In a recent project we faced a challange to authenticate user through asp. We needed only to authenticate user and not log him into the Magento store site.
After googling through the net I found this link http://www.magentocommerce.com/boards/viewthread/36143/#t119542
<?php
/**
* Magento validation starts here
*/
require_once (dirname(dirname(realpath(__FILE__))).’/app/Mage.php’);
umask(0);
Mage::app();
//Mage::getSingleton(’core/session’, array(’name’=>’frontend’));
$session = Mage::getSingleton(’customer/session’);
try{
$session->login($_REQUEST['login']['username'],$_REQUEST['login']['password']);
$session->setCustomerAsLoggedIn($session->getCustomer());
//Load the customer modules
$customer = Mage::getModel(’customer/customer’)
->setWebsiteId(Mage::app()->getStore()->getWebsiteId());
//Get customer attribbutes
$customer->loadByEmail($_REQUEST['login']['username']);
/**
* Do the stuff [...]