Openssh Private Key to RSA Private Key

You have an OpenSSH format key and want a PEM format key. It is not intuitive to me, but the suggested way to convert is by changing the password for the key and writing it in a different format at the same time.

The command looks like this:

ssh-keygen -p -N "" -m pem -f /path/to/key

It will change the file in place, so make a backup of your current key just in case. -N "" will set the passphrase as none. I haven't tested this with a passphrase.

The public key should be fine as is.

For full explanation of the above command, see the -m option here: https://man.openbsd.org/ssh-keygen#m

1. Install and open puttygen
2. Click on "Load an existing private key file"
3. Click on menu item "Conversions" -> "Export OpenSSH key"
4. Save file

Here's what worked for me for an in-place conversion of a key with a passphrase:

ssh-keygen -p -P "old passphrase" -N "new passphrase" -m pem -f path/to/key