authorization. Nothing can prevent an agent from sharing a credential with another, so a limit on how far authority may be delegated does not stop delegation; it only stops the delegation from being visible. Capability systems rely instead on holding whoever shares a credential responsible for everything done with it, in the hope that this gives the sharer reason to delegate properly.¶

A limit on what a record holds is different, because the record confers no authority. scope.max_hops (Section 3.3) sets how many hops the record holds. Once the chain is full it is not extended, and responsibility for anything delegated beyond that depth rests with the agent that appended the final hop, by the recursive accountability described in Section 9.1. That agent is usually the right one to hold responsible: an agent at the third hop that starts a swarm of a hundred sub-agents is accountable for all of them, whether or not each has a hop of its own. An issuer MAY therefore set max_hops to bound the size of a record, or to limit how much of an organisation's structure a record exposes. Opaque identifiers (Section 9.1) hide who is in a chain but not how deep it goes; max_hops limits the depth shown.¶

Earlier revisions argued against max_hops: where an application gated actions on a valid chain, an exhausted budget pushed an agent that still needed to delegate to do so without appending a hop. That argument depended on the token being an input to access decisions. It no longer is (Section 1.1). A full chain stops no action, so reaching max_hops gives no agent a reason to hide what it does.¶

Delegation without appending a hop remains possible whatever max_hops says, because nothing in HDP forces an agent to extend the token. A planner can hand its token unchanged to a sub-agent, or pass the task along in-process, and the sub-agent's actions then go unrecorded or are recorded as the planner's. The chain still verifies, because verification checks only what was recorded. HDP cannot prevent this on its own. Two things limit it: the agent that passed the task on without a hop answers for what was done with it, and a recording component outside the agent's control (Section 3.4) records the hops an agent would leave out.¶

Independently of max_hops, an auditor MAY flag chains longer than a locally configured limit, as a heuristic it can change without reissuing tokens. Chains are expected to grow longer as agents start agents, and Section 8.3 describes how a store can hold them without repeating each chain prefix.¶

10.6. Attribution Across Concurrent Tokens

An agent may hold more than one valid HDP token whose scope covers the same resource: for example, one issued for Alice and one issued for Carol, each authorizing an update to the same dataset. Applications MUST bind an action or attempted action to the task and token that actually triggered it, and agents MUST record it under that context. They MUST NOT select a different token merely because its scope would make the action appear authorized. A deviation from the triggering token's scope SHOULD be recorded under that token, explicitly identified as an attempted, blocked, or observed violation in action_summary. Recording the deviation does not amend scope or assert that the principal approved it. If the triggering context is unknown, the application MUST preserve that uncertainty in its audit record rather than assign an unrelated principal.¶

HDP cannot detect a violation of this rule. A hop appended to the wrong token passes integrity verification, because verification establishes that the hop was recorded, not that it was recorded in the right place. The consequence is borne at audit. An update performed in Carol's task but recorded under Alice's token fits Alice's scope, so nothing in the record gives it away: it implicates Alice and leaves Carol's token silent. An action recorded under a token whose scope does not cover it at least shows up as a deviation; the harmful case is an action both scopes cover. Equally, an action that occurred in Alice's task belongs in Alice's record even if it exceeds her scope; it MUST NOT be moved to Carol's token to make it appear permitted. A signed record alone cannot prove correct task attribution, and inclusion of an action MUST NOT be interpreted as proof that the principal approved it.¶

Where more than one task could legitimately initiate an action, selecting the initiating task is application policy. Applications SHOULD define and record that choice before execution, together with a request or event identifier. Once selected, the triggering context governs provenance even if the action deviates from its scope. Joint approval (Section 7) does not address this case: it covers tokens linked by parent_token_id that share a session_id, and the tokens here are unrelated. The neighbouring question of how a service decides which of several grants applies to a request is an authorization-layer question and is outside HDP (Section 1.1).¶

10.7. Prompt Injection

Prompt injection attacks attempt to cause an agent to act as if it received instructions from a legitimate principal, when in fact the instructions originate from adversarial content in the agent's environment (e.g., a malicious web page or document). HDP does not detect or prevent this attack.¶

