What is the purpose of the host key?

A host key is the server's public key. The host key is used by the client to decrypt an authentication message sent from the server when connecting. The basic purpose of the host key is to ensure that when you connect to a remote host, it is actually the host that you intended to connect to. Unfortunately, this presents something of a catch-22, as described later in this document. If you've never connected to the host before, how do you know the host you are connecting to is the right one?

In Secure Shell, host keys can be used for host-based authentication, but this paper will not be addressing host-based authentication since it is not widely used.