NIP-61:

Nut Zaps

A Nut Zap is a P2PK cashu token where the payment itself is the receipt.

High-level flow

Alice wants to nutzap 1 sat to Bob because of an event event-id-1 she liked.

Alice nutzaps Bob

Alice fetches event kind:10019 from Bob to see the mints Bob trusts.
She mints a token at that mint (or swaps some tokens she already had in that mint) p2pk-locked to the pubkey Bob has listed in his kind:10019.
She publishes a kind:7337 event to the relays Bob indicated with the proofs she minted.

Bob receives the nutzap

At some point, Bob's client fetches kind:7337 events p-tagging him from his relays.
Bob's client swaps the token into his wallet.

Nutzap informational event

{
    "kind": 10019,
    "tags": [
        [ "relay", "wss://relay1" ],
        [ "relay", "wss://relay2" ],
        [ "mint", "https://mint1" ],
        [ "mint", "https://mint2" ],
        [ "pubkey", "<p2pk-pubkey>" ]
    ]
}

kind:10019 is an event that is useful for others to know how to send money to the user.

relay - Relays where the user will be reading token events from. If a user wants to send money to the user, they should write to these relays.
mint - Mints the user is explicitly agreeing to use to receive funds on. Clients SHOULD not send money on mints not listed here or risk burning their money.
pubkey - Pubkey that SHOULD be used to P2PK-lock receiving nutzaps. If not present, clients SHOULD use the pubkey of the recipient.

Nutzap event

Event kind:7337 is a nutzap event published by the sender, p-tagging the recipient. The outputs are P2PK-locked to the pubkey the recipient indicated in their kind:10019 event or to the recipient pubkey if the kind:10019 event doesn't have a explicit pubkey.

Clients MUST prefix the recipient pubkey with "02" (for nostr<>cashu pubkey compatibility).

{
    kind: 7337,
    content: "[{\"amount\":1,\"C\":\"02277c66191736eb72fce9d975d08e3191f8f96afb73ab1eec37e4465683066d3f\",\"id\":\"000a93d6f8a1d2c4\",\"secret\":\"[\\\"P2PK\\\",{\\\"nonce\\\":\\\"b00bdd0467b0090a25bdf2d2f0d45ac4e355c482c1418350f273a04fedaaee83\\\",\\\"data\\\":\\\"02eaee8939e3565e48cc62967e2fde9d8e2a4b3ec0081f29eceff5c64ef10ac1ed\\\"}]\"}]",
    pubkey: "sender-pubkey",
    tags: [
        [ "amount", "1", "sats" ],
        [ "comment", "Great point" ],
        [ "u", "https://stablenut.umint.cash", ],
        [ "e", "<zapped-event-id>", "<relay-hint>" ],
        [ "p", "e9fbced3a42dcf551486650cc752ab354347dd413b307484e4fd1818ab53f991" ], // recipient of nut zap
    ]
}

.content is an array of p2pk-locked outputs.
amount is a shorthand for the combined amount of all outputs. -- Clients SHOULD validate that the sum of the amounts in the outputs matches.
u is the mint the URL of the mint EXACTLY as specified by the recipient's kind:10019.
e zero or one event that is being nutzapped.
p exactly one pubkey, specifying the recipient of the nutzap.

Sending a nutzap

The sender fetches the recipient's kind:10019.
The sender mints/swaps ecash on one of the recipient's listed mints.
The sender p2pk locks to the recipient's specified pubkey in their

Receiving nutzaps

Clients should REQ for nut zaps:

Filtering with #u for mints they expect to receive ecash from.
- this is to prevent even interacting with mints the user hasn't explicitly signaled.
Filtering with since of the most recent kind:7376 event the same user has created.
- this can be used as a marker of the nut zaps that have already been swaped by the user -- clients might choose to use other kinds of markers, including internal state -- this is just a guidance of one possible approach.

Clients MIGHT choose to use some kind of filtering (e.g. WoT) to ignore spam.

{ "kinds": [7337], "#p": "my-pubkey", "#u": [ "<mint-1>", "<mint-2>"], "since": <latest-created_at-of-kind-7376> }.

Upon receiving a new nut zap, the client should swap the tokens into a wallet the user controls, either a NIP-60 wallet, their own LN wallet or anything else.

Updating nutzap-redemption history

When claiming a token the client SHOULD create a kind:7376 event and e tag the original nut zap event. This is to record that this token has already been claimed (and shouldn't be attempted again) and as signaling to the recipient that the ecash has been redeemed.

Multiple kind:7337 events can be tagged in the same kind:7376 event.

{
    "kind": 7376,
    "content": nip44_encrypt([
        [ "direction", "in" ], // in = received, out = sent
        [ "amount", "1", "sats" ],
        [ "e", "<7375-event-id>", "relay-hint", "created" ] // new token event that was created
    ]),
    "tags": [
        [ "a", "37375:<pubkey>:my-wallet" ], // an optional wallet tag
        [ "e", "<7337-event-id>", "relay-hint", "redeemed" ], // nutzap event that has been redeemed
        [ "p", "sender-pubkey" ] // pubkey of the author of the 7337 event (nutzap sender)
    ]
}

Events that redeem a nutzap SHOULD be published to the sender's NIP-65 relays.

Verifying a Cashu Zap

Clients SHOULD check that the receiving user has issued a kind:10019 tagging the mint where the cashu has been minted.
Clients SHOULD check that the token is locked to the pubkey the user has listed in their kind:10019.

Final Considerations

Clients SHOULD guide their users to use NUT-11 (P2PK) compatible-mints in their kind:10019 event to avoid receiving nut zaps anyone can spend
Clients SHOULD normalize and deduplicate mint URLs as described in NIP-65.
A nut zap MUST be sent to a mint the recipient has listed in their kind:10019 event or to the NIP-65 relays of the recipient, failure to do so may result in the recipient donating the tokens to the mint since the recipient might never see the event.

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