The nice thing about computer standards are that there are so many to choose from.

I use PuTTY on Windows to login to my Bitnami Ubuntu  machines on the Amazon Cloud. PuTTY uses the .pkk key format.

I develop using Intellij. This can deploy to remote hosts but uses the OpenSSH key format. To convert a PuTTY key to OpenSSH.

1.  Open your private key in PuTTYGen
2.  Top menu “Conversions”->”Export OpenSSH key”.
3.  Save the new OpenSSH key when prompted (e.g. my-server-name.openssh)

Thanks to Jesse Yowell for the tip.