Knowing whether a particular use of a permission went against what the principal wanted is a hard problem, and HDP does not attempt it. What HDP does is record every use, including attempted and blocked ones, and leave the judgement to the principal or an auditor, who can read the principal's request and each hop's reading of it side by side. An agent or recording component (Section 3.4) SHOULD record each attempted action, any deviation from the declared scope it noticed, and whether the action was blocked, in action_summary under the triggering task's token. Any blocking is done by the application's own access control, not by HDP, and blocking an action MUST NOT suppress its record. A violation observed after execution SHOULD likewise be recorded as an observed violation, without claiming that the principal approved it or that the signature proves execution. The hop timestamp remains the extension time, not a backdated event time. If the token cannot be extended because its chain has reached max_hops, the application SHOULD retain an integrity-protected incident record linked to the token digest and the triggering request. HDP v0.1 adds no status field for this purpose; the distinction is explicit in the declaration.¶

The record depends on agents, or recording components, actually recording actions. An agent that omits a hop is discussed in Section 10.4 and Section 10.5.¶

10.8. Key Management

The security of all HDP guarantees depends on the confidentiality of the issuer's Ed25519 secret key. Implementations MUST:¶

• Store secret keys in a secrets manager, HSM, or equivalent secure enclave. Secret keys MUST NOT be stored in source code, configuration files, or environment variables in production.¶
• Use distinct key pairs per environment (development, staging, production).¶
• Support key rotation by issuing new tokens with a new kid, and retain every public key an auditor may need, with its compromise history, for the audit retention period (Section 5.1). This does not require retaining retired secret keys.¶

10.9. Offline Verification Guarantee

A correct implementation of integrity verification (Section 5) requires no network calls, no registry lookups, and no third-party contact. The complete trust state it requires is the issuer's Ed25519 public key (32 bytes).¶

This guarantee is a property of the verification procedure, not of the single-key signing model of v0.1. Per-agent hop signing on the pattern described in Section 4.2 would preserve it, since the verifier would still resolve only the issuer's key out of band.¶

This guarantee lets an auditor verify records long after they were made, in an air-gapped environment, and without the issuer's cooperation beyond having published its key.¶

11.1. UCAN

UCAN 1.0 delegations [UCAN-DELEGATION] and invocations [UCAN-INVOCATION] each have an optional meta field, a map that the issuer signs with the rest of the payload. The delegation specification describes meta as asserted, signed data that is not delegated authority, which is the status HDP content needs. This profile places HDP content under a single meta key, hdp. The example is shown in JSON for readability; UCAN 1.0 payloads are encoded in DAG-CBOR.¶

"meta": {
  "hdp": {
    "v"         : "0.1",
    "intent"    : "Analyze Q1 sales data and report.",
    "principal" : { "id": "usr_alice_opaque", "id_type": "opaque" },
    "session"   : "sess-20260326-abc123",
    "summary"   : "Decompose task; delegate the query to sub-agent.",
    "agent"     : { "type": "orchestrator" }
  }
}

The intent, principal, session, and limits members appear only in the first certificate signed by the principal or by the agent the principal prompted. The summary and agent members appear in that certificate and every one below it, including the invocation, as does recorder when present. An invocation's prf field lists the delegations it relies on, so a resource that retains its invocations together with their delegations holds the whole record, and UCAN receipts can record the result of each invocation alongside it.¶

11.2. ZCAP-LD

A ZCAP-LD [W3C.ZCAP-LD] delegated capability is a JSON-LD document signed by the delegator with a Data Integrity proof. An invocation carries the delegated capability, and its capabilityChain embeds the parent capability in full, so content placed in each delegated capability would be signed by its delegator and reach the resource with the chain.¶

The current ZCAP-LD draft does not say where such content belongs. It forbids additional fields in a root capability, says nothing either way about additional fields in a delegated capability, and provides the caveat property only for restrictions on use. HDP content is not a restriction, and MUST NOT be expressed as a caveat, since a caveat is an input to the access decision. The candidate this document puts forward is an additional property, hdp, on each delegated capability, with the same members as the UCAN form and defined in a JSON-LD context listed after the ZCAP-LD context. A root capability can carry no other fields, so a principal who controls the root records the request in the first capability the principal delegates. Whether ZCAP-LD implementations accept a delegated capability with such a property, and whether a context is the right way to define it, are open questions on which review is sought. So is where the invoking agent's content belongs: an invocation made with an HTTP signature has no document to carry it.¶

