Multi-Column Primary Key in MySQL 5

I’m trying to learn how to use keys and to break the habit of necessarily having SERIAL type IDs for all rows in all my tables. At the same time, I’m also doing many-to-many relationships, and so requiring unique values on either column of the tables that coordinate the relationships would hamper that.

What’s the best method to use / store encryption keys in MySQL

I plan on using MySQL and it’s built-in encryption functionality to encrypt / decrypt certain columns in certain tables. The concern I have is that I need to store the key somewhere. I could certainly store the key in a file and control the permissions of that file and the permissions of the application that accesses it, but is that enough? I could also create a web service to get the key or something.