multi: use btcd's btcec/v2 and btcutil modules

This commit was previously split into the following parts to ease
review:
 - 2d746f68: replace imports
 - 4008f0fd: use ecdsa.Signature
 - 849e33d1: remove btcec.S256()
 - b8f6ebbd: use v2 library correctly
 - fa80bca9: bump go modules

keychain/ecdh.go

@@ -3,7 +3,7 @@ package keychain
 import (
 	"crypto/sha256"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
 )
 
 // NewPubKeyECDH wraps the given key of the key ring so it adheres to the
@@ -72,10 +72,16 @@ func (p *PrivKeyECDH) PubKey() *btcec.PublicKey {
 //
 // NOTE: This is part of the SingleKeyECDH interface.
 func (p *PrivKeyECDH) ECDH(pub *btcec.PublicKey) ([32]byte, error) {
-	s := &btcec.PublicKey{}
-	s.X, s.Y = btcec.S256().ScalarMult(pub.X, pub.Y, p.PrivKey.D.Bytes())
+	var (
+		pubJacobian btcec.JacobianPoint
+		s           btcec.JacobianPoint
+	)
+	pub.AsJacobian(&pubJacobian)
 
-	return sha256.Sum256(s.SerializeCompressed()), nil
+	btcec.ScalarMultNonConst(&p.PrivKey.Key, &pubJacobian, &s)
+	s.ToAffine()
+	sPubKey := btcec.NewPublicKey(&s.X, &s.Y)
+	return sha256.Sum256(sPubKey.SerializeCompressed()), nil
 }
 
 var _ SingleKeyECDH = (*PubKeyECDH)(nil)