12.1. HTTP Header Field Registration

This document requests registration of the following HTTP header fields in the "Hypertext Transfer Protocol (HTTP) Field Name Registry" maintained at <https://www.iana.org/assignments/http-fields/>.¶

Header Field Name:

HDP-Token¶

Status:

provisional¶

Reference:

This document, Section 8.1¶

Comments:

Carries a base64url-encoded HDP token for agentic delegation provenance.¶

Header Field Name:

HDP-Token-Ref¶

Status:

provisional¶

Reference:

This document, Section 8.2¶

Comments:

Carries a UUID token_id identifying an immutable token snapshot, or a sha256 content-addressed reference to a complete HDP token.¶

12.2. Media Type Registration

This document requests registration of the application/hdp-token+json media type in the "Media Types" registry, following the procedures of [RFC6838].¶

Type name:

application¶

Subtype name:

hdp-token+json¶

Required parameters:

N/A¶

Optional parameters:

N/A¶

Encoding considerations:

binary; the token is a UTF-8 JSON object [RFC8259].¶

Security considerations:

See Section 10 of this document.¶

Interoperability considerations:

The token uses the "+json" structured syntax suffix [RFC6839]; generic JSON processors can parse it. HDP-specific semantics are defined in this document.¶

Published specification:

This document.¶

Applications that use this media type:

Agentic AI frameworks and services that exchange HDP delegation-provenance tokens.¶

Fragment identifier considerations:

N/A¶

Additional information:

Deprecated alias names: none. Magic number(s): none. File extension(s): none. Macintosh file type code(s): none.¶

Person & email address to contact for further information:

Asiri Dalugoda <protocol@helixar.ai>¶

Intended usage:

COMMON¶

Restrictions on usage:

None¶

Author:

Asiri Dalugoda¶

Change controller:

IETF¶

12.3. Well-Known URI Registration

This document requests registration of the following entry in the "Well-Known URIs" registry, per [RFC8615].¶

URI suffix:

hdp-keys.json¶

Change controller:

IETF¶

Specification document:

This document, Section 8.4¶

Status:

provisional¶

Related information:

Serves a JSON document listing an issuer's Ed25519 public keys for HDP token verification.¶

13.1. IPP (draft-haberkamp-ipp-01)

The Intent Provenance Protocol [I-D.haberkamp-ipp] and HDP address the same root problem with different architectural trade-offs. The key differences are:¶

1. Revocation model. IPP -01 Section 8 describes its revocation registry as a distributed service at an endpoint specified by the token. IPP requires agents to poll at the configured interval, with a recommended default of 5,000 milliseconds; for high-stakes actions IPP recommends an additional check immediately before acting. When the registry is unreachable, IPP permits action only if the token supplies offline_grace_period_ms and the offline duration remains within that period. Otherwise IPP prohibits proceeding. HDP has no revocation: an HDP token is a record and is never honoured or refused (Section 1.1), so there is nothing to revoke. Withdrawing authority belongs to the access control mechanism HDP travels alongside.¶
2. Trust anchor. IPP tokens contain a genesis object (the Genesis Seal), a cryptographic artifact linking every token to the specification author's public key at https://ipp.khsovereign.com/keys/founding_public.pem. Self-hosted IPP deployments are cryptographically bound to this third-party key. HDP tokens carry no genesis seal and no spec-level attribution; any organization can issue and verify HDP tokens without anchoring to a third party.¶
3. Identity model. IPP mandates W3C DID Core-conformant principal identifiers. HDP supports id_type: "opaque" as a first-class option, making DID infrastructure optional rather than required.¶

These are design choices, not defects. Deployments with reliable connectivity to a revocation service, existing DID infrastructure, and a requirement for revocation across token ancestry may prefer IPP. Deployments that prioritize offline operability, self-sovereignty, and minimal infrastructure may prefer HDP.¶

