added nostr cash

88.md

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+NIP-15
+======
+
+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 mints and relays they don't trust but for smaller amounts. 
+
+The relay must have a keypair and an additional keypair for its mint.
+
+Nostr Cash uses derived keys, much like [NIP26](https://github.com/nostr-protocol/nips/blob/master/26.md). 
+
+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 
+
+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
+
+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
+
+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
+
+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 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,
+}
+```