Add UMAD-07: payreq response.

umad-06-payreq-response.md

@@ -1 +1,42 @@
 # UMAD-06: Payreq Response
+
+The response to the payreq request is an extension of LNURL's [LUD-06](https://github.com/lnurl/luds/blob/luds/06.md).
+It contains the actual lightning invoice, as well as some additional fields for compliance and currency conversion.
+The full structure of the LNURLP response is:
+
+```raw
+{
+  // The encoded BOLT-11 invoice
+  "pr": string,
+  // Empty for legcy LNURL reasons.
+  "routes": [],
+  "compliance": {
+    // Public key of the recipient node for pre-screening.
+    "nodePubKey": string,
+    // A list of the expected UTXOs over which the receiver may receive the transaction (receiver's channels).
+    "utxos": string[],
+    // A url which the sending VASP should call on transaction completion to notify the receiving VASP of
+    // the utxos used to complete the transaction. See [UMAD-07](/umad-07-post-tx-hooks.md).
+    "utxoCallback": string
+  },
+  "paymentInfo": {
+    // The currency code of the receiving currency (eg. "USD"). This should match the requested currency in the payreq
+    // request.
+    "currencyCode": string,
+    // Millisats per "unit" of the receiving currency (eg. 1 cent in USD).
+    "multiplier": number,
+    // The fees charged (in millisats) by the receiving VASP to convert to the target currency.
+    // This is separate from the multiplier rate.
+    "exchangeFeesMillisatoshi": number
+  },
+  "umaVersion": "1.0",
+}
+```
+
+**NOTE:** The conversion rate is implicitly valid as long as the lightning invoice. VASPs can set a short invoice
+expiry to avoid large swings in conversion rate between invoice generation and payment. The invoice expiry is used as
+a hard cap on the conversion rate validity.
+
+As described in [UMAD-05](/umad-05-payreq-request.md), the `nodePubKey` and `utxos` fields are used for pre-screening
+by the sending VASP. The `utxoCallback` field is used by the receiving VASP to notify the sending VASP of the utxos
+used to complete the transaction. See [UMAD-07](/umad-07-post-tx-hooks.md) for details.