Diffie-Hellman for TLS

After successfully enabling Let's Encrypt for my domain, I ran the test over at SSL Labs and was disappointed to see I only scored a C grade.

It seems that because my Nginx configuration supported SSLv3 it was susceptible to the POODLE attack. And thus regardless of anything else the grade was capped at C. This was easily fixed however by adding the following line to nginx.conf:

http {  
    ...
    ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
    ...
}

I ran the test again but this time only scored a B grade. Here's why:

Warning! This site uses a commonly-shared 1024-bit Diffie-Hellman group, and might be in range of being broken by a nation-state. It might be a good idea to generate a unique, 2048-bit group for the site.

Luckily, the kind folks over at weakdh.org provide a Guide to Deploying Diffie-Hellman for TLS. To summarise, do the following:

First thing to do is generate a dhparams.pem. I put it in my /etc/nginx/sites-available directory, but it doesn't really matter where you put it as long as it's a fairly safe location:

openssl dhparam -out /etc/nginx/sites-available/dhparams.pem 2048  

Now in the server block for your domain (by default this is in /etc/nginx/sites-available/default) add the following lines:

server {  
    ...
    ssl_ciphers 'ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-DSS-AES128-GCM-SHA256:kEDH+AESGCM:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA:ECDHE-ECDSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-DSS-AES128-SHA256:DHE-RSA-AES256-SHA256:DHE-DSS-AES256-SHA:DHE-RSA-AES256-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256:AES128-SHA:AES256-SHA:AES:CAMELLIA:DES-CBC3-SHA:!aNULL:!eNULL:!EXPORT:!DES:!RC4:!MD5:!PSK:!aECDH:!EDH-DSS-DES-CBC3-SHA:!EDH-RSA-DES-CBC3-SHA:!KRB5-DES-CBC3-SHA';
    ssl_prefer_server_ciphers on;
    ssl_dhparam /etc/nginx/sites-available/dhparams.pem;
    ...

Now reload Nginx:

sudo nginx -s reload  

These simple steps have garnered me an A grade.

A grade on SSL Labs

Implementing Let's Encrypt

Updated Now that Let's Encrypt is in public beta I've updated this to suit.

These are exciting times for the web. Secure connections have been around for a long time and are now expected and trusted by the public. Unfortunately they're also very expensive, usually around $99 per year.

But huzzah and hooray, we now have Let's Encrypt, a service from the EFF, Mozilla, Cisco and others to provide FREE TLS encryption for any website. You can read more about the mission here: https://letsencrypt.org/about/

At the moment the service is in public beta which means they're still ironing out the bugs, but it's currently serving tens of thousands of certs and seems to be holding up fine.

For the purposes of this little walk-through I'm using Ubuntu 14.10 and Nginx.

One of the goals of Let's Encrypt is that it be automatic. In time this will include configuring the server too (be that Apache, Nginx, or another) and it already does a pretty good job but there's definitely more work to do.

I'm going to show how get just the cert files and configure Nginx manually. So to start with clone the Let's Encrypt software somewhere, I put it in my home directory:

git clone https://github.com/letsencrypt/letsencrypt  

Ideally before the next step make sure you stop any service using port 80. There is a 'webroot' option to circumvent this and you can read more on the How it works page. But in my case I'm happy killing Nginx for a couple of minutes:

sudo service nginx stop  

Now cd in to the new directory and run the software with the following parameters:

cd letsencrypt  
./letsencrypt-auto certonly

The first time you run the program it will ask for an email address and ask that you agree with the T's and C's.

Lets Encrypt email address screen

Lets Encrypt terms and conditions screen

Finally it will ask for your domain(s). Separate these either with a space or a comma.

That's it! If all goes well the necessary files will be created here: /etc/letsencrypt/live/<yourdomain>/

To add to Nginx, add or change your sites-available file to look like this:

server {  
    listen 443 ssl;
    server_name lewiswalsh.com;
    ssl_certificate     /etc/letsencrypt/live/lewiswalsh.com/fullchain.pem;
    ssl_certificate_key /etc/letsencrypt/live/lewiswalsh.com/privkey.pem;

Next I created a server block to redirect https://www.lewiswalsh.com to https://lewiswalsh.com and one to redirect the insecure versions of those domains:

server {  
    listen       443 ssl;
    server_name  www.lewiswalsh.com;
    return       301 https://lewiswalsh.com$request_uri;
}
server {  
    listen       80;
    server_name  lewiswalsh.com www.lewiswalsh.com;
    return       301 https://lewiswalsh.com$request_uri;
}

Restart Nginx and all should be well.

sudo service nginx restart  

Since Let's Encrypt certificates expire every ninety days you'll need to manually renew. Eventually this can be automated, but for now just run the following command again when your certs expire:

./letsencrypt-auto certonly

While you're at it, you may as well beef up the Diffie-Hellman cyphers to get that A-grade SSL.