How to make a secure download ?

Hi, I am trying to put a file to download. I want it secure so only the people having the password can access it. I made a page that requires a password.. if the password is correct it changes the password send it on my mail and redirects the user to another script. This script searches in the database for the id given with GET and finds the name of the file which is md5(something). it changes the name of the file in md5(smth else). After that it searches in the download folder for a file named md5(smth else). if it finds the file it gives you the link to the file. So i need to find a way to change the filename after the user downloads the file. to keep it secure so the user wont be able to share the link to oters. if you need more info ask :D thanks and sorry for my bad english .