Key Agreement vs. Key Exchange Definition

 In most cases, when data is being sent securely over a trusted network, it is protected using a symmetric encryption algorithm. These algorithms are more efficient than public key cryptography, making them a better choice for bulk data encryption and transmission.

However, these algorithms need the communicating parties to have a shared secret key. Key agreement and key exchange protocols are used to create this shared secret. However, while the terms are often used interchangeably, they work in subtly different ways.

What is Key Agreement?

A key agreement protocol is used to generate a shared secret key via the active participation of both parties. Each of the communicating parties has a say in what the final secret key will be.

Diffie-Hellman is an example of a key agreement protocol. In Diffie-Hellman, each party generates a public keypair and shares their public key with the other person. By combining their own private key with the other party’s public key, they produce the same secret key, which can be used for symmetric encryption.

What is Key Exchange?

A key exchange protocol is designed to share a secret key with another party. One party will create the secret key and securely transmit it to the other party.

Public key cryptography algorithms such as RSA can be used to perform key exchange. In this case, one user will generate a symmetric encryption key and encrypt this key with the other person’s public key. This allows it to be securely transmitted to the intended recipient, who can use their private key to decrypt it and access the symmetric key used for decryption.

Key Agreement vs. Key Exchange

Key agreement and key exchange have similar goals but operate in different ways. Some of the key differences between them include:

• Active vs. Passive Role: In a key agreement protocol, both parties actively participate in creating the final, shared key. In a key exchange protocol, one party just accepts the key generated by the other party.
• Unilateral Control: In a key exchange scheme, one party has full control over the secret key. This creates a greater probability of a weak key than a key agreement scheme where both parties contribute.
• Security Considerations: Key agreement schemes can offer protection against

man-in-the-middle (MitM) attacks and offer perfect forward secrecy (PFS) due to the involvement and authentication of both parties. Key exchange protocols don’t actively involve both parties, creating a higher risk of attack.

Conclusion

Key agreement and key exchange schemes are both designed to set up a shared secret used for symmetric cryptography. However, they do so in different ways. Both parties contribute in a key agreement scheme, while only one party sets the key in a key exchange algorithm.

These two approaches to generating a shared key may be used in different applications. For example, key agreement is often better for temporary keys, while key exchange may occur when a trusted authority — such as a server — establishes a key to communicate in a client-server model.