In theory you can't actually do it at all. The key is inside the chip, the attacker has physical control over the chip, an attacker with enough resources is going to be able to extract the key. You have no hope against a state-level attacker or even many university research departments. The assumption seems to be that the attacker won't be that sophisticated.

The problem is there are also likely to be attacks which won't require significant resources once published. Researchers are always coming up with new ways to extract keys from "tamper proof hardware" using timing or power consumption or whatever else. Some future version of the hardware will protect against that specific attack but that's too late for all the secrets you trusted to the current version.