Recently i had to work in Windows. Sometimes I do, especially if i need to attend a lot of Skype for Business meetings.
Anyhow, I found myself in the need of a linux VM. When setting up SSH, i used putty to create cert etc.
And today i was back in Linux and needed to log into my new server. Of course the putty cert is not out of the box compatible with open-ssh.
So, this is how to go from putty cert to openssh cert. Required software is putty(tools)
cd ~ puttygen new_cert_made_in_putty.ppk -O private-openssh -o new_cert_to_server_private_parts puttygen new_cert_made_in_putty.ppk -O public-openssh -o new_cert_to_server_public_parts.pub
Move files to .ssh folder, set chmod 600 on the new files.