Encrypting a local user credentials file

Question

I am writing a configuration tool that accesses different databases and it has two different views. One for the admins that have rights to see and edit the connection strings, and one for the employees that don't. The login data is stored in a local xml file and the whole file needs to be encrypted. The problem I am having is that I don't know how to safely hide the key inside the application. How do I make it so that the application can access the file to check user credentials without exposing the key inside of the executable?