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crypto/bip32/bip32.go

@@ -0,0 +1,66 @@
+package bip32
+
+import (
+	"crypto/hmac"
+	"crypto/sha512"
+	"encoding/binary"
+	"errors"
+	"math/big"
+
+	"github.com/btcsuite/btcd/btcec/v2"
+)
+
+// ComputePublicKey computes the public key of a child key given the extended public key, chain code, and index.
+func ComputePublicKey(extPubKey []byte, chainCode uint32, index int) ([]byte, error) {
+	// Check if the index is a hardened child key
+	if chainCode&0x80000000 != 0 && index < 0 {
+		return nil, errors.New("invalid index")
+	}
+
+	// Serialize the public key
+	pubKey, err := btcec.ParsePubKey(extPubKey)
+	if err != nil {
+		return nil, err
+	}
+	pubKeyBytes := pubKey.SerializeCompressed()
+
+	// Serialize the index
+	indexBytes := make([]byte, 4)
+	binary.BigEndian.PutUint32(indexBytes, uint32(index))
+
+	// Compute the HMAC-SHA512
+	mac := hmac.New(sha512.New, []byte{byte(chainCode)})
+	mac.Write(pubKeyBytes)
+	mac.Write(indexBytes)
+	I := mac.Sum(nil)
+
+	// Split I into two 32-byte sequences
+	IL := I[:32]
+
+	// Convert IL to a big integer
+	ilNum := new(big.Int).SetBytes(IL)
+
+	// Check if parse256(IL) >= n
+	curve := btcec.S256()
+	if ilNum.Cmp(curve.N) >= 0 {
+		return nil, errors.New("invalid child key")
+	}
+
+	// Compute the child public key
+	ilx, ily := curve.ScalarBaseMult(IL)
+	childX, childY := curve.Add(ilx, ily, pubKey.X(), pubKey.Y())
+	lx := newBigIntFieldVal(childX)
+	ly := newBigIntFieldVal(childY)
+
+	// Create the child public key
+	childPubKey := btcec.NewPublicKey(lx, ly)
+	childPubKeyBytes := childPubKey.SerializeCompressed()
+	return childPubKeyBytes, nil
+}
+
+// newBigIntFieldVal creates a new field value from a big integer.
+func newBigIntFieldVal(val *big.Int) *btcec.FieldVal {
+	lx := new(btcec.FieldVal)
+	lx.SetByteSlice(val.Bytes())
+	return lx
+}