===========================
Secure and Fast Downloading
===========================

-----------------------
Module: mod_secdownload
-----------------------

:Author: Jan Kneschke
:Date: $Date: 2004/08/01 07:01:29 $
:Revision: $Revision: 1.1 $

:abstract:
  authenticated file requests and a countermeasure against
  deep-linking can be achieved easily by using mod_secdownload

.. meta::
  :keywords: lighttpd, secure, fast, downloads

.. contents:: Table of Contents

Options
=======

::

  secdownload.secret        = <string>
  secdownload.document-root = <string>
  secdownload.uri-prefix    = <string>  (default: /)
  secdownload.timeout       = <short>   (default: 60 seconds)

Description
===========

there are multiple ways to handle secured download mechanisms:

1. use the webserver and the internal HTTP authentication
2. use the application to authenticate and send the file
   through the application

Both ways have limitations:

webserver:

- ``+`` fast download
- ``+`` no additional system load
- ``-`` inflexible authentication handling

application:

- ``+`` integrated into the overall layout
- ``+`` very flexible permission management
- ``-`` the download occupies an application thread/process

A simple way to combine the two ways could be:

1. app authenticates user and checks permissions to
   download the file.
2. app redirects user to the file accessable by the webserver
   for further downloading.
3. the webserver transfers the file to the user.

As the webserver doesn't know anything about the permissions
used in the app, the resulting URL would be available to every
user who knows the URL.

mod_secdownload removes this problem by introducing a way to
authenticate a URL for a specified time. The application has
to generate a token and a timestamp which are checked by the
webserver before it allows the file to be downloaded by the
webserver.

The generated URL has to have the format:

<uri-prefix><token>/<timestamp-in-hex><rel-path>

<token> is an MD5 of

1. a secret string (user supplied)
2. <rel-path> (starts with /)
3. <timestamp-in-hex>


As you can see, the token is not bound to the user at all. The
only limiting factor is the timestamp which is used to
invalidate the URL after a given timeout (secdownload.timeout).

.. Note::
  Be sure to choose a another secret than the one used in the
  examples, as this is the only part of the token that is not
  known to the user.



If the user tries to fake the URL by choosing a random token,
status 403 'Forbidden' will be sent out.

If the timeout is reached, status 408 'Request Timeout' will be
sent. (This does not really conform to the standard, but should
do the trick).

If token and timeout are valid, the <rel-path> is appended to
the configured (secdownload.document-root) and passed to the
normal internal file transfer functionality. This might lead to
status 200 or 404.

Example
=======

Application
-----------

Your application has to generate the correct URLs. The following sample
code for PHP should be easily adaptable to any other language: ::

  <?php

  $secret = "verysecret";
  $uri_prefix = "/dl/";

  # filename
  $f = "/secret-file.txt";

  # current timestamp
  $t = time();

  $t_hex = sprintf("%08x", $t);
  $m = md5($secret.$f.$t_hex);

  # generate link
  printf('<a href="%s%s/%s%s">%s</a>',
         $uri_prefix, $m, $t_hex, $f, $f);
  ?>

Webserver
---------

The server has to be configured in the same way. The URI prefix and
secret have to match: ::

  server.modules = ( ..., "mod_secdownload", ... )

  secdownload.secret          = "verysecret"
  secdownload.document-root   = "/home/www/servers/download-area/"
  secdownload.uri-prefix      = "/dl/"
  secdownload.timeout         = 120
  secdownload.algorithm       = "md5"