From 4003f252819840483ac81756d103fca26001954b Mon Sep 17 00:00:00 2001
From: Oliver Gugger <gugger@gmail.com>
Date: Tue, 28 Apr 2020 10:06:32 +0200
Subject: [PATCH] signrpc: use ECDH interface for shared key generation

---
 lnrpc/signrpc/signer_server.go | 29 ++++++-----------------------
 1 file changed, 6 insertions(+), 23 deletions(-)

diff --git a/lnrpc/signrpc/signer_server.go b/lnrpc/signrpc/signer_server.go
index 127d113e..1d53be2b 100644
--- a/lnrpc/signrpc/signer_server.go
+++ b/lnrpc/signrpc/signer_server.go
@@ -5,7 +5,6 @@ package signrpc
 import (
 	"bytes"
 	"context"
-	"crypto/sha256"
 	"fmt"
 	"io/ioutil"
 	"os"
@@ -515,31 +514,15 @@ func (s *Server) DeriveSharedKey(_ context.Context, in *SharedKeyRequest) (
 		locator.Index = uint32(in.KeyLoc.KeyIndex)
 	}
 
-	// Derive our node's private key from the key ring.
-	idPrivKey, err := s.cfg.KeyRing.DerivePrivKey(keychain.KeyDescriptor{
-		KeyLocator: locator,
-	})
+	// Derive the shared key using ECDH and hashing the serialized
+	// compressed shared point.
+	keyDescriptor := keychain.KeyDescriptor{KeyLocator: locator}
+	sharedKeyHash, err := s.cfg.KeyRing.ECDH(keyDescriptor, ephemeralPubkey)
 	if err != nil {
-		err := fmt.Errorf("unable to derive node private key: %v", err)
+		err := fmt.Errorf("unable to derive shared key: %v", err)
 		log.Error(err)
 		return nil, err
 	}
-	idPrivKey.Curve = btcec.S256()
 
-	// Derive the shared key using ECDH and hashing the serialized
-	// compressed shared point.
-	sharedKeyHash := ecdh(ephemeralPubkey, idPrivKey)
-	return &SharedKeyResponse{SharedKey: sharedKeyHash}, nil
-}
-
-// ecdh performs an ECDH operation between pub and priv. The returned value is
-// the sha256 of the compressed shared point.
-func ecdh(pub *btcec.PublicKey, priv *btcec.PrivateKey) []byte {
-	s := &btcec.PublicKey{}
-	x, y := btcec.S256().ScalarMult(pub.X, pub.Y, priv.D.Bytes())
-	s.X = x
-	s.Y = y
-
-	h := sha256.Sum256(s.SerializeCompressed())
-	return h[:]
+	return &SharedKeyResponse{SharedKey: sharedKeyHash[:]}, nil
 }