Questions and answers

An open protocol and a Java Card applet that put Cashu ecash on an NFC smart card. A wallet loads ecash onto the card, and its holder pays by tapping the card on a merchant's phone or till. The till can accept the tap while offline and settle with the mint once it is connected again.

Cashu is an ecash protocol for Bitcoin. A mint issues ecash backed by the bitcoin it holds, and wallets pass that ecash between people privately. Its specifications are called NUTs and live in cashubtc/nuts. cashu.space introduces it.

NUT-CCP, the Cashu Card Protocol, is a draft NUT written by this project and kept in cashucard/nut. It is written for proposal to cashubtc/nuts, where it is still to be proposed, numbered or merged. Until the Cashu maintainers assign it a number, it is called NUT-CCP. It has had little testing or review outside this project.

The protocol is a draft, the applet has run end to end on one card model with a few phones, and an independent security review is still to come. Expect changes. The move from the 0.x applet to 2.0 changed the card's commands, and cards on 0.x had to be emptied and reinstalled; a later change could need the same.

Its contributors, in the open, in the cashucard organization on GitHub. The code is under the MIT license, so anyone can read, use, change and ship it.

The software is free. Blank cards cost money: Where to get cards lists sellers, from about $6 to $12 a card. A mint can charge a fee on the swaps a payment makes, and a wallet or card issuer can charge for its own service. Those fees are theirs to set.

Buy blank NXP J3R180 cards (Where to get cards), install the applet with the Cashu Card Provisioner app or the install guide, then set the card up in a wallet that supports Cashu Card. Custom printed cards with the applet installed are coming soon.

Wallets that implement NUT-CCP and the card's profiles. The install guide's setup step lists the wallets it uses, under Apps at the foot of that page. Support is early, and the developer page is the place to start for a wallet that wants to add it.

A wallet that supports Cashu Card names the card's mint, then writes ecash onto it, each piece locked to a key the card made on its own chip. A card holds up to 128 pieces, all from one mint, in sats.

Only small amounts you can afford to lose. A lost or broken card loses its balance, and the project is experimental. The risks explain why.

The mint. The card's ecash is a claim on one mint, named on the card when it is set up, and that mint holds the bitcoin that backs it. You trust the mint to honor its ecash. A reader can check that each piece was signed by the mint; whether the mint stays solvent is a matter of trust in its operator.

Its balance is lost. The key that spends the card's ecash is made on the chip and stays there, so a backup of your wallet cannot recreate it. This is a deliberate design choice, recorded as decision D5. The applet has a dated refund path for a lost card, and the apps keep it switched off for now.

The October 2026 tests used named Android phones and iPhones with NFC, recorded in the end-to-end report and the reset report. Other phones with NFC may work and are untested.

A Cashu Card pays readers that speak its protocol, such as a merchant's phone running a till app that supports it. Bank card terminals use EMV, a different system, and they cannot read a Cashu Card's ecash.

Yes. The card issuer that installed it can reset it from the Cashu Card Provisioner, once the holder has moved its value off and pending payments have settled. The card is then installed again with a new key and set up again in a wallet.

Up to the limits the card's owner set. On a card with a PIN, a payment needs the PIN unless it fits the no-PIN allowance the owner chose. A card with no PIN pays whoever holds it, like cash, up to its daily limit and its limit on one payment.

Three wrong PINs block the card, and any reader in range can send them. The owner's wallet sets a new PIN with the card's owner key, which unblocks it. If the owner key is lost as well, a blocked card's balance is stranded.

When you set a card up, your wallet makes an owner key that administers the card: its PIN, its limits and its reset. Keep your wallet's backup. If the owner key is gone, you can still pay with the PIN, but a blocked PIN stays blocked, and the daily limit renews only a set number of times (7 by default) before the card stops paying. A balance above about eight daily limits is then stranded.

Yes. Any reader can read the card's balance, its pieces and its payment history without the PIN. Spending takes a signature that only the card's chip can make, and the card makes it only within the limits above.

A reader has to be within NFC range, and it can spend only up to the card's limits described above. The project has yet to measure the card's range or test whether draining a card at a distance is practical.

An independent security review is still to come. The project's security model lists each threat, its control and the open items, among them pulling a card mid-write and timing attacks, which are still untested on silicon.

The till reads the card's pieces and names the ones it wants. The card signs once for the whole payment and marks those pieces spent. The till checks the mint's signature on each piece without going online and accepts the payment. Back online, it swaps the pieces at the mint, and the payment settles. One tap, step by step shows it.

Until the till settles, the merchant carries the risk that a piece was spent elsewhere first, and that the mint fails. Settling promptly shortens that window, and a till that is online checks with the mint before it accepts. The card's limits and small balances keep the amount at stake small.

Yes. The protocol is open, and the developer page links the draft, the card's commands byte for byte, test vectors, a client library and the applet's source.

Report a bug as a GitHub issue. Report a security problem privately to security@cashucard.org or through a security advisory, as the security policy asks, so it can be fixed before it is public.

Start with the contributing guide. Documentation, the command-line tools, the client library and reading the spec against the code all need help, and none of them needs a card or a reader.

In GitHub Discussions, on the main repository. Questions about the draft NUT, the applet and the client library all go there.