The Cashu protocol has launched an experimental physical NFC card designed for offline Bitcoin-backed ecash transactions. The card utilizes a JavaCard chip to store bearer tokens, allowing users to make payments at terminals without an active internet connection.
The hardware is based on NXP J3R180 smart cards, which are available for purchase on Amazon in packs of two or five. Each card costs approximately $9 and features up to 128 slots for storing ecash tokens.
The system operates on the NUT-CCP specification. When a payment occurs, the card generates a signature that burns the spent tokens, preventing double-spending. Merchants settle the transactions with the issuing mint once their terminals reconnect to the internet.
Security is managed through blind signatures and P2PK spending conditions, ensuring that tokens are tied to a key stored on the chip. The project documentation notes that if a card is lost, the stored balance cannot be recovered.
The protocol relies on the solvency and integrity of the issuing mint. Merchants face a period of exposure to potential double-spending between the moment of the offline tap and the final settlement with the mint.
