To explain myself, what I need is to be able to have a way so that the central server can only interact with the contract only with valid identification from a user? This identification provided by the user may be a password or anything else that isn't too inconvenient for the user. However, most importantly, there should be no way for someone to hack the system into authenticating an invalid authentification. So ideally the contract would have to verify that the user has provided identification that allows the server to perform a certain task such as changing the user's balance value in the contract.
The goal is that a user can interact with the decentralized contract through the server which can then send additional information to the contract, if and only if a certain authentification that can not be forged and can be verified by the contract is provided.
To give a concrete example for my use case: I have a web app that users can access from their browser. However, I want users to be able to easily interact with a smart contract without having to set up anything on the client side. Therefore I want to be able to have a user which has an account on the web app to be able to click a simple button that sends a post request to the server. This post request will for instance transfer a user's balance into another user's balance in the contract. However, the server has to know to only call the smart contract if this is a legitimate and verified call. And the smart contract needs to be able to distingue between a legitimate call and a forged call. As to not have someone send a user's funds to his account on the smart contract without authorization. Which would be undesirable since what happens on the contract is out of my control.
Is there any smart way to do this?