code: func NewTransactor(keyin io.Reader, passphrase string) (*TransactOpts, error) {
  	json, err := ioutil.ReadAll(keyin)
  	if err != nil {
  		return nil, err
  	}
  	key, err := keystore.DecryptKey(json, passphrase)
  	if err != nil {
  		return nil, err
  	}
  	return NewKeyedTransactor(key.PrivateKey), nil
  }
docs: // NewTransactor is a utility method to easily create a transaction signer from
  // an encrypted json key stream and the associated passphrase.