DH Param File & Diffie-Hellman Groups

TL;DR

Yes, a DH (Diffie-Hellman) parameter .pem file can restrict the types of Diffie-Hellman key exchanges allowed. It does this by specifying only one or a limited set of prime groups that OpenSSL will use during negotiation. This enhances cyber security by reducing potential attack surfaces.

How DH Param Files Work

Diffie-Hellman (DH) relies on mathematical operations with large prime numbers. Different sets of these primes are called ‘groups’. Each group has different strengths and weaknesses. A DH parameter file contains the parameters for one or more specific groups.

Steps to Restrict Diffie-Hellman Groups

1. Generate a DH Param File for a Specific Group: You’ll use OpenSSL to create this file. The most common group is 2, but you can choose others (e.g., 5, 14). Using only one group restricts the exchange.

   openssl dhparam -out dh_group2.pem 2048

   This command creates a file named dh_group2.pem using group number 2 and a key length of 2048 bits.

2. Configure Your Server to Use the DH Param File: The exact method depends on your server software (e.g., Apache, Nginx). Here are examples:
   • Apache: In your virtual host configuration file (e.g., httpd.conf or vhost.conf), add or modify the following line within the <VirtualHost> block:

     DHParam /path/to/dh_group2.pem

   • Nginx: In your server configuration file (e.g., nginx.conf), add or modify the following line within the server block:

     ssl_dhparam /path/to/dh_group2.pem;

3. Restart Your Server: After making changes to your configuration, restart the server for them to take effect.
   • Apache: sudo systemctl restart apache2 (or similar command depending on your Linux distribution)
   • Nginx: sudo systemctl restart nginx
4. Verify the Configuration: Use an online SSL checker tool or OpenSSL to confirm that only the specified DH group is being offered.

   openssl s_client -connect yourdomain.com:443 -tls1_2

   Look for lines in the output indicating the supported Diffie-Hellman groups. You should see only group 2 (or whichever group you specified) listed.

Why Restrict DH Groups?

• Improved Security: Limiting the available groups reduces the attack surface. Older or weaker groups can be disabled, preventing attackers from exploiting them.
• Forward Secrecy: Using Ephemeral Diffie-Hellman (DHE) or Elliptic Curve DHE (ECDHE) with a restricted set of strong groups helps ensure forward secrecy, meaning past communications remain secure even if the server’s private key is compromised.
• Compliance: Some security standards require specific DH group configurations.

Important Considerations

• Group Choice: Group 2 (14) is widely supported but may be considered less secure than newer groups like 19, 20 or 21. Choose a strong group appropriate for your security needs.
• Key Length: Use at least 2048-bit DH keys for adequate security.
• Regular Updates: Keep your OpenSSL version up to date to benefit from the latest security patches and improvements.