fedimintcard Experimental prototype

Fedimint ecash on a card.

Fedimint Card is an experimental JavaCard applet. The card holds the spend keys for Fedimint ecash notes and signs each spend itself when you tap it at a till.

Back of the card: a signature panel, a QR code to fedimintcard.org and a short description Front of the card: the fedimintcard logo, a gold chip and the words Fedimint ecash card

How a payment works

The card does one job: it signs with keys that were made on its chip. The till and the federation do everything else.

  1. Load the card

    Ecash from a Fedimint wallet is reissued to keys the card generated on its own chip. The federation issues the new notes to those keys.

  2. Tap at the till

    The till builds the transaction and the card signs it with the note's key. The card sees only the 32-byte transaction hash.

  3. The federation settles

    The guardians check the signature and mark the note spent. Change goes back to keys on the card in the same transaction.

What the spike showed

Run in October 2026 on a stock Fedimint federation, with no change to its consensus code.

Card
NXP J3R180 (JCOP 4), a dual-interface JavaCard
Federation
Four guardians, Fedimint mintv2 module
Keys
Generated on the card; the applet has no command that returns a private key
Signing time
About 850 ms per BIP340 signature on the card
Rejected as expected
A signature from another key, a second spend of the same note, and outputs changed after signing
End to end
Ecash moved from a phone wallet to the card, paid a till by tap and was cashed out again

Fedimint Card is an experimental prototype and has had no outside security review. Keep only small amounts on a card.