The pre-authentication requires the requesting user to supply its secret key (DES, RC4, AES128 or AES256) derived from the user password. Technically, when asking the KDC (Key Distribution Center) for a TGT (Ticket Granting Ticket), the requesting user needs to validate pre-authentication by sending a timestamp encrypted with it's own credentials. It ensures the user is requesting a TGT for himself. Once validated, the TGT is then sent to the user in the KRB_AS_REP message, but that message also contains a session key. That session key is encrypted with the requested user's NT hash.