diff --git a/ecdsa.go b/ecdsa.go
index ffbcfffd..894de053 100644
--- a/ecdsa.go
+++ b/ecdsa.go
@@ -397,8 +397,8 @@ func (a *ecdsaContext) isLowS(s *big.Int) bool {
 func (a *ecdsaContext) signatureNormalizeS(sig []byte) ([]byte, bool) {
 	// read S
 	nLen := bitsToBytes(a.curveN.BitLen())
-	s := new(big.Int).SetBytes(sig[nLen:]) // S >= 0
-	if a.isLowS(s) {                       // S <= (n-1)/2
+	_, s := readTwoBigInts(sig, nLen) // S >= 0
+	if a.isLowS(s) {                  // S <= (n-1)/2
 		return sig, true // S is in the valid range and no need to flip it
 	}
 
diff --git a/ecdsa_test.go b/ecdsa_test.go
index 8c9e3c35..6f0819a5 100644
--- a/ecdsa_test.go
+++ b/ecdsa_test.go
@@ -549,7 +549,7 @@ func TestECDSAHighAndLowS(t *testing.T) {
 func (a *ecdsaContext) signatureFlipS(sig []byte) []byte {
 	// read S
 	nLen := bitsToBytes(a.curveN.BitLen())
-	s := new(big.Int).SetBytes(sig[nLen:])
+	_, s := readTwoBigInts(sig, nLen)
 	// compute N-S
 	sComplement := new(big.Int).Sub(a.curveN, s)
 	// write it into a new signature