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So I started to use ssh on a server and I noticed that there are two ways of authenticating. I can either just type ssh@username and then provide password or I can use -i command and then select a key file that I have on my drive to give as an argument?

I though the key was necessary to encrypt the communication. If the key file is not provided what is the key used to encrypt the communication?

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The users key is used for authentication of the user only, similar to the optional client certificate in TLS (which includes HTTPS). Encryption is done with a key created using a key exchange algorithm like Diffie-Hellman but only after authorizing the server to protect against man in the middle attacks. Insofar is SSH similar to TLS.
For more information have a look at Understanding the SSH Encryption and Connection Process.

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  • @SteffenUlrich Thanks, I will read the link to understand better
    – yoyo_fun
    Commented Dec 22, 2016 at 19:50

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