How to Enable Diffie-Hellman Key Exchange in Linux for Secure SSH

1. Introduction

Security, especially over communication, is of prime importance in the digital world. In fact, the cryptographic keys maintain the confidentiality and integrity of a system, and they form one of the critical aspects of securing communication. And the Diffie-Hellman exchange algorithm enhances the SSH security.

In this article, we’ll explore the fundamentals of Diffie-Hellman key exchange and the sequential approach to applying it in a SSH Configuration to improvise the server’s security over SSH transaction.

2. Understanding Diffie-Hellman Key Exchange

Before we dive into the technical intricacies, let’s first understand the Diffie-Hellman key exchange and its importance.

Basically, Diffie-Hellman key exchange is a protocol that enables two systems to securely exchange cryptographic keys over an untrusted network. This transpires without a direct key exchange but by enabling both parties to independently construct a shared secret key. Further, the data between the two systems is then encrypted and decrypted using this shared secret key.

Diffie-Hellman is a crucial tool for protecting data transfer because even if an attacker intercepts the communication between the two systems, he cannot quickly ascertain the shared secret key.

Thus, by allowing for secure key exchange over suspicious networks, this protocol helps protect sensitive data from eavesdropping and unauthorized access.

3. Enabling Diffie-Hellman Key Exchange in Linux

Now, let’s see how to work on Diffie-Hellman key exchange in a Linux environment in a step-by-step manner:

3.1. Access Linux Server

To begin with, let’s gain access to our Linux server. The general approach is establishing a connection via SSH (Secure Shell).

So, now let’s install the OpenSSH server on the Linux system using the apt distribution’s package manager:

$ sudo apt-get install openssh-server -y

Hence, we now connect to the server using PuTTY or other terminals through SSH.

3.2. Activate Diffie-Hellman Key Exchange

Next, we modify the SSH configuration file. Generally, this file can be placed anywhere, depending on the Linux distribution. Nevertheless, let’s go to /etc/ssh/sshd_config, where it’s most commonly located.

Let’s use a text editor such as nano or vi to open the file with superuser privileges:

$ sudo nano /etc/ssh/sshd_config

Now that we’ve located the SSH configuration file, the next step is to identify the line that starts with “KexAlgorithms”. Then, let’s add “diffie-hellman-group-exchange-sha256” to the list of key exchange algorithms:

KexAlgorithms [email protected],diffie-hellman-group-exchange-sha256

Let’s save the changes and exit the text editor. Use the grep command to validate the addition:

$ grep "KexAlgorithms" /etc/ssh/sshd_config
KexAlgorithms [email protected],diffie-hellman-group-exchange-sha256

This line specifies that both Curve25519 and Diffie-Hellman key exchange will be used for the negotiation process during the connection process.

3.3. Verify the Configuration

To enable the changes, we restart the SSH services:

$ sudo systemctl restart ssh
$ sudo systemctl status ssh

Lastly, let’s connect to the server using SSH and then inspect the negotiation process to verify the Diffie-Hellman key exchange:

$ ssh -vvv username@hostname
debug1: SSH2_MSG_KEXINIT sent
debug3: receive packet: type 20
debug1: SSH2_MSG_KEXINIT received
debug2: local client KEXINIT proposal
debug2: KEX algorithms: diffie-hellman-group14-sha256
...
... output truncated ...
...
debug2: first_kex_follows 0
debug2: reserved 0
debug2: peer server KEXINIT proposal
debug2: KEX algorithms: diffie-hellman-group-exchange-sha256
...
... output truncated ...
...
debug2: first_kex_follows 0
debug2: reserved 0
debug1: kex: algorithm: diffie-hellman-group-exchange-sha256
...
... output truncated ...
...

Here, we replace username and hostname with the actual username and server hostname or IP address.

The -vvv flag displays verbose debugging information, including the key exchange algorithms negotiated during the connection.

4. Conclusion

To summarize, now we know the importance of enabling Diffie-Hellman key exchange in Linux and how it enhances the security of our server’s communication. Further, we delved into the sequential approach to configuring this protocol.

Hence, by following the steps outlined in this article, we can ensure that the configuration of the Linux server using this powerful security feature keeps our data safe from prying eyes.