code: func (x *XC) RecipientIDs(ctx context.Context, ciphertext []byte) ([]string, error) {
  	msg := &xcpb.Message{}
  	if err := proto.Unmarshal(ciphertext, msg); err != nil {
  		return nil, err
  	}
  
  	ids := make([]string, 0, len(msg.Header.Recipients))
  	for k := range msg.Header.Recipients {
  		ids = append(ids, k)
  	}
  	sort.Strings(ids)
  	return ids, nil
  }
docs: // RecipientIDs reads the header of the given file and extracts the
  // recipients IDs