Ever heard of a 'shibboleth'?

The log in process to a university website, learn@uw if you want specifics, sometimes flashes a screen that says something like "Shibboleth authorization process." I'd never heard of it.

Turns out it "usually refers to features of language, and particularly to a word whose pronunciation identifies its speaker as being a member or not a member of a particular group." I didn't know there was an actual name for this sort of thing.

The examples Wikipedia has are mainly from WWII and how solders used them to tell their countrymen apart from disguised enemies.