Key agreement, also known as key exchange, is a cryptographic technique that allows two or more parties to establish a shared secret key for secure communication. In other words, it is the process of creating a secret key that is shared between two parties to ensure that only they can access the information being transmitted.

When two parties communicate over a network, they need a way to exchange information securely. Key agreement protocols provide a way for them to establish a shared secret key that can be used for encryption and decryption. This means that even if an attacker intercepts the communication, they will not be able to read the information because they do not have access to the secret key.

Key agreement protocols work by generating a random key that is shared between the two parties. This key can be used for encryption and decryption of messages, and is only known to the two parties involved. The key is typically generated using a combination of public and private keys, and is frequently used in conjunction with other cryptographic techniques, such as digital signatures.

One of the most widely used key agreement protocols is the Diffie-Hellman key exchange. This protocol allows two parties to establish a shared secret key without actually transmitting the key over the network. Instead, each party generates a public and private key pair, and then exchanges their public keys. Using these public keys, they can then generate a shared secret key that is only known to them.

Key agreement is an important concept in cryptography, as it allows for secure communication between two or more parties. It is used in a variety of applications, including SSL/TLS encryption for web communications, secure email, and secure messaging applications. By establishing a shared secret key, key agreement protocols ensure that sensitive information remains private and protected from attackers.