. PuTTY contacts the OpenSSH.
2. OpenSSH identifies itself to the PuTTY by sending it a Host key and some other parameters.
3. PuTTY, in turn, searches through its known hosts database to see whether the Host key that OpenSSH has sent in step 2 exits or not.
4. If NOT, then before terminating the current session, the security alert window (Figure 5) would appear; if YES, then proceed to step 5.
So, a Host key to PuTTY is basically what a valet's uniform and badge is to a car owner. It is simply a unique fingerprint to your server's SSH program (OpenSSH) that helps PuTTY in future sessions to recognize it by.
Back again to the 'Security Alert' window in (Figure 5), assuming you have entered your server's IP address correctly, it would be safe now to click 'Yes'. What that basically does is tell PuTTY to save your server's Host key in its known hosts database in windows registry to be used for future authentications.
5. Finally, server authentication is completed and secure connection is established.
Fastmailsender