Add cross-currency payment methods to NWC.

This will help serve use cases like e-cash wallets, bolt-12 offers which allow for other currency denominations, and [LUD-21](lnurl/luds#251 wallet providers like UMA VASPs.

47.md

@@ -75,6 +75,7 @@ If the command was successful, the `error` field must be null.
 - `QUOTA_EXCEEDED`: The wallet has exceeded its spending quota.
 - `RESTRICTED`: This public key is not allowed to do this operation.
 - `UNAUTHORIZED`: This public key has no wallet connected.
+- `EXPIRED`: The invoice or quote being paid has expired.
 - `INTERNAL`: An internal error.
 - `OTHER`: Other error.
 
@@ -396,6 +397,17 @@ Response:
             "block_height": 1,
             "block_hash": "hex string",
             "methods": ["pay_invoice", "get_balance", "make_invoice", "lookup_invoice", "list_transactions", "get_info"], // list of supported methods for this connection
+            // Optional fields:
+            "lud16": "string", // lightning address
+            // Preferred currencies for the user. Omission of this field implies only SATs.
+            "currencies": {
+                "name": "string",
+                "code": "string",
+                "symbol": "string",
+                "decimals": "number",
+                "min": "number",
+                "max": "number",
+            }[],
     }
 }
 ```
@@ -407,5 +419,297 @@ Response:
 2. **wallet service** verifies that the author's key is authorized to perform the payment, decrypts the payload and sends the payment.
 3. **wallet service** responds to the event by sending an event with kind `23195` and content being a response either containing an error message or a preimage.
 
+## Cross-Currency Extensions
+
+This section describes extensions to Nostr Wallet Connect to support payments across currencies through a connected wallet. This will help serve use cases like e-cash wallets, bolt-12 offers which allow for other currency denominations, and [LUD-21](https://github.com/lnurl/luds/pull/251)-compatible wallet providers like UMA VASPs.
+
+### `lookup_user`
+
+The `lookup_user` function can be used to fetch a list of preferred currencies for a given receiver.
+
+Request:
+
+```jsonc
+{
+    "method": "lookup_user",
+    "params": {
+        "receiver": {
+            // Exactly of the following fields is required:
+            "lud16": "string|undefined",
+            "bolt12": "string|undefined",
+            // ... extensible to future address formats (npub, etc).
+        },
+        // Optional, to retrieve FX rates for receiving currencies relative to a specific sending currency.
+        // This currency must be supported by the sender. If omitted, SATs will be used.
+        "base_sending_currency_code": "string|undefined", 
+    }
+}
+```
+
+Response:
+
+```jsonc
+{
+    "result_type": "lookup_user",
+    "result": {
+        "receiver": {
+            "lud16": "string",
+            "bolt12": "string",
+            // ... extensible to future address formats (npub, etc).
+        },
+        // Contains a list of preferred currencies like LUD-21
+        "currencies": {
+            "code": "string", // eg. "PHP",
+            "name": "string", // eg. "Philippine Pesos",
+            "symbol": "string", // eg. "₱",
+            // Estimated number of milli-sats per smallest unit of this currency (eg. cents)
+            // If base_sending_currency_code was specified, this is the rate relative to that currency instead of milli-sats.
+            "multiplier": "number",
+            // Number of digits after the decimal point for display on the sender side, and to add clarity around what the
+            // "smallest unit" of the currency is. For example, in USD, by convention, there are 2 digits for cents - $5.95.
+            // In this case, `decimals` would be 2. Note that the multiplier is still always in the smallest unit (cents).
+            // In addition to display purposes, this field can be used to resolve ambiguity in what the multiplier
+            // means. For example, if the currency is "BTC" and the multiplier is 1000, really we're exchanging in SATs, so
+            // `decimals` would be 8.
+            "decimals": "number",
+            // Minimum and maximium amounts the receiver is willing/able to convert to this currency in the smallest unit of
+            // the currency. For example, if the currency is USD, the smallest unit is cents.
+            "min": "number",
+            "max": "number",
+        }[],
+    },
+}
+```
+
+### `fetch_quote`
+
+The `fetch_quote` method retrieves a locked quote to send a specific amount of money to a specified receiver. This call corresponds to the `payreq` request and its response corresponds to the `converted` field in the payreq response with [LUD-21](https://github.com/lnurl/luds/pull/251). The caller must specify whether the sending or receiving currency amount is what’s being locked with this quote. For example, do I want to send exactly $5 on my side, or do I want the receiver to receive exactly 5 Pesos on the other side. This method is only required for receiver-locked sends, but is optional for sender-locked (where `pay_to_address` can be used without a quote).
+
+Request:
+
+```jsonc
+{
+    "method": "fetch_quote",
+    "params": {
+        "receiver": {
+            // Exactly of the following fields is required:
+            "lud16": "string|undefined",
+            "bolt12": "string|undefined",
+            // ... extensible to future address formats (npub, etc).
+        },
+        "sending_currency_code": "string",
+        "receiving_currency_code": "string",
+        "locked_currency_side": "SENDING"|"RECEIVING",
+        "locked_currency_amount": "number",
+    }
+}
+```
+
+Response:
+
+```jsonc
+{
+    "result_type": "fetch_quote",
