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Protocol Model

Prototype protocol

This page describes the endpoints implemented in the first Clear prototype. The planned governance layer replaces the operator token with signed treasurer instructions governed by a root-signed policy. See How Clear Is Governed.

Clear preserves the central Cashu model:

  • blinded Diffie-Hellman key exchange over secp256k1;
  • denomination-specific keys grouped into a keyset;
  • blinded outputs and blind signatures;
  • bearer proofs containing a secret and unblinded signature;
  • atomic proof swaps;
  • spent-secret tracking; and
  • proof-state checks.

In product and protocol language, the resulting bearer instruments are Mint Notes denominated in a Clear Mint Unit (CMU). In implementation language, each spendable Mint Note is represented by a Cashu proof. The distinction keeps the instrument understandable without renaming Cashu data structures.

Clear issuance method

Cashu NUT-04 provides a general quote flow and allows method-specific settlement mechanisms. Clear introduces the experimental method name clear:

  1. A wallet requests a clear mint quote for one exact cmu-<keyset-id>.
  2. The quote begins with no authorized amount.
  3. The prototype authorizes the quote through its protected operator boundary.
  4. The wallet submits blinded outputs up to the authorized amount.
  5. Clear returns blind signatures and records issued supply.

The prototype derives its initial keyset secret locally and never accepts it over the HTTP API. New keysets will instead be generated from independent random secrets inside the mint after a signed, bounded treasurer authorization. The mint encrypts those secrets at rest and never returns them through a routine API.

The prototype uses a separate operator token for routine actions. The intended model instead requires a signed authorization from a treasurer named in the active policy. The operational signer remains in mint custody and releases signatures only after authorization and ledger checks succeed.

The Clear Mint Unit is not operator-selected. The target protocol identifier is formed from the exact NUT-02 keyset ID:

cmu-<keyset-id>

Each new keyset creates a new CMU. A currency root may authorize several keysets, but it does not make their Mint Notes automatically interchangeable. A configured friendly name remains presentation metadata.

Canonical identifier implemented

The current service exposes cmu-<keyset-id> across API responses, database identity, and circulating tokens.

Clear redemption and retirement

Redemption and retirement are intentionally distinct from Cashu melt-to-payment behavior:

  1. The organization receives Mint Notes under its own policy.
  2. The treasurer submits their Cashu proofs to the protected retirement endpoint.
  3. Clear validates the notes and confirms they are unspent.
  4. Their secrets are atomically marked spent, completing protocol redemption.
  5. The amount is recorded as retired supply.

Retirement is the generalized terminal state, while redemption describes the normal holder-initiated event that reaches it. A future policy profile could also retire units because they expired, were revoked or cancelled, or required reconciliation. Those paths need explicit authorization, proof-selection, and accounting semantics. The current endpoint retires only proofs presented to the mint; it does not invalidate unseen bearer notes automatically.

Compatibility boundary

Mint Notes use normal Cashu proof structures and swaps. However, clear is not currently a published Cashu NUT, and many wallets assume familiar units or Lightning settlement. Compatible wallets must support keyset-bound CMU strings and the Clear quote workflow.

The implementation follows the public Cashu NUTs and takes architectural guidance from the Python Nutshell reference implementation without presenting Clear as a drop-in replacement.