What is the Diffie-Hellman Key Exchange Protocol?

The Diffie-Hellman Key Exchange Protocol is a key agreement protocol that allows authenticated parties to exchange keying material over an unsecured connection. This protocol is widely used in protocols like IPSec and SSL/TLS. Using this protocol, sending and receiving devices in a network derive a secret shared key using which data can be encrypted.

Two hosts over the Internet cannot use public-key encryption to send the actual encrypted data. If they do so, there won’t be any Perfect Forward Secrecy (What is Perfect Forward Secrecy?). So, the two hosts derive a secret shared symmetric key and encrypt the actual sensitive data with the symmetric key. Diffie-Hellman Key Exchange protocol is used to derive the shared secret key.

How does the Diffie-Hellman Key Exchange Protocol work?

Let’s understand that first in simple English.

1. Let’s assume, Alice and Bob want to share a color between them secretly.
2. Alice and Bob agree on a common color that needs not to be kept secret. Let’s assume that the agreed-on common color is yellow.
3. Now, each of them selects a secret color, red and aqua, respectively. They do not share this secret color with each other.
4. Now, they mix up their respective secret colors with the common color yellow. So, Alice will get orange, and Bob will get a yellow and blue mixture.
5. Alice and Bob now share these mixed colors, an orange and blue mixture, publicly with each other.
6. Now, Alice and Bob should mix up the colors they received with their secret color. So, Alice will mix up aqua and red to get brown. And Bob will mix up orange and aqua to get brown.
7. Hence, the agreed-on secret color is brown. Please note that it is impossible for any third party listening to their conversation to determine the common secret color.

How is the concept actually implemented in the Diffie-Hellman Key Exchange Protocol?

In the original implementation of the protocol, the multiplicative group of integers modulo a prime …