NIP-88
======

Nostr Cash (simple Nostr ecash)
-----------------------------------

`draft` `optional` `author:benarc`

> Nostr Cash is not blinded by default.

Ecash is useful and relays should have the option to offer ecash mints. Other mints could exist as stand-alone clients, but relays are ideally placed to package in with their service offerings.

Users can use relays offering mints they trust, or relays they don't trust but for smaller amounts. 

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 "ecash" 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 ecash wallet stores:

```
active:
    key: <string, derived private key/ecash>, 
    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/ecash>, 
    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 ecash 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/ecash>,
  "amount": <integer BTC millisat denomination>
}
```

User client stores:

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

### Claiming

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

User DMs relay:

```json
{
  "key": <String, derived private key/ecash>,
  "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 ecash to another nostr user.**

User DMs relay:

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

Mint `burns` the old record for the ecash 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/ecash. 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/ecash>,
  "amount": <integer BTC millisat denomination>
}
```

User(s) client stores:

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