Des del 2000 compartiendo sobre…

Secure download URLs with expiration time

Reading time: 4 – 6 minutes


Imagine a HTTP server with those restrictions:

  • only specific files can be downloaded
  • with a limited time (expiration date)
  • an ID allows to trace who download files
  • with minimal maintenance and dependencies (no databases, or things like that)

the base of the solution that I designed is the URL format:

  • signature: is calculated with the next formula, given a “seed”
    • seed = “This is just a random text.”
    • str = customer_id + expire_date + path_n_file
    • signature = encode_base64( hmac_sha1( seed, str))
  • customer_id: just an arbitrary identifier when you want to distinguish who use the URL
  • expire_date: when the generated URL stops working
  • path_n_file: relative path in your private repository and the file to share

Understanding the ideas explained before I think it’s enough to understand what is the goal of the solution. I developed the solution using NGINX and LUA. But the NGINX version used is not the default version is a very patched version called Openresty. This version is specially famous because some important Chinese webs works with that, for instance, Taobao.com

Expiration URL solution Architecture schema

In the above schema there is a master who wants to share a file which is in the internal private repository, but the file has a time restriction and the URL is only for that customer. Then using the command line admin creates a unique URL with desired constrains (expiration date, customer to share and file to share). Next step is send the URL to the customer’s user. When the URL is requested NGINX server evaluates the URL and returns desired file only if the user has a valid URL. It means the URL is not expired, the file already exists, the customer identification is valid and the signature is not modified.

NGINX Configuration

server {
 server_name downloads.local;

 location ~ ^/(?<signature>[^/]+)/(?<customer_id>[^/]+)/(?<expire_date>[^/]+)/(?<path_n_file>.*)$ {
 content_by_lua_file "lua/get_file.lua";

 location / {
 return 403;

This is the server part of the NGINX configuration file, the rest of the file can as you want. Understanding this file is really simple, because the “server_name” works as always. Then only locations command are relevant. First “location” is just a regular expression which identifies the relevant variables of the URL and passes them to the LUA script. All other URLs that doesn’t match with the URI pattern fall in path “/” and the response is always “Forbiden” (HTTP 403 code). Then magics happen all in LUA code.

LUA scripts

There are some LUA files required:

  • create_secure_link.lua: creates secure URLs
  • get_file.lua: evaluates URLs and serves content of the required file
  • lib.lua: module developed to reuse code between other lua files
  • sha1.lua: SHA-1 secure hash computation, and HMAC-SHA1 signature computation in Lua (get from https://github.com/kikito/sha.lua)

It’s required to configure “lib.lua” file, at the beginning of the file are three variables to set up:

lib.secret = "This is just a long string to set a seed"
lib.base_url = "http://downloads.local/"
lib.base_dir = "/tmp/downloads/"

Create secure URLs is really simple, take look of the command parameters:

$ ./create_secure_link.lua 

 ./create_secure_link.lua <customer_id> <expiration_date> <relative_path/filename>

Create URLs with expiration date.

 customer_id: any string identifying the customer who wants the URL
 expiration_date: when URL has to expire, format: YYYY-MM-DDTHH:MM
 relative_path/filename: relative path to file to transfer, base path is: /tmp/downloads/

Run example:

$ mkdir -p /tmp/downloads/dir1
$ echo hello > /tmp/downloads/dir1/example1.txt
$ ./create_secure_link.lua acme 2015-08-15T20:30 dir1/example1.txt
$ date
Wed Aug 12 20:27:14 CEST 2015
$ curl http://downloads.local:55080/YjZhNDAzZDY0/acme/2015-08-15T20:30/dir1/example1.txt
$ date
Wed Aug 12 20:31:40 CEST 2015
$ curl http://downloads.local:55080/YjZhNDAzZDY0/acme/2015-08-15T20:30/dir1/example1.txt
Link expired

Little video demostration


Disclaimer and gratefulness



5 thoughts on “Secure download URLs with expiration time

  1. Does this effect the performance of downloading procedure as it’s processing via Lua script rather than NGINX ?

  2. I don’t know if I understand your question, but you can create expiry dates for just one second which is the minimum resolution of the timestamp used in the URL.

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