Game software for use in video game systems can be downloaded from Internet servers to game consoles in encrypted form to protect the software from being copied by software pirates. A small but essential part of the game software can be encrypted for use with a larger amount software that is not encrypted. The encrypted portion is downloaded into a secure cryptoprocessor preferably in a memory cartridge that plugs into a game system. This cryptoprocessor decrypts the downloaded software, stores it in on-chip EEPROM and then executes it, all in the same cryptoprocessor. The non-encrypted software is processed in the game system by a conventional processor which depends on data generated by program instructions decrypted and executed in the secure cryptoprocessor.