Install putty-tools
$ sudo apt-get install putty-tools
generate the private key
from .ppk
$ puttygen idrsa.ppk -O private-openssh -o idrsa_b
generate the public key
from .ppk
$ puttygen idrsa_a.ppk -O public-openssh -o idrsa_b.pub
Install putty-tools
$ sudo apt-get install putty-tools
generate the private key
from .ppk
$ puttygen idrsa.ppk -O private-openssh -o idrsa_b
generate the public key
from .ppk
$ puttygen idrsa_a.ppk -O public-openssh -o idrsa_b.pub