A vehicle communication test system is described that includes a diagnostic computer having a computer program product stored thereon. The program product includes a non-transitory computer-readable medium for the diagnostic computer that includes an application software program stored on the computer-readable medium that includes instructions adapted to validate encrypted messages transmitted over a network connection in a vehicle. The instructions include: performing an initialization sequence that includes receiving initialization data at the diagnostic computer, wherein the initialization data is associated with a plurality of vehicle system modules (VSMs) which are coupled together via the vehicle network connection; receiving as a data input at the diagnostic computer an encrypted message transmitted over the network connection; and based on the initialization data, determining at the diagnostic computer whether the received encrypted message is valid.