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A related talk about the system you are up against.
Long story short you have 3 possible levels of difficulty:
1. pretty straight forward for all cars without imobilization system. That's for cars older than about 1991 but you basically don't even need a key to start those if you know which two wires to short)
2. Basic electric systems which are about 3 relays more difficult to hack than 1.
3. A modern electronic system. Typically the engine control unit and the key have some pin-challenges going on with more or less tight security (as shown in the video above). Given the right car and knowledge you can actually unlock the imobilization system and after that it's more or less the same as 1 or 2. Some of those system appear to work with properly implemented crypto which makes hacking it harder. If you are good with a lot of paper and implementation you could actually implement a new key yourself. or you try mechanical tricks such as operating the rfid tag out of your actual car key and electromagnetically shield it from the reader until your own rfid tag gets read correctly.
Lots of fun to be had here.