+    "result": {
+        "sending_currency_code": "string",
+        "receiving_currency_code": "string",
+        "payment_hash": "string", // used to execute the quote
+        "expires_at": "number",
+        "multiplier": "number", // receiving unit per sending unit
+        "fees": "number", // fees in the sending currency
+        "total_receiving_amount": "number",
+        "total_sending_amount": "number",
+    },
+}
+```
+
+### `execute_quote`
+
+Sends a payment corresponding to a quote retrieved from fetch_quote. If the quote has expired, the payment will fail.
+
+Request:
+
+```jsonc
+{
+    "method": "execute_quote",
+    "params": {
+        "payment_hash": "string",
+    }
+}
+```
+
+Response:
+
+```jsonc
+{
+    "result_type": "execute_quote",
+    "result": {
+        "preimage": "string",
+    },
+}
+```
+
+### `pay_to_address`
+
+This method directly pays the receiving user based on a fixed sending amount. The client app can complete the whole quote creation and execution exchange with this one call. Callers can optionally exclude the `receiving_currency` to allow just sending to the receiver's first preferred currency.
+
+Request:
+```jsonc
+{
+    "method": "pay_to_address",
+    "params": {
+        "receiver": {
+            // Exactly of the following fields is required:
+            "lud16": "string|undefined",
+            "bolt12": "string|undefined",
+            // ... extensible to future address formats (npub, etc).
+        },
+        "sending_currency_code": "string",
+        "receiving_currency_code": "string|undefined",
+        "sending_currency_amount": "number",
+    }
+}
+```
+
+Response:
+```jsonc
+{
+    "result_type": "pay_to_address",
+    "result": {
+        "preimage": "string",
+        "quote": {
+            "sending_currency_code": "string",
+            "receiving_currency_code": "string",
+            "payment_hash": "string",
+            "multiplier": "number", // receiving unit per sending unit
+            "fees": "number", // fees in the sending currency
+            "total_receiving_amount": "number",
+            "total_sending_amount": "number",
+        },
+    },
+}
+```
+
+### Extension of `get_balance`
+
+The `get_balance` request can take an optional `currency_code` field to specify which currency to look up. If none is specified the sats balance is returned.
+
+```jsonc
+{
+    "method": "get_balance",
+    "params": {
+        "currency_code": "string|undefined",
+    }
+}
+```
+
+Response:
+```jsonc
+{
+    "result_type": "get_balance",
+    "result": {
+        "balance": "number", // user's balance in the smallest unit of the currency
+        "currency_code": "string"
+    }
+}
+```
+
+### Extension of `Invoice` and `Payment` objects
+
+The invoice/payment objects returned by `lookup_invoice` and `list_transactions` should include some new info about other currencies if applicable:
+
+```jsonc
+{
+    // ...Existing fields...
+
+    // Optional field:
+    "fx": {
+        "receiving_currency_code": "string",
+        "total_receiving_amount": "number",
+        "multiplier": "number", // receiving unit per sending unit (SATs if incoming)
+        // Remaining fields only available for outgoing payments:
+        "sending_currency_code": "string", 
+        "fees": "number", // fees in the sending currency
+        "total_sending_amount": "number",
+    },
+}
+```
+
+### Example Cross-Currency Payments
+
+#### Directly send exactly $1 USD to a user
+
+_If you don’t care about the receiving amount or currency:_
+
+```jsonc
+{
+    "method": "pay_to_address",
+    "params": {
+        "receiver": { "lud16": "$alice@vasp.net" },
+        "sending_currency_code": "USD",
+        "sending_currency_amount": 100, // Denominated in ISO 4712 decimals (cents)
+        // Can add receiving_currency_code if you want to let the sender choose
+    }
+}
+```
+
+_If you want to show the user how much the receiver will receive:_
+
+```jsonc
+{
+    "method": "fetch_quote",
+    "params": {
+        "receiver": { "lud16": "$alice@vasp.net" },
+        "sending_currency_amount": "USD",
+        "locked_currency_side": "SENDING",
+        "locked_currency_amount": 100, // Denominated in ISO 4712 decimals (cents)
+        // Can set receiving_currency_code if known. Otherwise, the receiver's preferred currency will be used.
+    }
+}
+
+// Show the quote to the user with expiration time...
+
+// User confirms the quote and executes it
+{
+    "method": "execute_quote",
+    "params": {
+        "payment_hash": "hash from fetch_quote",
+    }
+}
+```
+
+#### Paying for some service such that the receiver receives exactly MX$5
+
+```jsonc
+// First retrieve the receiver currency list if not yet known.
+{
+    "method": "lookup_user",
+    "params": {
+        "receiver": { "lud16": "$alice@vasp.net" },
+    },
+}
+
+{
+    "method": "fetch_quote",
+    "params": {
+        "receiver": { "lud16": "$alice@vasp.net" },
+        "sending_currency_code": "USD",
+        "receiving_currency_code": "MXN",
+        "locked_currency_side": "RECEIVING",
+        "locked_currency_amount": 500, // Denominated in ISO 4712 decimals (cents)
+    },
+}
+
+// Show the quote to the user with expiration time...
+
+// User confirms the quote and executes it
+{
+    "method": "execute_quote",
+    "params": {
+        "payment_hash": "hash from fetch_quote",
+    }
+}
+```
+
 ## Using a dedicated relay
 This NIP does not specify any requirements on the type of relays used. However, if the user is using a custodial service it might make sense to use a relay that is hosted by the custodial service. The relay may then enforce authentication to prevent metadata leaks. Not depending on a 3rd party relay would also improve reliability in this case.