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Private keys are never exchanged, and are used to decrypt the information encrypted with the associated public key. Your client encrypts information to the server using the servers public key and the server encrypts information sent to you using your public key. For the latter to work, a small public key is given to the server and the server gives your client its public key. SSH operates just as telnet with a user/password combination or on a Public/Private key infastructure. Unless changed, everything SSH operates on port 22. It can also be used for creating secure tunnels, somewhat akin to Virtual Private Networks, and for use as a network file system ( Sshfs). SSH, or Secure Shell, is an encrypted protocol and associated program intended to replace telnet.
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