why SFTP client only need to know the private key, doesn't requires public key?

Question

When I connect a SFTP server from a client, I only need to provide to the client software the private key, not the public key. I don't know how this works, because from [1] and [2], both requires the client software also know the public key.

My understanding of how SFTP work is

setup

• client: generate private/public key pairs.

• client: send public key to server

• server: add this public key to its authorized keys list

during connection

• client: tell server the public key

• server: check if the public key is authorized, if not, refuse, otherwise, send a random session id to client

• client: client use private key to encode the random session id and send to server

• server: decode it by the public key and confirm the identity.

In this case, the client requires public key. However, I didn't provide this but SFTP client still able to connect the sever. So how this SFTP protocol really works?

[1] http://www.openssh.org/txt/rfc4252.txt

[2] https://www.eldos.com/security/articles/1962.php

Answer 1

While @Pascal Cuoq's comment is the correct answer, it is only half of the answer, since Actually, the client does need a public key - but only the server's public key, and you already have that in your known_hosts file.

If you don't - you are asked to trust the one the server is presenting on your first connection to the server. This is a thing that most people overlook completely. If you are DNS-poisoned (or there is some other form of Man-in-the-Middle attack ongoing) during the first connection - or don't pay attention to Server identity has changed notifications - you are vulnerable to MitM attacks.