http://l3tczdiiwoo63iwxty4lhs6p7eaxop5micbn7vbliydgv63x5zrrrfyd.onion/user-manual/howto.html
If this check passes, the browser sends a random number encrypted with the server's public key to the
server, and both compute a shared secret using the Diffie-Hellman key exchange algorithm. Now server and
browser can communicate, but no one else can break that communication because it's encrypted between them. 11.1.2.