keys/hd/address.go
func DerivePrivateKey(privKeyBytes []byte, chainCode []byte, index uint32, prime bool) ([]byte, []byte) {
	var data []byte
	if prime {
		index = index | 0x80000000
		data = append([]byte{byte(0)}, privKeyBytes...)
	} else {
		public := PubKeyBytesFromPrivKeyBytes(privKeyBytes, true)
		data = public
	}
	data = append(data, uint32ToBytes(index)...)
	data2, chainCode2 := I64(chainCode, data)
	x := addScalars(privKeyBytes, data2)
	return x, chainCode2
}
func DerivePrivateKey(privKeyBytes []byte, chainCode []byte, index uint32, prime bool) ([]byte, []byte) {
	var data []byte
	if prime {
		index = index | 0x80000000
		data = append([]byte{byte(0)}, privKeyBytes...)
	} else {
		public := PubKeyBytesFromPrivKeyBytes(privKeyBytes, true)
		data = public
	}
	data = append(data, uint32ToBytes(index)...)
	data2, chainCode2 := I64(chainCode, data)
	x := addScalars(privKeyBytes, data2)
	return x, chainCode2
}
// DerivePrivateKey derives the private key with index and chainCode.
// If prime is true, the derivation is 'hardened'.
// It returns the new private key and new chain code.