function doPasswordStuff(){
    if(isset($_POST['homepagepassword'])){
    	global $wpdb;
        $post_password = trim($_POST['passwordfield']);
        $post_id = $wpdb->get_var( $wpdb->prepare("SELECT ID FROM $wpdb->posts WHERE post_password = %s", $post_password) );
        if (!empty($post_id)) {
        	if ( empty( $wp_hasher ) ) {
        		require_once( ABSPATH . 'wp-includes/class-phpass.php' );
		        // By default, use the portable hash from phpass
		        $wp_hasher = new PasswordHash(8, true);
		    }
		    	setcookie( 'wp-postpass_' . COOKIEHASH, $wp_hasher->HashPassword( stripslashes( $post_password ) ), time() + 864000, COOKIEPATH );
                wp_redirect(get_permalink($post_id));
                exit();
        } else {
            // oh dear, there isnt a post with this 'password', put a redirect to a fallback here
            wp_redirect('http://address');
            exit();
        }
        wp_reset_query();
    }
}
add_action('template_redirect','doPasswordStuff');