If you don't care about security, just save the file together with a hash. Ie:
your.xml
and
your.xml.hash
You can use System.Security.Cryptography.MD5Managed for example. It's just to save the xml file, and then save a hash of the file itself. When reading, just calculate the hash, compare with what's saved, and then use your xml file as regular.
Of course, the information in the xml file isn't encrypted, it's possible to read, but if you edit the file then the hash won't be correct, and your program will discover the attempt. Keep it simple :)
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