NIP-88
======

Nostr Cash (simple Nostr cash/token/cheque)
-----------------------------------

`draft` `optional` `author:benarc`

Electronic cash is useful and relays should have the option to offer minting/custodial services. Other mints could exist as stand-alone clients, but relays are ideally placed to package in with their service offerings.

Nostr Cash is simple and flexible. User gives a mint some money and the mint issues the user electromic cash/cheque/token that can be used as money in nostr.

Nostr Cash is entirely custodial and trust based, and is just a common standard for users to send/receive value in nostr. Keeping mints simple means more users can run, for themselves, their family, or the whole world.

Relays offering mints must have a keypair and an additional nostr keypair for its mint.

Nostr Cash uses derived keys, much like [NIP26](https://github.com/nostr-protocol/nips/blob/master/26.md). Each "cash" is actually just a derived private key that can be burned or passed to another user.

All communication between the relay and user happen over [NIP04](https://github.com/nostr-protocol/nips/blob/master/04.md).

The mints ledger is two tables `active` and `spent`:

```
active:
    key: <integer, keypair deriviation number from master>, 
    amount: <integer BTC millisat denomination>, 
    user: <string, public-key of user>,
    timestamp: <integer timestamp>

spent:
    key: <integer, keypair deriviation number from master>, 
    amount: <integer BTC millisat denomination>, 
    user: <string, public-key of user>,
    timestamp: <integer timestamp>
```

User client cash wallet stores:

```
active:
    key: <string, derived private key/cash>, 
    amount: <integer BTC millisat denomination>, 
    relay: <string, relay ws address>,
    relaykey: <string, public-key of relay>,
    timestamp: <integer timestamp>

spent:
    key: <string, derived private key/cash>, 
    amount: <integer BTC millisat denomination>, 
    relay: <string, relay ws address>,
    relaykey: <string, public-key of relay>,
    timestamp: <integer timestamp> 
```

## Workflows

### Mint details 

**For fetching info about the mint.**

User DMs the relay:

```json
{
    "mint": <bool true>
}
```

Relay DMs user (maybe after a check if the relays mint has restricted access):

```json
{
    "title": <string mints title>,
    "description": <string mints description>,
    "liquiity": <integer optional public millisat balance of mint>,
    "type": <list different offered payment types bolt11 bolt12 onchain stripe paypal shitcoin etc>,
    "fee": <integer millisat fee for internal nostr payments>,
}
```

### Minting

**For creating new cash from a mint.**

User DMs the relay:

```json
{
  "amount": <integer millisats>
}
```

Relay responds with payment request, which user pays:
    
```json
{
  "request": <string payment request bolt11 bolt12 onchain stripe paypal shitcoin etc>
}
```

Once paid the mint generates derived keypair from the mints private key, storing in table `active`:

```json
{
  "key": <integer derived keypair number>,
  "amount": <integer BTC millisat denomination>, 
  "user": <string public-key of user>
}
```

Mint replies:

```json
{
  "key": <string derived private key/cash>,
  "amount": <integer BTC millisat denomination>
}
```

User client stores:

```json
{
  "key": <string derived private key/cash>,
  "amount": <integer BTC millisat denomination>,
  "relay": <string relay ws address>,
  "relaykey": <string public-key of relay>,
  "timestamp": <integer timestamp>
}
```

### Claiming

**For cashing in cash with the mint.**

User DMs relay:

```json
{
  "key": <String, derived private key/cash>,
  "amount": <Integer, BTC millisat denomination>,
  "type": <String, preffered payment method bolt11 bolt12 onchain stripe paypal shitcoin etc>
}
```

Relay DMs user:

```json
{
  "type": <String, bolt11 bolt12 onchain stripe paypal shitcoin etc>
}
```

User DMs relay:

```json
{
   "request": <String, payment-request bolt11 bolt12 onchain stripe paypal shitcoin etc>,
}
```

### Sending

**For sending cash to another nostr user.**

User DMs relay:

```json
{
  "key": <String, derived private key/cash>,
  "amount": <Integer, BTC millisat denomination>,
  "type": <String, public key of recipient>
}
```

Mint `burns` the old record for the cash and creates a new record(s), depending on if sending user is owed change (the old record moves from `active` to `spent` table). The mint creates a new record in `active` and DMs the recipient user the private-key/cash. If change is due, another record is created in `active` and sent back to the sender user.

Relay DMs user(s):

```json
{
  "key": <string derived private key/cash>,
  "amount": <integer BTC millisat denomination>
}
```

User(s) client stores:

```json
{
  "key": <string derived private key/cash>,
  "amount": <integer BTC millisat denomination>,
  "relay": <string, relay ws address>,
  "relaykey": <string public-key of relay>,
  "timestamp": <integer timestamp>
}
```