What do I need to change in this code to have it not require static? [Extension Methods]

I'm a beginner programming student, and I was given a class file by my teacher as an example of basic cryptography. I'm using an edited version of it in one of my programs.

The teacher's version has everything static, where as I want to instantiate like a normal class, as in Class objectname = new Class();.

So I tried to remove the statics, but apparently it uses "extension methods", which require static, and which I'm having trouble understanding from my google/stack research.

In any case, I couldn't find a solution. Could any of you help me?
Here is the code on pastebin: http://ift.tt/1MfrXOS

by AntediluvianChilde via /r/csharp

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