Skip to content

Mint Notes and CMU

Clear distinguishes the issuer, the bearer instrument it issues, and the unit used to denominate that instrument.

A transferable unit is the general category for a fungible unit that can move between holders under an issuer's policy. A Clear Mint Unit (CMU) is Clear's concrete form of a transferable unit. A Mint Note is the unique bearer instrument that represents a fixed quantity of that CMU.

Mint
└── operates a keyset
    └── defines cmu-<keyset-id>
        └── issues Mint Notes denominated in that CMU

Mint

A Mint is a cryptographic issuer and redeemer of bearer instruments. It operates cryptographic keysets, issues Mint Notes, validates returned notes, and prevents the same note from being redeemed twice.

Mint Note

A Mint Note is a cryptographically issued bearer instrument representing a fixed denomination. It is issued by one mint keyset, transferable by the protocol, and redeemable by the issuing mint.

Examples include:

  • a 1 CMU Mint Note;
  • a 64 CMU Mint Note; and
  • a 1024 CMU Mint Note.

Clear Mint Unit

A Clear Mint Unit, abbreviated CMU and pronounced as the letters “C-M-U,” is Clear's concrete form of a transferable unit. It is defined by a mint keyset, and its name reflects Clear's role in issuing, redeeming, and clearing Mint Notes under an explicit policy. Its canonical protocol identifier is:

cmu-<keyset-id>

For example:

cmu-00a1b2c3d4e5f6

The complete CMU and authenticated issuer policy identify the balance. A mint endpoint is a service route, and one logical mint may expose a mint cluster of authorized instances. Notes from different logical mints or keysets are not interchangeable merely because both display CMU.

Portable keysets and several instances

A treasurer may be separately appointed as the custodian or delegated signer for a keyset. With mint-operator approval, that keyset can be enrolled in an existing Clear deployment. Possession of the keyset secret is cryptographic issuance power, so the role must be explicit and the secret strongly protected.

The same keyset can be served through a mint cluster, allowing Mint Notes in one CMU to be issued or redeemed at different endpoints. The active policy identifies which member mints must be checked for double-spend state. Cluster members need synchronous shared state or a reserve-and-commit protocol; periodic synchronization alone leaves a race in which two mints could accept the same note. Independent instances must use different keysets and CMUs.

Keyset rotation creates a new CMU

A new keyset defines a new Clear Mint Unit. The old and new notes remain separate unless the issuer offers an explicit migration or exchange:

cmu-<old-keyset-id> != cmu-<new-keyset-id>

A durable currency root may authorize both keysets, but it does not silently make their notes equivalent. Wallets must keep the balances distinct and show the terms of any conversion.

Mint Notes and Cashu proofs

Mint Note is the product and protocol term for the bearer instrument. Cashu proof remains the technical term for the structure containing its amount, keyset ID, secret, and unblinded signature. A Mint Note issued using Chaumian blind signatures may also be described as a Chaumian Note.

Redemption and retirement

Redemption means returning a Mint Note to its issuing mint for validation and consumption. Retirement is the broader terminal outcome: units are permanently removed from circulation. Redemption is the normal path to retirement, but an issuer's policy may also require retirement following expiration, revocation, cancellation, reconciliation, or another authorized event.

The current retire operation consumes presented notes and records retired supply. Automatic expiration and administrative revocation of unpresented bearer notes require separate enforcement mechanisms and are not yet implemented. Retirement does not by itself promise a Bitcoin, fiat, goods, or service payout; the issuer’s policy defines any external consequence.

Canonical unit

Clear now exposes cmu-<keyset-id> as the protocol contract across its API, database identity, and circulating tokens.

The complete normative vocabulary is maintained in the repository specification.