If I were building a tiny number of auth servers relative to my frontend servers, I'd potentially just use an HSM and symmetric crypto vs. a computationally har

If I were building a tiny number of auth servers relative to my frontend servers, I'd potentially just use an HSM and symmetric crypto vs. a computationally hard KDF. Less subject to resource consumption DoS, somewhat more flexible for even regular authentication, and allows you to do things like hold credentials for services which don't support oauth using the same system.