Server authentication is done during Diffie-Hellman key exchange through a
single public-key operation. When public-key authentication is used to
authenticate the server, the first connection is very important. During the
first connection the client will display a message similar to
the one in Figure First connection to a remote host.
Figure : SSH Tectia Client on Windows – first connection to a remote host
At this point, you should verify the validity of the fingerprint, for
example by contacting the administrator of the remote host computer
(preferably by telephone) and asking the administrator to verify that the key
fingerprint is correct. If the fingerprint is not verified, it is
possible that the server you are connecting to is not the intended one
(this is known as a man-in-the-middle attack).
After verifying the fingerprint, it is safe to continue
connecting. The server public key will then be stored on the
client machine. The location depends on the client
implementation (on Unix it is the
After the first connection, the local copy of the server public key will
be used in server authentication.