An experimental NFC card stores Bitcoin-backed ecash on a chip, letting merchants accept a tap without internet and settle later
Cashu wants your Bitcoin to work like a transit card. The open-source ecash protocol has introduced an experimental physical tap-to-pay card that lets users make offline payments with Bitcoin-backed ecash.
The idea targets one of the oldest complaints about paying with Bitcoin at a shop counter. Lightning payments are fast, but both sides usually need to be online. Cashu’s card is built for the moment when the internet isn’t.
How the Cashu card works
Cashu is a Chaumian ecash protocol designed for Bitcoin and the Lightning Network. Think of it as digital cash tokens issued by a “mint,” which acts a bit like a bank vault holding the underlying Bitcoin.
The new card takes those bearer tokens and stores them directly on a JavaCard chip. A wallet loads ecash onto the card, the card gets tapped at a terminal, and value moves without either party touching the internet in that moment.
Merchants can accept the tap offline. Once their terminal reconnects, they settle the tokens with the issuing mint.
The chip can hold ecash across up to 128 slots, which wallets fill when loading the card. Each tap produces a single signature, and that signature burns the tokens being spent.
The project runs on a draft specification called NUT-CCP. Cashu’s existing technical standards are known as NUTs, so this one slots into the same family. The project lives at cashucard.org, which provides the applets needed for compatible hardware.
The hardware, and the DIY part
This is not a card that arrives in a branded envelope with a welcome letter. The cards are built on dual-interface NXP J3R180 smart cards, and blank versions are sold on Amazon.
The listing comes in packs of 2 or 5, at about $9 a card. It names both the dual interface and the default SCP02 keys, which matters to anyone planning to write software onto the chip.
Users install the open-source application on the blank card themselves.
Privacy and security under the hood
The card leans on features Cashu already uses. The first is blind signatures, the core idea behind Chaumian ecash.
With blind signatures, the mint signs tokens without being able to link them to the person who later spends them. That gives ecash its privacy properties.
The second is P2PK spending conditions, short for pay-to-public-key. Tokens can be locked so that only the holder of a specific key can spend them.
Cashu’s card ties stored value to a key on the chip. That on-chip key is never released, so the card itself must produce the signature for any spend. Copying the token data alone shouldn’t be enough to drain the balance.
The single-signature burn on each tap is designed to stop the same tokens from being used twice.
The risks Cashu spells out
The project’s documentation is frank about trade-offs. The first is loss. If you lose the card, the balance on it is gone permanently.
The second risk is mint trust. Ecash depends on the mint that issued it, so holders rely on that mint staying solvent and honest. The card does not change this. It just lets the tokens travel offline.
The third risk falls on merchants. Because settlement happens after the tap, a merchant accepting an offline payment carries exposure to double-spending until they reconnect and redeem with the mint.
The on-chip burn mechanism is meant to prevent tokens from being spent twice. Still, the merchant only gets final confirmation once the mint accepts the tokens, and the documentation flags that gap. End-to-end testing of the cards has so far been limited.
Part of a bigger offline push
The card is one piece of a wider Cashu ecosystem effort aimed at offline and contactless payments. Additional projects, including merchant applications, are planned.
