Secure Persistance Data on Open dedicated server

I want to make an online Multiplayer Game with the posibility for Users to set up their own dedicated servers.
For the Players it should be possible to Travel across the servers. I want the Player stats and the Inventory to be
Persistance across the servers (eg Save them in MySQL Database)
So the servers are Open to Download, how can i make the Save for persistance Data on a Secure way?
The Clients i should Not Trust to Send their Data, but can i Trust the servers?
Any idea how to solve this?