clmscript: add DecrementKey

clmscript/script.go

@@ -3,6 +3,7 @@ package clmscript
 import (
 	"bytes"
 	"crypto/sha256"
+	"math/big"
 
 	"github.com/btcsuite/btcd/btcec"
 	"github.com/btcsuite/btcd/txscript"
@@ -180,6 +181,25 @@ func IncrementKey(key *btcec.PublicKey) *btcec.PublicKey {
 	}
 }
 
+// DecrementKey is the opposite of IncrementKey, it "subtracts one" from the
+// current key to arrive at the key used before the IncrementKey operation.
+func DecrementKey(key *btcec.PublicKey) *btcec.PublicKey {
+	//  priorKey = key - G
+	//  priorKey = (key.x, key.y) + (G.x, -G.y)
+	curveParams := btcec.S256().Params()
+	negY := new(big.Int).Neg(curveParams.Gy)
+	negY = negY.Mod(negY, curveParams.P)
+	x, y := key.Curve.Add(
+		key.X, key.Y, curveParams.Gx, negY,
+	)
+
+	return &btcec.PublicKey{
+		X:     x,
+		Y:     y,
+		Curve: btcec.S256(),
+	}
+}
+
 // LocateOutputScript determines whether a transaction includes an output with a
 // specific script. If it does, the output index is returned.
 func LocateOutputScript(tx *wire.MsgTx, script []byte) (uint32, bool) {

clmscript/script_test.go

@@ -0,0 +1,37 @@
+package clmscript
+
+import (
+	"testing"
+
+	"github.com/btcsuite/btcd/btcec"
+	"github.com/stretchr/testify/require"
+)
+
+const (
+	numOperations = 1000
+)
+
+// TestIncrementDecrementKey makes sure that incrementing and decrementing an EC
+// public key are inverse operations to each other.
+func TestIncrementDecrementKey(t *testing.T) {
+	t.Parallel()
+
+	privKey, err := btcec.NewPrivateKey(btcec.S256())
+	require.NoError(t, err)
+
+	randomStartBatchKey := privKey.PubKey()
+
+	// Increment the key numOperations times.
+	currentKey := randomStartBatchKey
+	for i := 0; i < numOperations; i++ {
+		currentKey = IncrementKey(currentKey)
+	}
+
+	// Decrement the key again.
+	for i := 0; i < numOperations; i++ {
+		currentKey = DecrementKey(currentKey)
+	}
+
+	// We should arrive at the same start key again.
+	require.Equal(t, randomStartBatchKey, currentKey)
+}