Convert putty key to openssh on linux

I had a putty generated ssh key that I wanted to use on my linux box (long story). Turns out it's simple to convert it using linux tools. Here are the steps:

sudo aptitude install putty-tools

Then, to convert a private putty key named id_dsa.ppk to an openssh key named id_dsa:

puttygen id_dsa.ppk -O private-openssh -o id_dsa

Then just type in the key's passphrase and you're done. You can type puttygen --help to see similar options for public keys.