From af01571fc61dfab2f5813050dc59c40e7c00be20 Mon Sep 17 00:00:00 2001
From: Yaacov Akiba Slama <ya@slamail.org>
Date: Sun, 28 Jun 2020 16:51:42 +0300
Subject: [PATCH] Let invoice.Encode receive a function which hashes itself the
 message

Modify the SignCompact function passed to invoice.Encode to receive the
message before it's hashed and hash it itself.
With this modification, the SignMessage rpc function from the signrpc
subserver can be used and an invoice can be encoded outside of lnd.
---
 lnrpc/invoicesrpc/addinvoice.go | 6 +++++-
 zpay32/encode.go                | 4 ++--
 zpay32/invoice.go               | 6 +++---
 zpay32/invoice_test.go          | 7 ++++---
 4 files changed, 14 insertions(+), 9 deletions(-)

diff --git a/lnrpc/invoicesrpc/addinvoice.go b/lnrpc/invoicesrpc/addinvoice.go
index 573d2f38..7daf599c 100644
--- a/lnrpc/invoicesrpc/addinvoice.go
+++ b/lnrpc/invoicesrpc/addinvoice.go
@@ -10,6 +10,7 @@ import (
 	"time"
 
 	"github.com/btcsuite/btcd/chaincfg"
+	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcutil"
 	"github.com/davecgh/go-spew/spew"
 
@@ -387,7 +388,10 @@ func AddInvoice(ctx context.Context, cfg *AddInvoiceConfig,
 
 	payReqString, err := payReq.Encode(
 		zpay32.MessageSigner{
-			SignCompact: cfg.NodeSigner.SignDigestCompact,
+			SignCompact: func(msg []byte) ([]byte, error) {
+				hash := chainhash.HashB(msg)
+				return cfg.NodeSigner.SignDigestCompact(hash)
+			},
 		},
 	)
 	if err != nil {
diff --git a/zpay32/encode.go b/zpay32/encode.go
index 9084e230..92783f65 100644
--- a/zpay32/encode.go
+++ b/zpay32/encode.go
@@ -70,12 +70,11 @@ func (invoice *Invoice) Encode(signer MessageSigner) (string, error) {
 	}
 
 	toSign := append([]byte(hrp), taggedFieldsBytes...)
-	hash := chainhash.HashB(toSign)
 
 	// We use compact signature format, and also encoded the recovery ID
 	// such that a reader of the invoice can recover our pubkey from the
 	// signature.
-	sign, err := signer.SignCompact(hash)
+	sign, err := signer.SignCompact(toSign)
 	if err != nil {
 		return "", err
 	}
@@ -95,6 +94,7 @@ func (invoice *Invoice) Encode(signer MessageSigner) (string, error) {
 				"signature: %v", err)
 		}
 
+		hash := chainhash.HashB(toSign)
 		valid := signature.Verify(hash, invoice.Destination)
 		if !valid {
 			return "", fmt.Errorf("signature does not match " +
diff --git a/zpay32/invoice.go b/zpay32/invoice.go
index 8444e3ee..7f2d86c7 100644
--- a/zpay32/invoice.go
+++ b/zpay32/invoice.go
@@ -100,11 +100,11 @@ var (
 // MessageSigner is passed to the Encode method to provide a signature
 // corresponding to the node's pubkey.
 type MessageSigner struct {
-	// SignCompact signs the passed hash with the node's privkey. The
-	// returned signature should be 65 bytes, where the last 64 are the
+	// SignCompact signs the hash of the passed msg with the node's privkey.
+	// The returned signature should be 65 bytes, where the last 64 are the
 	// compact signature, and the first one is a header byte. This is the
 	// format returned by btcec.SignCompact.
-	SignCompact func(hash []byte) ([]byte, error)
+	SignCompact func(msg []byte) ([]byte, error)
 }
 
 // Invoice represents a decoded invoice, or to-be-encoded invoice. Some of the
diff --git a/zpay32/invoice_test.go b/zpay32/invoice_test.go
index 41687a69..389c350d 100644
--- a/zpay32/invoice_test.go
+++ b/zpay32/invoice_test.go
@@ -101,9 +101,9 @@ var (
 	}
 
 	testMessageSigner = MessageSigner{
-		SignCompact: func(hash []byte) ([]byte, error) {
+		SignCompact: func(msg []byte) ([]byte, error) {
 			sig, err := btcec.SignCompact(btcec.S256(),
-				testPrivKey, hash, true)
+				testPrivKey, chainhash.HashB(msg), true)
 			if err != nil {
 				return nil, fmt.Errorf("can't sign the "+
 					"message: %v", err)
@@ -915,7 +915,8 @@ func TestInvoiceChecksumMalleability(t *testing.T) {
 
 	privKey, _ := btcec.PrivKeyFromBytes(btcec.S256(), privKeyBytes)
 	msgSigner := MessageSigner{
-		SignCompact: func(hash []byte) ([]byte, error) {
+		SignCompact: func(msg []byte) ([]byte, error) {
+			hash := chainhash.HashB(msg)
 			return btcec.SignCompact(btcec.S256(), privKey, hash, true)
 		},
 	}