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Keystone Enabled A receipt or audit chain tells an auditor what was decided. It does not, on its own, tell them under which ruleset that decision was made — and if the issuer quietly rotates its policy, an old record gives no signal that the rules changed underneath it. Policy Binding closes that gap. It binds a content hash of the policy snapshot to an existing, frozen reference (a settlement-action binding_ref, or a retention_chain_ref) so the bound record becomes version-provable (you can prove exactly which policy was in force) and rotation-detectable (a record sealed under policy P fails recomputation under a rotated policy P'). It is strictly additive over the frozen Layer 1. It introduces no new cryptographic primitive and changes nothing in the canonicalisation substrate — it composes the same RFC 8785 JCS + SHA-256 already in use.
Apache-2.0 open source. Install via pip install algovoi-policy-binding. Python and TypeScript are byte-for-byte identical on the same input. Adopters who pin and verify against the canonical vectors qualify for a free v0 licence key for the mandate auditor — see Adopters below.

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How it works

Two references, both computed with RFC 8785 JCS canonicalisation and SHA-256:
  • policy_ref is the content hash of the policy/ruleset/mandate snapshot itself. Because the policy is JCS-canonicalised first, key order in the policy document does not affect the result.
  • policy_bound_ref binds that policy hash to a subject_ref — the frozen reference of the thing the policy governed. The construction is version-agnostic: the subject_ref is imported by hash, so the same construction binds a policy to a settlement-action binding_ref (v1) or to a retention_chain_ref (v0 or v1) without any change.
Because subject_ref is imported by hash, Policy Binding does not need to know — and is not coupled to — the internal shape of whatever it binds. The subject can evolve independently; the binding stays valid as long as the subject hash is unchanged.

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What an auditor can verify

No issuer call. No registry lookup. No AlgoVoi service. RFC 8785 JCS, SHA-256, and a JSON parser are the entire dependency.

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Invariants

The construction holds five invariants, each backed by a conformance check:

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Conformance vectors

policy_binding_v1 — 14 checks across three subject types and both rotation directions. Cross-validated Python + TypeScript, byte-for-byte. Subjects (the published canonical references from the conformance corpus): Policy references: Bound references (policy_bound_ref), six positives — every subject under both P and P': Plus three negative rotation cases (one per subject: sealed under P, must reject under P') and two invariant assertions (key-order-invariance, subject-binding).

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Adopters

If you build on algovoi-policy-binding and verify against the canonical vectors, you qualify for a free v0 licence key for algovoi-mandate-auditor — AlgoVoi’s production-grade MiCA/DORA compliance audit service for payment mandate charge chains. Qualification criteria (enforced by scripts/check_v0_adoption.py):
  1. algovoi-policy-binding declared as a dependency, pinned to a specific version (==0.1.0) — a bare version range is rejected
  2. At least one canonical vector hash from policy_binding_v1 in your conformance tests
  3. A NOTICE file preserving the Apache-2.0 attribution
To apply: email [email protected] with your dependency file, a snippet showing the canonical hash in your test suite, and a copy of your NOTICE file. If all criteria are met, a v0 key is issued within one business day.

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Relationship to the open substrate

Policy Binding sits directly on top of the open JCS Canonicalisation Substrate and composes with the Retention Chain and settlement-action binding. It uses the same RFC 8785 JCS and SHA-256 primitives — no additional cryptographic dependencies.
It enables the security; it is realized only when a verifier or gate rejects on a policy_bound_ref mismatch — that is a runtime posture decision, not a property of the substrate itself. Also available natively in the commercial Substrate 2 (substrate2.policy_binding), carried to adopters through the existing pin-and-licence channel.

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Specification

The normative specification is IETF Internet-Draft draft-hopley-x402-retention-chain-05 (AlgoVoi, sole authorship, 19 June 2026), which adds:
  • Section 7.7 — Policy Binding: the policy_ref / policy_bound_ref construction and its invariants (Stability, Snapshot-binding, Rotation-detection, Subject-binding, Key-order-invariance)
  • Section 8.10 — Policy Binding Vectors: the exact preimage inputs, JCS canonical forms, and expected byte sequences for policy_binding_v1