4.20.7 ‘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.

If a key file is specified here, and Pageant is running (see chapter 9), 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.