I worked for a company that makes test taking / proctoring software that attempts to do this, I didn't work on the product myself but it seemed a bit of a trainwreck as they would always be having to do overrides for people who couldn't make it past the typing authentication (which was based on a previous sample of their typing), it measured pace, speed, etc.. the company itself wasn't that great so not surprising their implementation of this wasn't optimal, however its an interesting concept.