Recently I’ve had to migrate an SSH key that was generated on a Windows 10 machine to a Debian workstation. The instructions on how to convert an SSH key generated on PuTTY to OpenSSH are as simple as:
sudo apt install putty-tools puttygen your-windows-10-private-key.ppk -O private-openssh -o your-openssh-private-key-name
And that’s it! This will get your private key on a format that openssh will recognize.
To check your public key just run:
puttygen your-windows-10-public-key -L
Put it wherever you like!