Converting a Putty Private Key (.ppk) to Openssh Format on a Mac

Ensure putty is installed on the system (I use hombrew for Mac package management):

Kyles-MacBook-Air:~ kjones$ brew install putty

Installing putty pulls in puttygen as well, which is the command we will use to convert the private key into the openssh format we need. Next execute the puttygen command with the -O and -o switches in order to specify the format to convert to, and the name of the resulting file respectively:

Kyles-MacBook-Air:~ kjones$ puttygen ~/Dropbox/Keys/private_key.ppk -O private-openssh -o private_key