Apps and AI agents increasingly need to pay for things, but today that usually means handing them a wallet key, which lets them spend everything, anywhere. Firebreak gives them a limited key instead.
The owner sets aside an allowance as a handful of fixed-value vouchers, say 50, 20, 20 and 10. Each voucher is locked on chain so that only two things can ever happen to it: the app‘s key pays it, whole, to one merchant the owner chose up front, or the owner takes it back. The app can’t change the destination, take change, or spend more than the vouchers hold. This holds even if its key is stolen, because the chain rejects anything else, not the app.
Payments stay private: the public sees that transactions happened, but not the amounts. The merchant can open its receipts and spend the money normally.
In the demo, an owner with 1,000 gives an app 100. The leaked app key is then used to try to steal it, and every attempt is rejected. The app pays 60, and the owner reclaims the unused 40, ending at 940 for the owner and 60 for the merchant.
The trade-off is that payments must add up exactly from the remaining vouchers, like paying with exact cash.