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SSL improvement

September 23, 2016

The report

Recently I decided to enable SSL for my blog. Of course, I checked the quality of my SSL settings, which weren’t too bad, but not A+ either:

The clue is in the report:

This server supports weak Diffie-Hellman (DH) key exchange parameters. Grade capped to B.

The cause

The problem is explained in detail at https://weakdh.org. What it boils down to is that by default, a Diffie-Helman group of 1024 bits is generated, which is considered weak by today’s standards.

This is the main reason for the ‘B’ grade. Other SSL configuration parameters are important as well, so read on :)

The solution

Here’s how I remediated this problem. First, I created a 4096 bit DH group:

openssl dhparam -out /etc/ssl/certs/dhparams.pem 4096

The output would be something like:

Generating DH parameters, 4096 bit long safe prime, generator 2
This is going to take a long time
........................................................................................................................
<snip>
.........+.............................................................................................................++*++*

After generating the 4096 bit DH group, I changed the following parameters in the server block of my nginx vhost config:

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/ssl/certs/dhparams.pem;

    ...
}

The nginx configuration can be checked with

nginx -t

Any problems with the configuration will be reported. If you’ve done it right, it shows something like:

nginx: the configuration file /etc/nginx/nginx.conf syntax is ok
nginx: configuration file /etc/nginx/nginx.conf test is successful

Make sure you include all configuration files the right way, otherwise you might be troubleshooting configuration that isn’t actually active at all!

After reloading nginx, all this resulted in the following report:

Close, but no sigar..!

A few more SSL settings need to be modified in order to get the A+ rating. So, I edited the nginx config again:

server {
    listen 443 ssl default_server;
    listen       [::]:443 ssl http2 default_server;

    ...

    ssl_certificate ""/etc/..../fullchain.pem"";
    ssl_certificate_key ""/etc/..../privkey.pem"";

    ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
    ssl_prefer_server_ciphers on;
    ssl_ciphers ""EECDH+AESGCM:EDH+AESGCM:AES256+EECDH:AES256+EDH"";
    ssl_ecdh_curve secp384r1;
    ssl_session_cache shared:SSL:10m;
    ssl_session_tickets off;
    ssl_stapling on;
    ssl_stapling_verify on;

    add_header Strict-Transport-Security ""max-age=63072000; includeSubdomains"";
    add_header X-Frame-Options DENY;
    add_header X-Content-Type-Options nosniff;

    ssl_dhparam /etc/ssl/certs/dhparams.pem;

    ...
}

Which resulted in A+ (yay):

A lot of information about these nginx settings can be found here: https://raymii.org/s/tutorials/Strong_SSL_Security_On_nginx.html and here: https://cipherli.st.

Mitigating TLS-FALLBACK-SCSV would be possible, if it weren’t for the openssl version on CentOS 7, which is 1.0.1e. OpenSSL 1.0.1 has TLSFALLBACKSCSV in 1.0.1j and higher though.