What is Diffie Hellman Key Exchange & How Does It Work?

Data encryption is the method of translating data into another form giving access to only people who have the decryption key or password to read it. It is one of the most secure methods used widely for encrypting the data. The main aim of encryption is to protect the digital data stored on computer systems. The modern algorithms have replaced the outdated data encryption standard (DES) and play a crucial role in IT systems’ security.

One such algorithm that has been popularly used by the industries is the Diffie Hellman key exchange. It is also called an exponential key exchange, is a digital encryption method that produces a decryption key with numbers raised to specific powers depending on the non-transmitted components.

The algorithm provides confidentiality and drives key security initiatives such as authentication and integrity. These signify the message verification, no change in the message content, and ensure that the sender cannot deny sending the message.

What is Diffie Hellman Key Exchange?

The Diffie Hellman key exchange method was first used to develop and exchange keys over an insecure channel safely. It set a milestone in cryptography and is still used today in various applications where encryption is required.

Let’s understand the mechanism with the following example,

You want to communicate with a spy from an allied nation who is not known to you. There is no secure channel to talk to them. Messages sent without encryption can make any undesirable person read the contents. If the encryption is done on the message, no one will be able to read it.

This issue can be handled easily with the Diffie Hallman key exchange, which makes this method unique. The algorithm provisions safely create a shared key on a secure channel and even over an insecure channel tracked by the adversaries.

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Where Did The Diffie Hellman Key Exchange Come From?

Whitfield Diffie and cryptologist Martin Hellman together coined the term Diffie-Hellman key exchange to provide a sound mechanism for cryptography. This invention was inspired by the earlier developments made by scientist Ralph Merkle who had generated some puzzles that would take some computational resources to solve.

The Diffie-Hellman key exchange used some of those ideas and made them complicated for creating a secured method for public-key cryptography.

How does the Diffie-Hellman Key Exchange Mechanism Work?

The Diffie-Hellman key exchange uses large numbers and a lot of computations for cryptography. This can be understood with the diagram depicting an analogy below.

Suppose two people are deciding on the color of a paint named Alice and Bob. First, they agree on a random color to begin. Let’s assume they decide on yellow as a standard color.

Each chooses his secret color and does not disclose their choice. Let’s say Alice chooses red, while Bob chooses light blue.

The next step is mixing the secret colors by Alice and Bob, with yellow that was agreed mutually. 

As per the diagram, Alice selects orange color while Bob goes for a deep blue color.

Once they are done mixing, they send the outcome to the other party. Alice receives deep blue, while Bob gets the orange paint.

Once each receives a mixed result, they add their secret color to it. Alice takes the deeper blue and adds the secret red paint, while Bob the hidden deep blue to the orange he has received. In the end, they find that they have received the same color, which is brown here. This shared color is called the common secret.

With the Diffie-Hellman key exchange, both parties get the same result, without sending the shared secret across the communication channel. If some adversary peeps into the exchange, they could only access the standard yellow color, and the exchanged mixed colors.

Implementing the Diffie-Hellman key exchange securely would take enormous time and computational resources for the attacker to break the secret.

This structure of the Diffie-Hellman key exchange allows the two parties to communicate over an unsecured connection and still come up with a shared secret that can be used for making encryption keys for future communications. The complete shared secret is never sent over the connection; thus, attackers cannot monitor it. Both Alice and Bob do the same operation but in alternate order but come up with the same output.

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How Is The Diffie-Hellman Key Exchange Implemented?

The Diffie-Hellman key exchange is implemented in multiple ways. It serves as a basis for several other algorithms. All of the types of algorithms are implemented with a different purpose. Some provide authorization, while others offer various cryptographic features such as perfect forward secrecy.

1. Elliptic-curve Diffie-Hellman

The Elliptic-curve Diffie-Hellman follows an algebraic structure of elliptic curves to make the implementations achieve a similar security level with a smaller key size. A 224-bit elliptic-curve key provides equal security as a 2048-bit RSA key, making the exchanges more efficient, and the storage requirements are also reduced.

2. TLS  

TLS is a protocol used to secure the internet that uses the Diffie-Hellman exchange in three different ways: anonymous, static, and temporary.

  • Anonymous Diffie-Hellman Key Exchange– This category doesn’t use any authentication, leaving it vulnerable to attacks. Hence it is not preferable to be used or implemented.
  • Static Diffie-Hellman Key Exchange – Static Diffie-Hellman uses certificates for authenticating the server. It does not take part in authenticating the client, and also does not provide forward secrecy.
  • Ephemeral Diffie-Hellman Key Exchange – This is the most secure implementation as it allows for perfect forward secrecy. It is combined with an algorithm such as DSA or RSA to authenticate one or both parties present in the connection. Ephemeral Diffie-Hellman gives the connection perfect forward secrecy as it uses different key pairs every time the protocol is run. As a result, if a key is disclosed in any case, it can’t decrypt all the past messages.

3. ElGamal

ElGamal is a public-key algorithm. It contains no authentication and is generally combined with other mechanisms for the same. ElGamal was primarily used in PGP, GNU Privacy Guard, and other systems. Since 2000, ElGamal has not been implemented much frequently as its rival, RSA, was made to be implemented freely after that.

4. Station-to-Station (STS)

The Station-to-Station (STS) protocol is another critical agreement protocol that protects against the middleman attacks and perfect forward secrecy.

It demands the parties in a connection to have a key pair, used to authenticate each side.

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The Diffie-Hellman key exchange was crafted as an innovative method for assisting two unknown parties to communicate safely. There have been newer versions introduced in modern technology with larger keys. Still, the protocol is so efficient that it will continue securing the advanced attacks in the future.

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