Look at the Tor Rendezvous Specification rend-spec-v3.txt, the onion addresses that a user enter are Self authenticating, Because the onion address is the public key of the hidden service. 
Roger explains this in the DEF CON talk here https://youtu.be/Di7qAVidy1Y?t=1124


On Oct 14, 2017, at 12:33 AM, Toralf Förster <toralf.foerster@gmx.de> wrote:

do wonder how Tor handl