BUD-07
======

Paid upload and download
---------------

`draft` `optional`

Payment requirements for blob storage.

## Payment Required

Some servers MAY require payment for uploads, downloads, or any other endpoint. In such cases, these endpoints MUST return a **402 Payment Required** status code.

Some endpoints a server may require payment for:

- [`HEAD /upload`](./06.md) to signal that payment is required for the `PUT` request ( if [BUD-06](./06.md) is supported )
- [`PUT /upload`](./02.md#put-upload---upload-blob) to require payment for uploads
- [`HEAD /<sha256>`](./01.md#head-sha256---has-blob) to signal that payment is required for the `GET` request
- [`GET /<sha256>`](./01.md#get-sha256---get-blob) to require payment for downloads ( maybe charge by MB downloaded? )
- [`HEAD /media`](./05.md) and [`PUT /upload`](./05.md) to require payment for media optimizations ( if [BUD-06](./06.md) is supported )

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 402 status code and `X-{payment_method}` header is used by the server to inform the client that a payment is required for the requested operation. The server MUST provide specific headers for each supported payment method.

Supported payment methods:

- `X-Cashu`: Payment details for the cashu payment method, adhering to the [NUT-24](https://github.com/cashubtc/nuts/blob/main/24.md) standard.
- `X-Lightning`: Payment details for the lightning payment method, adhering to the [BOLT-11](https://github.com/lightning/bolts/blob/master/11-payment-encoding.md) standard.

If a server supports multiple payment methods, it MAY send multiple `X-{payment_method}` headers in the same response.

Schema:

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

### `X-Cashu` Header

When using the X-Cashu header, the server MUST adhere to the [NUT-24](https://github.com/cashubtc/nuts/blob/main/24.md) standard.

Example for cashu:

```http	
HTTP/1.1 402 Payment Required
X-Cashu: creqApWF0gaNhdGVub3N0cmFheKlucHJvZmlsZTFxeTI4d3VtbjhnaGo3dW45ZDNzaGp0bnl2OWtoMnVld2Q5aHN6OW1od2RlbjV0ZTB3ZmprY2N0ZTljdXJ4dmVuOWVlaHFjdHJ2NWhzenJ0aHdkZW41dGUwZGVoaHh0bnZkYWtxcWd5ZGFxeTdjdXJrNDM5eWtwdGt5c3Y3dWRoZGh1NjhzdWNtMjk1YWtxZWZkZWhrZjBkNDk1Y3d1bmw1YWeBgmFuYjE3YWloYjdhOTAxNzZhYQphdWNzYXRhbYF4Imh0dHBzOi8vbm9mZWVzLnRlc3RudXQuY2FzaHUuc3BhY2U
```

### `X-Lightning` Header

When using the X-Lightning header, the server MUST adhere to the [BOLT-11](https://github.com/lightning/bolts/blob/master/11-payment-encoding.md) standard.
Example for lightning:

```http
HTTP/1.1 402 Payment Required
X-Lightning: lnbc30n1pnnmw3lpp57727jjq8zxctahfavqacymellq56l70f7lwfkmhxfjva6dgul2zqhp5w48l28v60yvythn6qvnpq0lez54422a042yaw4kq8arvd68a6n7qcqzzsxqyz5vqsp5sqezejdfaxx5hge83tf59a50h6gagwah59fjn9mw2d5mn278jkys9qxpqysgqt2q2lhjl9kgfaqz864mhlsspftzdyr642lf3zdt6ljqj6wmathdhtgcn0e6f4ym34jl0qkt6gwnllygvzkhdlpq64c6yv3rta2hyzlqp8k28pz
```

### Client implementation

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

The client MUST provide the payment proof when re-trying the request using the same `X-{payment_method}` header that was chosen. The payment proof MUST align with the payment method specification:

- For cashu the payment proof should be a serialized `cashuB` token in the `X-Cashu` header according to [NUT-24](https://github.com/cashubtc/nuts/blob/main/24.md#client-payment).
- For lightning the payment proof should be the preimage of the payment request according to [BOLT-11](https://github.com/lightning/bolts/blob/master/11-payment-encoding.md).

Schema:
  
```http
X-{payment_method}: "<encoded_payment_proof_according_to_{payment_method}_spec>"
```

Example for Cashu:

```http
X-Cashu: cashuBo2F0gqJhaUgA_9SLj17PgGFwgaNhYQFhc3hAYWNjMTI0MzVlN2I4NDg0YzNjZjE4NTAxNDkyMThhZjkwZjcxNmE1MmJmNGE1ZWQzNDdlNDhlY2MxM2Y3NzM4OGFjWCECRFODGd5IXVW
```

Example for Lightning:

```http
X-Lightning: 966fcb8f153339372f9a187f725384ff4ceae0047c25b9ce607488d7c7e93bba
```

**Special Note on HEAD Requests**

The HEAD endpoints are only used to retrieve blob or server information. They MUST NOT be retried with payment proof. Instead, clients should complete the payment and proceed with the `PUT` or `GET` request.

### Error handling

If the client fails to provide the payment proof (expired invoice, invalid token, etc.) the server MUST respond with **400 Bad request** status code and include a `X-Reason` header with a human-readable message. The client SHOULD inform the user about the error and provide a way to retry the request.

### 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 the specific payment details.

New methods MUST adhere their own specification, which MUST be publicly available and linked in the header.
