21 BTCCashu
Cashu is an open-source ecash protocol for Bitcoin. A mint holds bitcoin and issues blinded tokens that are private, instant and can be passed around as plain text; the trade-off is that the mint is custodial.
How it works
You pay the mint over Lightning and receive tokens signed with blind signatures (Chaum, 1982), so the mint cannot link who later spends them. Tokens move between wallets as text and are redeemed back to Lightning. Specifications are numbered NUTs; NUT-24 describes paying for HTTP resources with an X-Cashu header after a 402 reply.
For AI agents a token works like a prepaid API key: Routstr, for example, accepts a Cashu token as the key of an OpenAI-compatible API. Keep balances small: a mint can disappear with its reserves. Fedimint spreads that trust across a federation.
See also
- FedimintFedimint is an open-source protocol for federated ecash: a group of guardians jointly holds bitcoin and issues private ecash notes, so no single operator controls the funds. It connects to Lightning through gateways.
- Lightning NetworkLightning is a payment network built on top of Bitcoin. Two parties lock bitcoin in a shared channel and then pay each other instantly and cheaply off-chain; payments can hop across channels to reach anyone.
- L402L402 is a protocol for paying for web resources with Lightning inside an ordinary HTTP request. The server replies 402 "Payment Required" with an invoice and a token; the client pays, retries with the proof and gets access. It needs no account or API key, which suits software agents.
- AI agents and BitcoinAI agents are programs that do work on their own and increasingly need to pay for it: an API call, a page of data, a minute of compute. They cannot open a bank account, but they can hold a Bitcoin key and pay over Lightning in amounts far smaller than a card can handle. That is the case for why the next billion Bitcoin users may not be people. It is a strong argument with real caveats.
References
- Cashu NUT specifications github.com
- NUT-24: HTTP 402 Payment Required github.com
- cashu.space cashu.space
- Routstr Core (GitHub) github.com