Is full protection possible when synchronizing with the server?

Question

Data synchronization

There is an idea to create a small MMORPG for mobile platforms with offline mode.
That is, if there is no Internet, the user can upgrade the character offline, and then synchronize the data with the server.
How much can you protect the data sent by the user from forgery? And how.

Answer 1

Everyone's favorite and respected company Blizzard considers the user's device "enemy territory". I think that says it all.

Answer 2

The user cannot be trusted, and from the word "absolutely".
Absolutely everything that happens on the client side can be faked.
UPD:
You can somehow try to take into account the results of offline actions in the form of an analysis of the full log of user actions, according to which the results will be calculated on the server, with additional verification that such actions can be honestly performed.
And of course you can not accept any ready-made results from the client.
UPD2:
If you really implement a scheme with recalculation on the server of results according to the user's log, then a problem may arise even for normal users when, after synchronization, the real result will turn out to be different from that one. what was calculated online. And if it differs not in favor of the player, then there may be a sickly seething.
So this option can really be considered as an extreme one. Best of all, of course, full settlement on the server and the client online.

Answer 3

In general, nothing, everything that can be hacked will be hacked. No action logs will save. In any case, they can write a bot, as a result, they won’t even open the code, but will simply emulate the player’s actions.