NIP-69: Nostr Offer STRings
===========================

Nostr Offer STRings
-------------------

`wip` `optional`

This NIP proposes a format for static payment codes in Nostr as a successor to LNURL-Pay, enabling users to initiate Lightning Network payments by scanning or clicking a string.

## Motivation

Reliance on LNURL has led to centralization via custodial solutions due to the legacy baggage of IP4-NAT/Domain/SSL requirements. Nostr's use-cases are already centered around Lightning, whether as a wallet connector, zap receipts, or its overlapping network effects, rendering Nostr as a de facto "3rd layer" for Lightning and a natural successor to LNURL.

When layered over kind 21000, NWC, or other potential Lightning RPCs, this specification enables a seamless user experience similar to legacy LNURL-Pay, without the domain and SSL requirements. Additionally, the signed nature of Nostr communications eliminates the trust requirements inherent in LNURL when a node must outsource the serving of web requests.

## Specification

### Static Payment Code Format

The static payment code is a bech32 (per NIP-19) encoded string prefixed with `noffer`. The encoded string will include the following TLV (Type-Length-Value) items:

- `0`: The 32 bytes of the receiving service's public key, encoded in hex.
- `1`: The relay URL where the receiving service subscribes to payment requests.
- `2`: The offer string.

### NIP-05 Integration

To support trust-optimized Lightning Addresses, services can add a `nip69` field in the NIP-05 content. This field will provide the offer on name-based lookups.

### Process Flow

1. **Payer Scans or Clicks the Static Payment Code**
2. **Payer's Wallet Decodes the Payment Code**
3. **Payer's Wallet Sends a Nostr Event to the Specified Relay**
   - Addressed to the receiver's public key, containing the identifier string.
   - Event Type: Ephemeral Kind 21001 | NIP-44 Encrypted
   - Optional: Include additional payer data.
4. **Receiver Responds with Lightning Invoice**
   - Generates a Lightning invoice and responds with a Nostr event containing the invoice details.
   - Event Type: Ephemeral Kind 21001 | NIP-44 Encrypted
   - Optional: Include additional purchase data.
5. **Payer Pays the Invoice**
   - Completes the payment by settling the Lightning invoice.
6. **Optional: Receiver Emits Payment Receipt**
   - This step is out of scope for this NIP, but potential receipt considerations include:
     - Nostr Event as Receipt: Emitting a public Nostr event as a "zap" receipt for verifiable record-keeping.
     - Lightning Pre-Image: Sharing the Lightning pre-image as proof of payment.
     - External / WebHook: The receiver may finalize the flow out-of-band.

### Nostr Event

This NIP specifies the use of event kind `21001` with the following structure:

- `content`: NIP-44 encrypted payment details.
- `tags`: 
  - `p`: Receiver's public key (hex).

Example event:

```json
{
  "id": "<event_id>",
  "pubkey": "<sender_pubkey>",
  "created_at": 1234567890,
  "kind": 21001,
  "tags": [
    ["p", "<receiver_pubkey>"],
    ["e", "<event_id>"] // used only in response by the receiver to identify the original payment request
  ],
  "content": "<NIP-44 encrypted offer and optional payer data>",
  "sig": "<signature>"
}
```

The `content` field, after NIP-44 decryption, should contain an object with the following structure:

```json
{
    "offer": "<offer_string>",
    "payer_data": "<optional_payer_data>"
}
```

## Client Behavior

### Mandatory

Clients implementing this NIP must:

1. Decode and validate the structure of the `noffer` bech32-encoded static payment code.
2. Use NIP-44 encryption for all communication between payer and receiver.
3. Generate and send kind 21001 events as specified in this NIP.
4. Parse the `content` field of received events by decrypting and JSON-parsing the stringified content.
5. Handle potential errors gracefully, providing clear feedback to users.
6. Respect the relay URL specified in the static payment code for sending payment request events.

### Optional

Clients implementing this NIP may:

1. Support both this protocol and LNURL for backward compatibility during the transition period.
2. Implement additional features such as multi-recipient payments or integration with other Lightning-related NIPs.
3. **Use Addressable Events**: Implement addressable events to allow for updates to the recipient key or relay URL associated with the offer.
4. **Attempt Payments by Name**: Use the combination of NIP-05 and NIP-69 to attempt payments by name-based lookups.

### Prohibited

Clients implementing this NIP MUST NOT:

1. Send unencrypted sensitive information in event content or tags.
2. Ignore or modify any fields from the encoded static payment code.

## Service Behavior

### Optional

Services implementing this NIP may:

1. Use an account identifier as a default offer string for spontaneous payments without explicit user setup.
2. **NIP-05 Integration**: Add a `nip69` field in the NIP-05 content to provide the offer on name-based lookups for trust-optimized Lightning Addresses.

## Error Handling

To ensure consistent error handling across implementations, this NIP defines the following error responses, similar to other NIPs:

1. **Invalid Offer**: When the offer ID is invalid or no longer available.
2. **Temporary Failure**: When the receiver is temporarily unable to process the request.
3. **Expired Offer**: When the offer has expired and is no longer valid.
4. **Unsupported Feature**: When the receiver doesn't support a feature requested by the payer.

Error responses should be sent as kind 21001 events with the following structure in the encrypted content:

```json
{
  "error": "<error_message>",
  "code": "<error_code>"
}
```

Error codes:

- 1: Invalid Offer
- 2: Temporary Failure
- 3: Expired Offer
- 4: Unsupported Feature

Clients MUST handle these error responses gracefully and display appropriate messages to users.

## Transition from LNURL

LNURL services wishing to migrate to Nostr Offer STRings should consider adding a `noffer` tag to their responses.

## Future Considerations

Future versions of this NIP may consider additional features such as:

1. Multi-recipient payments.
2. Integration with other Lightning-related NIPs.
3. Extended metadata for more complex payment scenarios.
4. **Addressable Events**: In Nostr apps where the offer is used, consider using addressable events to allow for updates to the recipient key or relay URL associated with the offer.

## Reference Implementation

Lightning.Pub / ShockWallet offers branch