Authenticate Password:
PayPal will generate a username/password pair for each subscriber and communicate these variables to you in your Instant Payment Notification posts.
Your code will activate or deactivate the pairs based on your password protection process.
Note: Passwords sent through IPN will be hashed. The hash is generated by the UNIX crypt() function which is based on the DES hashing algorithm. For more information, type "man crypt" on any UNIX terminal.
Here is a link to a site if you would like to do it manually:
http://www.euronet.nl/~arnow/htpasswd/
Stephen Ivaskevicius
PayPal/eBay, Inc.
www.paypal.com/pdn