Mutual SSL authentication

http://www.codeproject.com/Articles/326574/An-Introduction-to-Mutual-SSL-Authentication

http://www.cafesoft.com/products/cams/ps/docs30/admin/SSLTLSPrimer.html

http://www.cafesoft.com/products/cams/ps/docs30/admin/ConfiguringApache2ForSSLTLSMutualAuthentication.html

http://stackoverflow.com/questions/454048/what-is-the-difference-between-encrypting-and-signing-in-asymmetric-encryption