A personal verification scheme might be impossible.

Any password you make ahead of time could be compromised through mind reading, detective work, super advanced psychological profiling or mental simulation based off later neural scans of your mind that guesses your password, etc.

On the other hand if you randomly generate a password and having him give it to you before you read it, I don't see how he could have the information. He comes from a future where you didn't generate the password, right? Only the future you that comes from the timeline where you avoided the reason for the visit would have that password. Wicked paradox.