This NIP defines two new event types for recording lightning payments between users. `9734` is a `zap request`, representing a payer's request to a recipient's lightning wallet for an invoice. `9735` is a `zap receipt`, representing the confirmation by the recipient's lightning wallet that the invoice issued in response to a zap request has been paid.
Having lightning receipts on nostr allows clients to display lightning payments from entities on the network. These can be used for fun or for spam deterrence.
1. Client calculates a recipient's lnurl pay request url from the `zap` tag on the event being zapped (see Appendix G), or by decoding their lud06 or lud16 field on their profile according to the [lnurl specifications](https://github.com/lnurl/luds). The client MUST send a GET request to this url and parse the response. If `allowsNostr` exists and it is `true`, and if `nostrPubkey` exists and is a valid BIP 340 public key in hex, the client should associate this information with the user, along with the response's `callback`, `minSendable`, and `maxSendable` values.
2. Clients may choose to display a lightning zap button on each post or on a user's profile. If the user's lnurl pay request endpoint supports nostr, the client SHOULD use this NIP to request a zap receipt rather than a normal lnurl invoice.
3. When a user (the "sender") indicates they want to send a zap to another user (the "recipient"), the client should create a `zap request` event as described in Appendix A of this NIP and sign it.
4. Instead of publishing the `zap request`, the `9734` event should instead be sent to the `callback` url received from the lnurl pay endpoint for the recipient using a GET request. See Appendix B for details and an example.
5. The recipient's lnurl server will receive this request and validate it. See Appendix C for details on how to properly configure an lnurl server to support zaps, and Appendix D for details on how to validate the `nostr` query parameter.
6. If the request is valid, the server should fetch a description hash invoice where the description is this note and this note only. No additional lnurl metadata is included in the description. This will be returned in the response according to [LUD06](https://github.com/lnurl/luds/blob/luds/06.md).
7. On receiving the invoice, the client MAY pay it or pass it to an app that can pay the invoice.
8. Once the invoice is paid, the recipient's lnurl server MUST generate a `zap receipt` as described in Appendix E, and publish it to the `relays` specified in the `zap request`.
9. Clients MAY fetch `zap receipt`s on posts and profiles, but MUST authorize their validity as described in Appendix F. If the zap request note contains a non-empty `content`, it may display a zap comment. Generally clients should show users the `zap request` note, and use the `zap receipt` to show "zap authorized by ..." but this is optional.
A `zap request` is an event of kind `9734` that is _not_ published to relays, but is instead sent to a recipient's lnurl pay `callback` url. This event's `content` MAY be an optional message to send along with the payment. The event MUST include the following tags:
-`relays` is a list of relays the recipient's wallet should publish its `zap receipt` to. Note that relays should not be nested in an additional list, but should be included as shown in the example below.
-`amount` is the amount in _millisats_ the sender intends to pay, formatted as a string. This is recommended, but optional.
-`lnurl` is the lnurl pay url of the recipient, encoded using bech32 with the prefix `lnurl`. This is recommended, but optional.
A signed zap request event is not published, but is instead sent using a HTTP GET request to the recipient's `callback` url, which was provided by the recipient's lnurl pay endpoint. This request should have the following query parameters defined:
-`amount` is the amount in _millisats_ the sender intends to pay
-`nostr` is the `9734` zap request event, JSON encoded then URI encoded
-`lnurl` is the lnurl pay url of the recipient, encoded using bech32 with the prefix `lnurl`
This request should return a JSON response with a `pr` key, which is the invoice the sender must pay to finalize his zap. Here is an example flow:
```javascript
const senderPubkey // The sender's pubkey
const recipientPubkey = // The recipient's pubkey
const callback = // The callback received from the recipients lnurl pay endpoint
const lnurl = // The recipient's lightning address, encoded as a lnurl
1. Add a `nostrPubkey` to the lnurl-pay static endpoint `/.well-known/lnurlp/<user>`, where `nostrPubkey` is the nostr pubkey your server will use to sign `zap receipt` events. Clients will use this to validate zap receipts.
When a client sends a zap request event to a server's lnurl-pay callback URL, there will be a `nostr` query parameter where the contents of the event are URI- and JSON-encoded. If present, the zap request event must be validated in the following ways:
A `zap receipt` is created by a lightning node when an invoice generated by a `zap request` is paid. Zap receipts are only created when the invoice description (committed to the description hash) contains a zap request note.
1. Get the description for the invoice. This needs to be saved somewhere during the generation of the description hash invoice. It is saved automatically for you with CLN, which is the reference implementation used here.
2. Parse the bolt11 description as a JSON nostr event. This SHOULD be validated based on the requirements in Appendix D, either when it is received, or before the invoice is paid.
3. Create a nostr event of kind `9735` as described below, and publish it to the `relays` declared in the zap request.
- The `created_at` date SHOULD be set to the invoice `paid_at` date for idempotency.
-`tags` MUST include the `p` tag AND optional `e` tag from the zap request.
- The zap receipt MUST have a `bolt11` tag containing the description hash bolt11 invoice.
- The zap receipt MUST contain a `description` tag which is the JSON-encoded invoice description.
-`SHA256(description)` MUST match the description hash in the bolt11 invoice.
- The zap receipt MAY contain a `preimage` tag to match against the payment hash of the bolt11 invoice. This isn't really a payment proof, there is no real way to prove that the invoice is real or has been paid. You are trusting the author of the zap receipt for the legitimacy of the payment.
The zap receipt is not a proof of payment, all it proves is that some nostr user fetched an invoice. The existence of the zap receipt implies the invoice as paid, but it could be a lie given a rogue implementation.
A client can retrieve `zap receipts` on events and pubkeys using a NIP-01 filter, for example `{"kinds": [9735], "#e": [...]}`. Zaps MUST be validated using the following steps:
When an event includes a `zap` tag, clients SHOULD calculate the lnurl pay request based on it's value instead of the profile's field. An optional third argument on the tag specifies the type of value, either `lud06` or `lud16`.
Zaps can be extended to be more private by encrypting zap request notes to the target user, but for simplicity it has been left out of this initial draft.
