1. Connecting to a remote machine via SSH

... machine. You can generate RSA key pair for SSH2 protocol with OpenSSH's ssh-keygen, or puttygen (but putty private key .ppk is not compatible with OpenSSH key format, so you have to convert it to OpenSSH's ...

2. How to convert Putty's private key (.ppk) into OpenSSH's private key format in Linux?

If you want to use your Putty generated private key (.ppk) in Linux with not only Putty, but other SSH or SFTP client (Terminal, Nautilus, GFTP, etc),  you have to convert it to OpenSSH's private key format, ...