sonr/internal/address/bip32.go

package address

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, coin type, and index.
func ComputePublicKey(extPubKey []byte, chainCode []byte, coinType uint32, index int) ([]byte, error) {
	// Check if the index is a hardened child key
	if uint32(index) >= HardenedOffset {
		return nil, errors.New("cannot derive hardened child key from public key")
	}

	// 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, 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: pubKey + IL * G
	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
}