4.22.8 ‘Private key file for authentication’

This box is where you enter the name of your private key file if you are using public key authentication. See chapter 8 for information about public key authentication in SSH.

This key must be in PuTTY's native format (*.PPK). If you have a private key in another format that you want to use with PuTTY, see section 8.2.12.

You can use the authentication agent Pageant so that you do not need to explicitly configure a key here; see chapter 9.

If a private key file is specified here with Pageant running, PuTTY will first try asking Pageant to authenticate with that key, and ignore any other keys Pageant may have. If that fails, PuTTY will ask for a passphrase as normal. You can also specify a public key file in this case (in RFC 4716 or OpenSSH format), as that's sufficient to identify the key to Pageant, but of course if Pageant isn't present PuTTY can't fall back to using this file itself.