Extra complexity. I imagine if there is a presumably existing secret key, that it's generated at run time by properties of the environment combined with the lo

Extra complexity. I imagine if there is a presumably existing secret key, that it's generated at run time by properties of the environment combined with the logic in the update to have known results for each chip, then matching logic is presumably part of an operation where the math is dependent on the key matching or "fitting". Thus you can get (some of) the security of an arbitrary length key without explicitly recording what that key exactly is. You could spread out the mechanisms and various individual incremental tests of the "lock" throughout the update operation code so that an attacker would have to completely understand the entire codebase just to understand the locking mechanism instead of the locking mechanism being self contained function with a pass/fail output.