blossom/buds/07.md

BUD-07
======

Paid storage
---------------

`draft` `optional`

Payment requirements for blob storage.

## Payment Required

Some servers MAY require payment for file storage. In such cases, these endpoints MUST return a **402 Payment Required** status code:

- [HEAD /upload](./01.md#head-sha256---has-blob) (Optional, if [BUD-06](./06.md) is implemented)
- [PUT /upload](./02.md#put-upload---upload-blob)
- [HEAT /<sha256>](./01.md#head-sha256---has-blob)
- [GET /<sha256>](./01.md#get-sha256---get-blob)

When payment is required, the server MUST include one or more `X-{payment_method}` header(s), each corresponding to a supported payment method. 

## Server headers

The `X-{payment_method}` header is used by the server to inform the client that payment is required for the requested operation. The server MUST provide specific headers for each supported payment method, using the following conventions:

- `X-{payment_method}`: Base64 UrlSafe encoded JSON with the payment details.

Supported payment methods:

- `X-Cashu`: Payment details for the cashu payment method.
- `X-Lightning`: Payment details for the lightning payment method.

If a server supports multiple payment methods, it MAY send multiple `X-{payment_method}` headers in the same response. The client MUST choose one of the provided methods and proceed with the payment accordingly.

### `X-Cashu` Header

Fields included in the X-Cashu header:

- `identifier`: The unique identifier for the payment.
- `amount`: The amount of ecash being requested
- `mints`: An array of mints that this server uses
- `unit`: The cashu `unit` from the `mints`
- `pubkey`: (optional) a 33 byte pubkey to lock the tokens too. see [NUT-11](https://github.com/cashubtc/nuts/blob/main/11.md)

The `identifier` field is a unique reference to a specific payment request. It allows the server to distinguish between different payments for the same file or blob. The server MUST ensure that each identifier is unique and link it internally to the relevant file or blob.

Example for cashu:

```json
{
  "identifier": "23537",
  "amount": "300",
  "mints": ["https://example.com", "https://example2.com"],
  "unit": "sat",
  "pubkey": "0a3bce8b6005175d25432244340912dfa431a0d62681d9720cda9196398d7222"
}
```

```http	
HTTP/1.1 402 Payment Required
X-Cashu: ewogICJpZGVudGlmaWVyIjogIjIzNTM3IiwKICAiYW1vdW50IjogIjMwMCIsCiAgIm1pbnRzIjogWyJodHRwczovL2V4YW1wbGUuY29tIiwgImh0dHBzOi8vZXhhbXBsZTIuY29tIl0sCiAgInVuaXQiOiAic2F0IiwKICAicHVia2V5IjogIjBhM2JjZThiNjAwNTE3NWQyNTQzMjI0NDM0MDkxMmRmYTQzMWEwZDYyNjgxZDk3MjBjZGE5MTk2Mzk4ZDcyMjIiCn0
```

### `X-Lightning` Header

Fields included in the X-Lightning header:

- `identifier`: The unique identifier for the payment.
- `amount`: The amount to be paid.
- `payment_request`: The lightning payment request.

The `identifier` field is a unique reference to a specific payment request. It allows the server to distinguish between different payments for the same file or blob. The server MUST ensure that each identifier is unique and link it internally to the relevant file or blob.

Example for lightning:

```json
{
  "identifier": "23537",
  "amount": "300",
  "payment_request": "lnbc300n1pw4..:",
}

```

```http
HTTP/1.1 402 Payment Required
X-Lightning: ewogICJpZGVudGlmaWVyIjogIjIzNTM3IiwKICAiYW1vdW50IjogIjMwMCIsCiAgInBheW1lbnRfcmVxdWVzdCI6ICJsbmJjMzAwbjFwdzQuLjoiLAp9
```

Schema:

```http
HTTP/1.1 402 Payment Required
X-Payment-{payment_method}: "<base64-UrlSafe_encoded_server_json>"
```


### Client implementation

Clients MUST parse and validate all available `X-{payment_method}` headers received from the server. The client SHOULD provide a way for the user to complete the payment and retry the request using the appropiate `X-Payment-{payment_method}` header.

- `X-{payment_method}-Payment`: Base64 UrlSafe encoded JSON with the client payment proof.

- `identifier`: The unique identifier for the payment.
- `payment_proof`: The proof of payment, specific to the method:

  - For cashu the payment should be a serialized cashu token according to [NUT-00](https://github.com/cashubtc/nuts/blob/main/00.md#v4-tokens)
  - For lightning the payment should be the preimage of the payment request according to [BOLT-11](https://github.com/lightning/bolts/blob/master/11-payment-encoding.md)

Example for Cashu:

```json
{
  "identifier": "23537",
  "payment_proof": "c3b9e2..."
}
```	

```http
X-Cashu-Payment: ewogICJpZGVudGlmaWVyIjogIjIzNTM3IiwKICAicGF5bWVudF9wcm9vZiI6ICJjM2I5ZTIiCn0
```

Example for Lightning:

```json
{
  "identifier": "23537",
  "payment_proof": "lnbc300n1pw4..."
}
```

```http
X-Lightning-Payment: ewogICJpZGVudGlmaWVyIjogIjIzNTM3IiwKICAicGF5bWVudF9wcm9vZiI6ICJsbmJjMzAwbjFwdzQuLjoiCn0
```

Schema:
  
```http
X-{payment_method}-Payment: "<base64-UrlSafe_encoded_client_json>"
```

### Error handling

If the client fails to provide the payment proof (expired invoice, invalid token, etc.) the server MUST respond with **402 Payment Required** status code and include the relevant `X-{payment_method}` header(s) with updated payment details.


### Extending with Future Payment Methods

To support future payment methods (e.g., other Layer 2 solutions), the specification allows the addition of new X-{payment_method} headers. Each new method MUST adhere to the following:

New methods MUST use a unique `X-{payment_method}` header containing Base64 UrlSafe encoded JSON.

**Required Fields:**

- `identifier`: Unique reference for the payment.
- `amount`: Payment amount in the method’s smallest unit.