code: func (c *controller) getPrimaryKeyTag(subsys string) ([]byte, uint64, error) {
  	c.Lock()
  	defer c.Unlock()
  	sort.Sort(ByTime(c.keys))
  	keys := []*types.EncryptionKey{}
  	for _, key := range c.keys {
  		if key.Subsystem == subsys {
  			keys = append(keys, key)
  		}
  	}
  	return keys[1].Key, keys[1].LamportTime, nil
  }
docs: // getPrimaryKeyTag returns the primary key for a given subsystem from the
  // list of sorted key and the associated tag