Convert openssh keys to rsa keys

from something that starts with

-----BEGIN OPENSSH PRIVATE KEY-----

to something that starts with

-----BEGIN RSA PRIVATE KEY-----

Prerequisites

You need puttygen.

sudo apt install putty-tools

Procedure

# convert from openssh to ssh2 encrypted format
puttygen originalKeyFile -O private-sshcom -o /tmp/tmp.putty
# convert from ssh2 encrypted format to rsa
ssh-keygen -i -f /tmp/tmp.putty > rsaKeyFile
rm /tmp/tmp.putty

Voilà