pem: supports both PKCS#1 and PKCS#8
openssh: new post-OpenSSH 6.5 internal format, produced by
pkcs8: variants of
rfc4253: raw OpenSSH wire format
.private format output by
Start PuTTYgen. For Actions, choose Load, and then navigate to your .ppk file.
Choose the .ppk file, and then choose Open.
If you don't have a key passphrase skip to 4. For Key passphrase, enter a passphrase. For Confirm passphrase, re-enter your passphrase.
From the menu at the top of the PuTTY Key Generator, choose Conversions, Export OpenSSH Key. Note: If you didn't enter a passphrase, you receive a PuTTYgen warning. Choose Yes.
Name the file and add the .pem extension.
Run the puttygen command to convert a .ppk file into a .pem file:
$ sudo puttygen ppkkey.ppk -O private-openssh -o pemkey.pem