Documentation
System StatusPortal Login

How to enable SSL/TLS perfect forward secrecy in Apache or Nginx

Suggest Edits

This article provides an overview of perfect forward secrecy (PFS) and how
to enable it on Apache® or Nginx® web servers.

What is PFS?

PFS protects data shared between the client and the server even if the private key is compromised.
You can accomplish this by generating a session key for each transaction made.

Why implement PFS on a website?

A TLS or SSL certificate works by using a public key and a private key. When the web browser and
the server exchange keys, the system creates a session key by using a key exchange mechanism
called RSA, where all the information between the client and the server is encrypted. RSA
creates a link between the server private key and the session key created for each unique
secure session.

The session might get brute-force attacked—this consists of an attack that injects the
server with combinations of security keys until it finds the correct one. Even though this
process might take a long time, if the server's private key is compromised, the attackers
can see both the session data and all client transactions.

How PFS protects a website

PFS enables the server not to rely on a single session key. Instead of using the same
encryption key whenever a user or service makes a connection, PFS generates a unique
session key for each connection.

Enable PFS by using exchange mechanisms—Ephemeral Diffie-Hellman (DHE) and
Elliptic Curve Diffie-Hellman (ECDHE). If the attackers brute force the session key,
they can only decrypt the information from that one session and not the others.

Requirements to implement PFS in a web server

Use one of the following tools to implement PFS:

  • OpenSSL 1.0.1c+
  • Apache 2.4 or
  • Nginx 1.0.6+ and 1.1.0+

You can check the versions of these packages by running the following commands:

Note: The results might vary as the vendors release new versions.

[root@rackspace-test ~]$ openssl version
OpenSSL 1.1.1g FIPS  21 Apr 2020

[root@rackspace-test ~]$ httpd -v
Server version: Apache/2.4.37 (centos)
Server built:   Nov  4 2020 03:20:37

For Debian® or Ubuntu® operating systems servers, the command is apache2ctl -v.

[root@rackspace-test ~]$ nginx -v
nginx version: nginx/1.14.1

SSL protocol configuration

Check what websites have SSL implemented by running the commands in the
following sections.

These samples implement PFS in a domain called example.com.

Apache instructions

There are two options to check what websites have an SSL certificate in place:

[root@rackspace-test ~]# grep -ir "SSLEngine" /etc/httpd/
/etc/httpd/conf.d/example.com.conf:     SSLEngine on

Note: The default path for Apache Virtual Hosts are under the
directory /etc/httpd/conf.d/. Directories might vary for your configuration.

Or, you can use the commands httpd -S or apachectl -S for CentOS ® or Red
Hat® Enterprise Linux ® (RHEL) and apache2ctl -S for Debian or Ubuntu
operating systems.

[root@rackspace-test ~]# httpd -S | grep 443
*:443                  is a NameVirtualHost
     port 443 namevhost www.example.com (/etc/httpd/conf.d/example.com.conf:10)

Add the following parameters to the vhost configuration with your
favorite text editor:

SSLProtocol all -SSLv2 -SSLv3
SSLHonorCipherOrder on
SSLCipherSuite "EECDH+ECDSA+AESGCM EECDH+aRSA+AESGCM EECDH+ECDSA+SHA384 EECDH+ECDSA+SHA256 EECDH+aRSA+SHA384 EECDH+aRSA+SHA256 EECDH+aRSA+RC4 EECDH EDH+aRSA RC4 !aNULL !eNULL !LOW !3DES !MD5 !EXP !PSK !SRP !DSS"

When you search for the word SSL in the vhost, the output should look be similar to the
following after the implementation:

[root@rackspace-test ~]# egrep 'SSL' /etc/httpd/conf.d/example.com.conf
     SSLEngine on
     SSLProtocol all -SSLv2 -SSLv3
     SSLHonorCipherOrder on
     SSLCipherSuite "EECDH+ECDSA+AESGCM EECDH+aRSA+AESGCM EECDH+ECDSA+SHA384 EECDH+ECDSA+SHA256 EECDH+aRSA+SHA384 EECDH+aRSA+SHA256 EECDH+aRSA+RC4 EECDH EDH+aRSA RC4 !aNULL !eNULL !LOW !3DES !MD5 !EXP !PSK !SRP !DSS"
     SSLCertificateFile       /etc/ssl/certs/2022-example.com.crt
     SSLCertificateKeyFile    /etc/ssl/private/2022-example.com.key

Make sure the syntax is correct and restart Apache.

[root@rackspace-test ~]# httpd -t
Syntax OK
[root@rackspace-test ~]# apachectl -k restart
Nginx Instructions

List the websites that have an SSL certificate installed:

[root@rackspace-test ~]# egrep -ir 'SSL' /etc/nginx/conf.d/
/etc/nginx/conf.d/example.com.conf:        listen 443 ssl;
/etc/nginx/conf.d/example.com.conf:        ssl_certificate       /etc/ssl/certs/2022-example.com.chained.crt;
/etc/nginx/conf.d/example.com.conf:        ssl_certificate_key   /etc/ssl/private/2022-example.com.key;

Note: The default path for Nginx Blocks are under the directory
/etc/nginx/conf.d/. Directories might vary for your configuration.

Add the following parameters to the vhost configuration with your
favorite text editor:

ssl_protocols TLSv1.2 TLSv1.1 TLSv1;
ssl_prefer_server_ciphers on;
ssl_ciphers "EECDH+ECDSA+AESGCM EECDH+aRSA+AESGCM EECDH+ECDSA+SHA384 EECDH+ECDSA+SHA256 EECDH+aRSA+SHA384 EECDH+aRSA+SHA256 EECDH+aRSA+RC4 EECDH EDH+aRSA RC4 !aNULL !eNULL !LOW !3DES !MD5 !EXP !PSK !SRP !DSS";

When you search for the word SSL in the vhost, the output should look be similar to the
following after the implementation:

[root@racksapce-test ~]# egrep -ir 'SSL' /etc/nginx/conf.d/example.com.conf
     listen          443 ssl;
     ssl_protocols TLSv1.2 TLSv1.1 TLSv1;
     ssl_prefer_server_ciphers on;
     ssl_ciphers "EECDH+ECDSA+AESGCM EECDH+aRSA+AESGCM EECDH+ECDSA+SHA384 EECDH+ECDSA+SHA256 EECDH+aRSA+SHA384 EECDH+aRSA+SHA256 EECDH+aRSA+RC4 EECDH EDH+aRSA RC4 !aNULL !eNULL !LOW !3DES !MD5 !EXP !PSK !SRP !DSS";
     ssl_certificate         /etc/ssl/certs/2022-example.com.chained.crt;
     ssl_certificate_key     /etc/ssl/private/2022-example.com.key;

Make sure the syntax is correct and restart Nginx.

[root@rackspace-test ~]# nginx -t
nginx: the configuration file /etc/nginx/nginx.conf syntax is ok
nginx: configuration file /etc/nginx/nginx.conf test is successful
[root@rackspace-test ~]# nginx -s reload

By using the preceding steps, you can implement PFS correctly for your websites.

Use the Feedback tab to make any comments or ask questions. You can also start a conversation with us.

Updated about 1 year ago