BUD-07
======

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

`draft` `optional`

Cashu payments for uploads and downloads

## Paid Upload

The server may require payment for uploading blob by returning a `402` status code the `PUT /upload` endpoint (and `HEAD /upload` if [BUD-06](./06.md) is supported)

## Paid Downloads

The server may also require payment for downloads by responding with a `402` status code for `GET /<sha256` and `HEAD /<sha256>` endpoints

## Payment Flow

When the server is requesting payment for an endpoint it MUST respond with `402` and a `X-Cashu` header containing a base64 encoded json object (payment request)

The payment request should contain an `amount`, `mints`, `unit`, and `pubkey` fields

 - `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)

When the client receives a `402` response and with a `X-Cashu` header it may retry the request with a payment

The payment should be a serialized cashu token according to [NUT-00](https://github.com/cashubtc/nuts/blob/main/00.md#v4-tokens)

## Payment Checks

Optionally a server may respond with `402` to the `HEAD /upload`, `HEAD /<sha156>` if it supports [BUD-06](./06.md)