Even in the mid-1990s the key-to-BECM protocols used in old Range Rovers was frankly massively overengineered, with a 48-bit rolling code key based off the vehicle's VIN and a 24-bit key code. The actual encryption routine is just a bunch of shifts, adds, and XORs, but so far it has resisted any attempt at spoofing keys.
There's a somewhat simple trick to get the engine to start without the immobiliser (but it requires special tools), but if the body ECU is immobilised most of the vehicle electrics will be locked out too.
There's a somewhat simple trick to get the engine to start without the immobiliser (but it requires special tools), but if the body ECU is immobilised most of the vehicle electrics will be locked out too.