fix incorrect padding in NewPublicKeyFromBigInt (#143)

dleq/cgo_dleq_test.go

@@ -1,8 +1,14 @@
 package dleq
 
 import (
+	"bytes"
+	"crypto/ecdsa"
+	"math/big"
 	"testing"
 
+	ethsecp256k1 "github.com/ethereum/go-ethereum/crypto/secp256k1"
+
+	"github.com/noot/atomic-swap/common"
 	mcrypto "github.com/noot/atomic-swap/crypto/monero"
 
 	ethcrypto "github.com/ethereum/go-ethereum/crypto"
@@ -25,3 +31,41 @@ func TestCGODLEq(t *testing.T) {
 	ed25519Pub := sk.Public().Bytes()
 	require.Equal(t, res.ed25519Pub[:], ed25519Pub)
 }
+
+func TestProofSecretComputesVerifyPubKeys(t *testing.T) {
+	// It would be nice to increase the number of iterations, but it's pretty slow even at 128. We
+	// previously had an issue when X or Y needed at least one high order padding byte. The chance
+	// of that happening is around (1/256+1/256)=1/128, so this loop will see values like that
+	// frequently, even if it doesn't happen on every run.
+	const iterations = 128
+
+	toBigInt := func(point [32]byte) *big.Int { return new(big.Int).SetBytes(point[:]) }
+
+	for i := 0; i < iterations; i++ {
+		proof, err := (&CGODLEq{}).Prove()
+		require.NoError(t, err)
+		res, err := (&CGODLEq{}).Verify(proof)
+		require.NoError(t, err)
+
+		// The ETH library needs the secret in big-endian format, while the monero library wants it
+		// in little endian format.
+		secretLE := proof.secret[:]
+		secretBE := common.Reverse(secretLE)
+
+		// Secp256k1 check
+		ethCurve := ethsecp256k1.S256()
+		xPub, yPub := ethCurve.ScalarBaseMult(secretBE)
+		ethPubFromSecret := &ecdsa.PublicKey{Curve: ethCurve, X: xPub, Y: yPub}
+		ethPubFromVerify := &ecdsa.PublicKey{Curve: ethCurve,
+			X: toBigInt(res.Secp256k1PublicKey().X()), Y: toBigInt(res.Secp256k1PublicKey().Y()),
+		}
+		require.True(t, ethPubFromSecret.Equal(ethPubFromVerify))
+
+		// ED25519 Check
+		sk, err := mcrypto.NewPrivateSpendKey(secretLE)
+		require.NoError(t, err)
+		xmrPubFromSecret := sk.Public().Bytes()
+		xmrPubFromVerify := res.ed25519Pub[:]
+		require.True(t, bytes.Equal(xmrPubFromSecret, xmrPubFromVerify))
+	}
+}