Hi and welcome to another Office Jesus tutorial. I just want to go ahead and say that if you are an ABSOLUTE beginner with no experience debugging or coding (Delphi in this case), you should NOT read this tutorial! I recommend watching Lena151's tutorials; they can be found at www.tuts4you.com. In this tutorial we will be exploring a simple serial checking routine that uses the MD5 hash and the RSA algorithm. I will not go into detail on how MD5 works or how it could be implemented. For more reading about MD5, check out the MD5 tutorials by Encrypto and ARTeam. I will try to explain RSA to the best of my ability. For further reading on RSA, visit RSA Wiki.