How to prevent a user from changing a file manually?

Question

In a WPF application I use .txt files for holding some information. An application can read and write data from/to .txt file. Everything is OK, but the problem is that, to achieve this purpose, I have to grant writing access rights to these files for a user of an application and so, he/she gets the possibility to edit these files manually.

How could I set editing .txt files access rights for an application without granting the same level of rights to a user?

Edited (added):

After getting some comments and answers, I put the question this way (just to make my question more clear and not restricted to user access rights scope): How I could prevent the user from changing the file manually?

Answer 1

Encrypt it, or digital sign it