How to get a 100% score on SSL Labs (Nginx, Let’s Encrypt)

This post should be titled “How to get a 100% score on SSL Labs (Nginx, Let’s Encrypt) as of April 2018“, since SSL Labs‘s test evolves all the time (and a good thing it does, too.) But that would be too long a title. 🙂

The challenge:

As we’ve seen before, it’s relatively easy to get an A+ rating on SSL Labs. However, even that configuration will only give a score like this:

Zurgl.com's current SSL Labs score
Zurgl.com’s current SSL Labs score

Certainly more than good enough (and, incidentally, almost certainly much better than whatever home banking site you use…), but your weirdo of a host can’t look at those less-than-100% bars and not see a challenge. 🙂

First, note that all of the following will be just for fun, as the settings to get 100% will demand recent browsers (and, in some cases, recent operating systems/devices), so you probably don’t want this in a world-accessible site. It would be more applicable, say, for a webmail used by you alone, or by half a dozen people. But the point here isn’t real-world use, it’s fun (OK, OK, and learning something new). So, let’s begin.

Getting that perfect SSL Labs score:

For this example, I’ll be using a just-installed Debian 9 server, in which I did something like:

apt -y install nginx
echo "deb http://ftp.debian.org/debian stretch-backports main" >> /etc/apt/sources.list.d/stretch-backports.list # if you don't have this repository active already
apt -y install python-certbot-nginx -t stretch-backports

(The reason for the above extra complexity is the fact that the default certbot in Debian Stretch is too old and tries to authenticate new certificates in an obsolete way, but there’s a newer one in stretch-backports.)

I also created a simple index.html file on /var/www/html/ that just says “Hello world!”. Right now, the site doesn’t even have HTTPS.

So, let’s create a new certificate and configure the HTTPS server in Nginx:

certbot --nginx --rsa-key-size 4096 --no-redirect --staple-ocsp -d mysite.mydomain.com

(replacing mysite.mydomain.com, obviously.)

Testing it on SSL Labs, it gives… exactly the same bars as in the image above, but with an A rating instead of A+. Right, we need HSTS to get A+ (and currently certbot doesn’t support configuring it automatically in Nginx), so we add this to the virtual host’s server section:

add_header Strict-Transport-Security "max-age=63072000; includeSubdomains; preload";

(Note: remove “includeSubDomains;” if you have HTTP sites on any subdomain. Also, HSTS makes your site HTTPS-only, so you can’t use it if you want to keep an HTTP version — in which case you can’t get better than an A rating.)

This upgrades the rating to A+, as expected, but the bars still didn’t change. Let’s make them grow, shall we?

Certificate:

It’s already at 100%, so yay. 🙂

Protocol Support:

Just disable any TLS lower than 1.2. In this case, edit /etc/letsencrypt/options-ssl-nginx.conf and replace the ssl_protocols line (or comment it and add a new one) with:

ssl_protocols TLSv1.2;

WARNING: any change you make to the file above will affect all virtual hosts on this Nginx server. If you need some of them to support older protocols, it’s better to just comment out that option in that file, and then include it in each virtual host’s server section.

EDIT: actually, it turns out that ssl_protocols affects the entire server, with the first option Nginx finds  (first in /etc/nginx.conf, then in the default website (which possibly includes /etc/letsencrypt/options-ssl-nginx.conf)) taking precedence. So just choose whether you want TLS 1.0 to 1.2 (maximum compatibility) or just 1.2 only (maximum security/rating); if you need both, use two separate servers.

Key Exchange:

We’ve already created the new certificate with a 4096-bit key (with “rsa-key-size 4096“, see above), and according to SSL Labs’ docs this should be enough… but it isn’t. After some googling, I found out that you also need to add the following option inside the server section in Nginx:

ssl_ecdh_curve secp384r1;

Since the default Nginx+OpenSSL/LibreSSL setting, either “X25519” or “secp256r1” (actually “prime256v1“), also lowers the score.

EDIT: again, ssl_ecdh_curve affects the entire server, so you can’t use different default curves for each virtual host. So I’d suggest (in /etc/letsencrypt/options-ssl-nginx.conf):

ssl_ecdh_curve secp384r1:X25519:prime256v1;

if you want the 100% rating, or:

ssl_ecdh_curve X25519:prime256v1:secp384r1:

otherwise. This one doesn’t affect compatibility, by the way; it’s just a question of the preferred order.

The certificate’s key size (4096 or 2048) is, like the certificate itself, specific to each virtual host.

Cipher Strength:

For 100% here, you need to disable not only any old protocols, but also any 128-bit ciphers. I started from Mozilla’s SSL tool (with the “Modern” option selected), then removed anything with “128” in its name, then moved ChaCha20 to the front just because, and ended up with this line, which I added to /etc/letsencrypt/options-ssl-nginx.conf (replacing the current setting with the same name):

ssl_ciphers 'ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384::ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA384';

(This time, this setting can indeed be specified for each virtual host, though you’d need to comment it out in the file above (which affects all virtual hosts with Let’s Encrypt certificates) if you want to do so, otherwise they’ll conflict.)

The result:

100% on SSL Labs

As said near the beginning, this is not something you likely want to do (yet) for a production, world-accessible site, as it will require relatively modern browsers, and you typically can’t control what visitors use1. According to SSL Labs’ list, it’s not actually that bad: all versions of Chrome, Firefox, or Safari from the past couple of years are fine, but no Microsoft browser older than IE11 will work, nor will Android’s default browser before 7.0 (Chrome on Android, which is not the same thing, will do fine.) I’d suggest it for a site where you can “control” the users, such as a small or medium-sized company’s webmail or employee portal (where you, as a sysadmin, can and should demand up-to-date security from your users).

But the main point of this exercise was, of course, to see if I could do it. Challenging yourself is always good. 🙂 And if you can share what you learned with others, so much the better.

(You can check out SSL Labs’ rating for the test site created in this post here. Among other things, you can see which browsers/versions are compatible with a site like this.)

  1. insert rant about Windows XP and IE

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.