13.2. OAuth 2.0 Token Exchange (RFC 8693)

OAuth 2.0 Token Exchange [RFC8693] defines a mechanism for exchanging one security token for another, including delegation and impersonation use cases. HDP and RFC 8693 are complementary rather than competing: RFC 8693 governs access token issuance and delegation in an OAuth 2.0 authorization server context, while HDP governs the provenance record that travels with an agentic task regardless of the authentication mechanism used.¶

HDP tokens do not replace OAuth access tokens. An agent framework MAY use OAuth 2.0 for resource authorization and HDP for delegation provenance simultaneously.¶

13.3. JSON Web Token (RFC 7519)

JSON Web Token [RFC7519] provides a general-purpose signed claims format. HDP differs from JWT in three respects:¶

• HDP tokens carry an append-only, per-hop-signed delegation chain (chain) that has no equivalent in the JWT standard claims set.¶
• HDP uses RFC 8785 canonical JSON for signing payloads, rather than the base64url-encoded header.payload convention used by JWS [RFC7515]. This allows direct JSON manipulation without base64 decoding.¶
• HDP's integrity verification is specific to agentic delegation records (per-hop signatures chained to the root signature) rather than general-purpose.¶

13.4. UCAN (User Controlled Authorization Networks)

UCAN [UCAN] defines a capability-based authorization token system with chained delegation. HDP and UCAN share the concept of delegation chains but differ significantly in scope: UCAN is a general capability authorization system, while HDP is specifically a provenance record for human-authorized agentic tasks. HDP makes no claims about capability enforcement; UCAN tokens carry executable capabilities that are enforced by receiving systems.¶

A UCAN delegation records who delegated what to whom, and UCAN's Invocation and Receipt objects record individual invocations and their results. A resource can retain them, and then holds a record of the authority exercised. What those objects do not record by default is the principal's request and each agent's reading of the task. A UCAN deployment can carry that content in its meta fields (Section 11.1) rather than in a separate HDP token. The standalone HDP token is intended for deployments with no capability chain to carry it, such as those using bearer tokens or API keys, or handing tasks between agents in-process.¶

13.5. ZCAP-LD (Authorization Capabilities for Linked Data)

ZCAP-LD [W3C.ZCAP-LD] expresses delegated authorization capabilities as Linked Data, with invocation and delegation rooted in a controller's key. As with UCAN, a resource that retains the capability chains presented to it holds a record of the authority exercised. What the chain does not record is the principal's request or each agent's reading of the task. HDP neither defines nor enforces capabilities. Section 11.2 discusses carrying HDP content in delegated capabilities; the form it takes there is an open question.¶

13.6. ODRL and the Verifiable Credentials Data Model

The Open Digital Rights Language (ODRL) [W3C.ODRL] is a W3C Recommendation for expressing permissions, prohibitions, and constraints. Several fields in HDP's scope object (Section 3.3) overlap with concepts ODRL already defines: authorized_tools and authorized_resources correspond to ODRL actions and targets, and network_egress and persistence map to ODRL permissions or prohibitions.¶

HDP v0.1 deliberately retains a small, self-contained scope object rather than embedding an ODRL policy. The trade-off is explicit: the minimal object keeps tokens compact and implementable with only JSON and Ed25519, at the cost of the vocabulary reuse, policy composability, and tooling interoperability that ODRL provides. Deployments that already reason over ODRL policies will require a separate mapping to interpret HDP scopes.¶

A further limitation of the v0.1 scope object is that it is fixed at issuance. HDP has no attenuation: a delegate cannot narrow the scope at its own hop, because hops record actions rather than grants and a hop record has no field in which a narrower scope could be expressed. A delegate that wishes to pass on less than it received must obtain a new token from the issuer with a narrower scope. Per-hop caveats, which only the verifier and any attenuating agent would need to interpret, are planned for a future version.¶

Because the chain-of-custody mechanism is payload-agnostic (Section 1.5), a future HDP profile MAY carry an ODRL policy as its payload in place of the native scope object. Such a profile would gain a natural binding to the Verifiable Credentials Data Model 2.0 [W3C.VC-DATA-MODEL-2.0], whose termsOfUse property can carry ODRL policies. This binding is identified as future work and is not specified in this document.¶