Install the puttygen
tools
brew install putty
Generate the private key
puttygen /path/to/your/file.ppk -O private-openssh-new -o id_rsa.private.key
Generate the public key
puttygen /path/to/your/file.ppk -O public-openssh -o id_rsa.public.key
Change the right permission
chmod 600 id_rsa.private.key
chmod 644 id_rsa.public.key