rpc: add SignMessage and VerifyMessage interface

This commit allows users to sign messages with their node's private key
with the SignMessage interface. The signatures are zbase32 encoded for
human readability/paste-ability.  Others users can verify that a message
was signed by another node in their channel database with the
VerifyMessage interface.

lnrpc/README.md

@@ -37,6 +37,12 @@ description):
      * Returns a new address, the following address types are supported:
        pay-to-public-key-hash (p2pkh), pay-to-witness-key-hash (p2wkh), and
        nested-pay-to-witness-key-hash (np2wkh).
+  * SignMessage
+     * Signs a message with the node's identity key and returns a
+       zbase32 encoded signature.
+  * VerifyMessage
+     * Verifies a signature signed by another node on a message. The other node
+       must be an active node in the channel database.
   * ConnectPeer
      * Connects to a peer identified by a public key and host.
   * DisconnectPeer

lnrpc/rpc.proto

@@ -38,6 +38,9 @@ service Lightning {
         };
     }
 
+    rpc SignMessage (SignMessageRequest) returns (SignMessageResponse);
+    rpc VerifyMessage (VerifyMessageRequest) returns (VerifyMessageResponse);
+
     rpc ConnectPeer (ConnectPeerRequest) returns (ConnectPeerResponse) {
         option (google.api.http) = {
             post: "/v1/peers"
@@ -247,6 +250,21 @@ message NewAddressResponse {
     string address = 1 [json_name = "address"];
 }
 
+message SignMessageRequest {
+    bytes msg = 1 [ json_name = "msg" ];
+}
+message SignMessageResponse {
+    string signature = 1 [ json_name = "signature" ];
+}
+
+message VerifyMessageRequest {
+    bytes msg = 1 [ json_name = "msg" ];
+    string signature = 2 [ json_name = "signature" ];
+}
+message VerifyMessageResponse {
+    bool valid = 1 [ json_name = "valid" ];
+}
+
 message ConnectPeerRequest {
     LightningAddress addr = 1;
     bool perm = 2;

lnrpc/rpc.swagger.json

@@ -1706,6 +1706,23 @@
     "lnrpcSetAliasResponse": {
       "type": "object"
     },
+    "lnrpcSignMessageRequest": {
+      "type": "object",
+      "properties": {
+        "msg": {
+          "type": "string",
+          "format": "byte"
+        }
+      }
+    },
+    "lnrpcSignMessageResponse": {
+      "type": "object",
+      "properties": {
+        "signature": {
+          "type": "string"
+        }
+      }
+    },
     "lnrpcStopRequest": {
       "type": "object"
     },
@@ -1754,6 +1771,27 @@
         }
       }
     },
+    "lnrpcVerifyMessageRequest": {
+      "type": "object",
+      "properties": {
+        "msg": {
+          "type": "string",
+          "format": "byte"
+        },
+        "signature": {
+          "type": "string"
+        }
+      }
+    },
+    "lnrpcVerifyMessageResponse": {
+      "type": "object",
+      "properties": {
+        "valid": {
+          "type": "boolean",
+          "format": "boolean"
+        }
+      }
+    },
     "lnrpcWalletBalanceRequest": {
       "type": "object",
       "properties": {