code: func (w *Worker) keyFromChannelData(logger log.Logger,
  	channel string) (data.Base64String, error) {
  	ch, err := w.channel(logger, channel)
  	if err != nil {
  		return "", err
  	}
  
  	offering, err := w.offering(logger, ch.Offering)
  	if err != nil {
  		return "", err
  	}
  
  	key, err := data.ChannelKey(ch.Client, ch.Agent,
  		ch.Block, offering.Hash)
  	// internal
  	if err != nil {
  		logger.Add("channel", ch, "offering", offering).Error(err.Error())
  		return "", ErrInternal
  	}
  	return data.FromBytes(key), nil
  }
docs: // keyFromChannelData returns the unique channel identifier
  // used in a Privatix Service Contract.