Are you sure they aren't protected against writing? The Atmel EEPROMs that I use have that capability. You can send some commands to them and they are locked against writing. The microprocessor in your ECU reads the PROM to get the data. If the chip was protected against reading, the microprocessor can't see the data either. [what follows is from my recollection] Old Arcade cabinet video games used to scramble the data on the EPROM. They would use special microprocessors that would descramble the data on the fly. The EPROM would still output data and wasn't special in any way, but you needed a key to descramble the data. You also needed the key to encrypt the data on the EPROM. I think each uP had a unique key. This has slowed some of the video game historians from preserving a few of the video games.