Key exchange protocols enable two or more parties to establish a shared encryption key that they can use to encrypt or sign data that they plan to exchange. Key exchange protocols typically employ cryptography to achieve this goal. Different cryptographic techniques can be used to achieve this goal.
In order for two parties to communicate confidentially, they must first exchange the secret key that will be used to encrypt and decrypt messages. This initial exchange of the encryption key is called the key exchange.
Key exchange protocols are designed to solve the problem of confidentially establishing a secret key between two or more parties without letting an unauthorized party somehow intercept, infer or otherwise obtain the key.
A naive example of a key exchange protocol is for one party to write down a secret key, place it in a tamper-evident envelope and send it to the receiver. If the envelope is intact, then the secret key can be used by both parties to encrypt and decrypt messages.
Commonly used key-agreement protocols include Diffie-Hellman, or protocols that are based on RSA or ECC.