Try setting the properties of your class to "Global MultiUse".
[right click on the Class file in the Project Explorer window, then go Properties - you set the "instancing" property there - change it to Global Multiuse from what it is now].

This setting will let your code call the "methods" in the class [e.g. EncryptString] *AS IF* you had already instanced the class and were calling the methods the usual "COM way".

You may have to go Classname.EncryptString(...) in your code where Classname is the "class" name of the .CLS file rather than just "Encryptstring" [see the properties of the .CLS file in Project Explorer for the Classname it has - usually its the same as the .CLS file, "base" filename (sans the .CLS extension), but can be different if you so decide to make it different].

Either of both of these should solve your problem.

Note, as the .cls file is a VB Class, you have to do it this way.
If you want to call EncryptString without needing to instance it or fiddle with Instancing parameters, then move all the code from in EncryptString into a Public Function in the ".bas" (module) file, and have EncryptString call this function (merely being a "wrapper" for the module function).