A contactless card the size of a bank card, built on the same chip technology. It keeps your OneEur key on its chip and signs payments when you tap it on your phone.
Bank cards carry a small, tamper-resistant computer (a smart card chip, often running Java Card) that talks to readers without contact over NFC. The OneEur card uses this same well-proven technology and the standard bank-card size, so it fits in any wallet.
The card creates your wallet’s key itself and keeps it inside the chip, which is built so that keys can’t be read out. Your phone only ever gets the signature.
Banks lock the chips on the cards they issue, so only the bank can put software on them. To pay with OneEur by card you need a OneEur card.
In the OneEur app, enter the address and amount, or scan a QR code, as you do today. The app builds the block.
Hold the card to the back of your phone. After you enter your PIN, the chip hashes the block with SHA-256 and signs it with Ed25519. This takes about a second.
The phone sends the signed block to the network. Chain holders check it exactly like any other payment, so card wallets and phone wallets can pay each other.
The key isn’t stored on your phone, so malware or a stolen phone can’t take it. Without the card, nothing can be signed.
The chip itself checks your PIN and stops working after too many wrong tries, so a lost card can’t be guessed open.
A card signature is an ordinary Ed25519 signature on the same ONEEUR-BLOCK-V1 bytes. Nothing changes for chain holders or the explorer.