Saving an Idle Game and offline progression

I am in the progress of creating an Incremental Idle game its my first in unity 5 using C#. I have the barebones structure in place got everything working money per sec, money per click, upgrades ect. the usual elements of a idle game. I am at the point where i'm attempting to save all the game info after exiting, then having the money progress while they are not playing the game or offline. I also want to be able to keep the same values in the upgrades the way they were when the player exited/offline. I do not want to save using PlayerPrefs as its unsecured and allows people to cheat. I am assuming I have to use serialization and equate code based on the computer clock and compare it to a world clock so they cannot cheat and modify the time/date on their computer. Anyways I hope someone can give me some sort of direction here. I am not asking for the answer just a pointing hand to guide the way. I have attempted searching for this but did not find any answer. Thank you in advance.