code: func (k *Keyring) UseKey(key []byte) error {
  	for _, installedKey := range k.keys {
  		if bytes.Equal(key, installedKey) {
  			k.installKeys(k.keys, key)
  			return nil
  		}
  	}
  	return fmt.Errorf("Requested key is not in the keyring")
  }
docs: // UseKey changes the key used to encrypt messages. This is the only key used to
  // encrypt messages, so peers should know this key before this method is called.