multi: refactor SignDigestCompact into SignMessageCompact

To make it possible to use a remote lnrpc server as a signer for our
wallet, we need to change our main interface to sign the message instead
of the message's digest. Otherwise we'd need to alter the
lnrpc.SignMessage RPC to accept a digest instead of only the message
which has security implications.

lnrpc/invoicesrpc/addinvoice.go

@@ -10,7 +10,6 @@ import (
 	"time"
 
 	"github.com/btcsuite/btcd/chaincfg"
-	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/wire"
 	"github.com/btcsuite/btcutil"
 	"github.com/davecgh/go-spew/spew"
@@ -426,14 +425,11 @@ func AddInvoice(ctx context.Context, cfg *AddInvoiceConfig,
 		return nil, nil, err
 	}
 
-	payReqString, err := payReq.Encode(
-		zpay32.MessageSigner{
-			SignCompact: func(msg []byte) ([]byte, error) {
-				hash := chainhash.HashB(msg)
-				return cfg.NodeSigner.SignDigestCompact(hash)
-			},
+	payReqString, err := payReq.Encode(zpay32.MessageSigner{
+		SignCompact: func(msg []byte) ([]byte, error) {
+			return cfg.NodeSigner.SignMessageCompact(msg, false)
 		},
-	)
+	})
 	if err != nil {
 		return nil, nil, err
 	}

lnrpc/lightning.proto

@@ -1086,6 +1086,12 @@ message SignMessageRequest {
     base64.
     */
     bytes msg = 1;
+
+    /*
+    Instead of the default double-SHA256 hashing of the message before signing,
+    only use one round of hashing instead.
+    */
+    bool single_hash = 2;
 }
 message SignMessageResponse {
     // The signature for the given message

lnrpc/lightning.swagger.json

@@ -6187,6 +6187,10 @@
           "type": "string",
           "format": "byte",
           "description": "The message to be signed. When using REST, this field must be encoded as\nbase64."
+        },
+        "single_hash": {
+          "type": "boolean",
+          "description": "Instead of the default double-SHA256 hashing of the message before signing,\nonly use one round of hashing instead."
         }
       }
     },