How to redirect users based on random URL string?

For security purposes, I need to only display a page to users if they are coming from a specific site, and not direct visitors.

The way I currently have it set up, there will be a random 12-character string generated and appended to the url. For example:

http://ift.tt/1QhHSM5

The skey parameter will always be random, but always have 12 characters. So basically, if my URL has a &key= parameter with 12 random characters, the page is displayed correctly. If not, the visitors go somewhere else.

Any help in regards to how to code this into the page, or what I should Google to find the necessary instructions? I have looked all over and haven't found how to go on about this, nor do I really know exactly what to look for. So any help would be greatly appreciated.

Thanks!