cln: add CLN funding key derivation

Author: guggero

cln/derivation.go

@@ -0,0 +1,59 @@
+package cln
+
+import (
+	"crypto/sha256"
+	"encoding/binary"
+
+	"github.com/btcsuite/btcd/btcec/v2"
+	"golang.org/x/crypto/hkdf"
+)
+
+var (
+	InfoPeerSeed   = []byte("peer seed")
+	InfoPerPeer    = []byte("per-peer seed")
+	InfoCLightning = []byte("c-lightning")
+)
+
+// FundingKey derives a CLN channel funding key for the given peer and channel
+// number (incrementing database index).
+func FundingKey(hsmSecret [32]byte, peerPubKey *btcec.PublicKey,
+	channelNum uint64) (*btcec.PublicKey, error) {
+
+	channelBase, err := HkdfSha256(hsmSecret[:], nil, InfoPeerSeed)
+	if err != nil {
+		return nil, err
+	}
+
+	peerAndChannel := make([]byte, 33+8)
+	copy(peerAndChannel[:33], peerPubKey.SerializeCompressed())
+	binary.LittleEndian.PutUint64(peerAndChannel[33:], channelNum)
+
+	channelSeed, err := HkdfSha256(
+		channelBase[:], peerAndChannel[:], InfoPerPeer,
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	fundingKey, err := HkdfSha256(channelSeed[:], nil, InfoCLightning)
+	if err != nil {
+		return nil, err
+	}
+
+	_, pubKey := btcec.PrivKeyFromBytes(fundingKey[:])
+	return pubKey, nil
+}
+
+// HkdfSha256 derives a 32-byte key from the given input key material, salt, and
+// info using the HKDF-SHA256 key derivation function.
+func HkdfSha256(key, salt, info []byte) ([32]byte, error) {
+	expander := hkdf.New(sha256.New, key, salt, info)
+	var outputKey [32]byte
+
+	_, err := expander.Read(outputKey[:])
+	if err != nil {
+		return [32]byte{}, err
+	}
+
+	return outputKey, nil
+}

cln/derivation_test.go

@@ -0,0 +1,37 @@
+package cln
+
+import (
+	"encoding/hex"
+	"testing"
+
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/stretchr/testify/require"
+)
+
+var (
+	hsmSecret = [32]byte{
+		0x3f, 0x0a, 0x06, 0xc6, 0x38, 0x5b, 0x74, 0x93,
+		0xf7, 0x5a, 0xa0, 0x08, 0x9f, 0x31, 0x6a, 0x13,
+		0xbf, 0x72, 0xbe, 0xb4, 0x30, 0xe5, 0x9e, 0x71,
+		0xb5, 0xac, 0x5a, 0x73, 0x58, 0x1a, 0x62, 0x70,
+	}
+	peerPubKeyBytes, _ = hex.DecodeString(
+		"02678187ca43e6a6f62f9185be98a933bf485313061e6a05578bbd83c54e" +
+			"88d460",
+	)
+	peerPubKey, _ = btcec.ParsePubKey(peerPubKeyBytes)
+
+	expectedFundingKeyBytes, _ = hex.DecodeString(
+		"0326a2171c97673cc8cd7a04a043f0224c59591fc8c9de320a48f7c9b68a" +
+			"b0ae2b",
+	)
+)
+
+func TestFundingKey(t *testing.T) {
+	fundingKey, err := FundingKey(hsmSecret, peerPubKey, 1)
+	require.NoError(t, err)
+
+	require.Equal(
+		t, expectedFundingKeyBytes, fundingKey.SerializeCompressed(),
+	)
+}