Alternatively if you want to grab the private and public keys from a PuTTY formated key file you can use puttygen
on *nix systems. For most apt-based systems puttygen
is part of the putty-tools
package.
Outputting a private key from a PuTTY formated keyfile:
$ puttygen keyfile.pem -O private-openssh -o avdev.pvk
For the public key:
$ puttygen keyfile.pem -L