code: func (api *HandshakeAPI) GetHandshakeKeyCapacity(symkeyid string) (uint16, error) {
  	storekey := api.ctrl.getSymKey(symkeyid)
  	if storekey == nil {
  		return 0, fmt.Errorf("invalid symkey id %s", symkeyid)
  	}
  	return storekey.limit - storekey.count, nil
  }
docs: // Returns the amount of messages the specified symmetric key
  // is still valid for under the handshake scheme