One of the useful features of Apache (or indeed any competent web server) is the ability to use client side certificates. All this means is that a certificate from each end of the TLS transaction is verified: the browser verifies the website certificate, but the website requires the client also to present one and verifies it. Using client certificates, when linked to your own client certificate CA gives web transactions the strength of two factor authentication if you do it on the login page. I use this feature quite a lot for all the admin features my own website does.[…]
https://blog.hansenpartnership.com/using-tpm-based-client-certificates-on-firefox-and-apache/
Firmware Security 