Wordpress XMLRPC Authentication from external app

functions.php

<?php
add_filter('xmlrpc_methods', 'wpse39662_add_login_method');

function wpse39662_add_login_method($methods)
{
    $methods['wpse39662.login'] = 'wpse39662_check_login';
    return $methods;
}

function wpse39662_check_login($args)
{
    $username = $args[0];
    $password = $args[1];

    $user = wp_authenticate($username, $password);

    if (is_wp_error($user)) {
      return false;
    }
    return true;
}
?>