htaccess will work in a way. You'll have to have a password to get the file. so weather or not they are authenticated with stuff at yourdomain.com if they have the password they can get the file.
By your post I assumbed you wanted authentication to carry over from something you had going at your domain. To do this, you'll have to add on some extra functionality to the software. Perhaps there is already a mod, hack, or 'community featuere' that someone has made that you can use for this.
If not, you'll have to look around for some coding you can add in your self to accomplish this. I was talking about the secure downloader as this concept might carry over into your request easly.
don't worry, you'll learn greek eventually.