Key-Exchange (also known as Key-Establishment) is any method in Cryptography by which cryptographic keys are exchanged between users, allowing use of a cryptographic algorithm.[1]

Key-Exchange in Communication contexts if Successful results in Key agreement. Often Key-Exchange and Key agreement are used interchangeably. (As Ldapwiki sees it)

If Alice and Bob wish to exchange encrypted messages, each must be equipped to perform encryption on messages to be sent and perform decryption messages received. The nature of the equipping they require depends on the Encryption technique they might use.

If they use a code, both will require a copy of the same codebook. If they use a cipher, they will need appropriate keys. If the Cipher is a Symmetric Key cipher, both will need a copy of the same key.

If an Asymmetric Key cipher with the Public Key/Private Key property, both will need the other's Public Key.

Preferably RSA should only be used for Authentication and the Key-Exchange should be performed using Diffie-Hellman key-exchange.

In TLS, the Key-Exchange is specified in the Cipher Suite the is negotiated with the ServerKeyExchange and the ClientKeyExchange Messages.

Diffie-Hellman or RSA

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