Generate .ppk out of .pem with Linux (Ubuntu)

Here is a example how to convert .pem to .ppk using Ubuntu.

First you need to install package putty-tools

 sudo apt-get install putty-tools  

After install, all you really need to do is this:

 puttygen key.pem -o key.ppk  

But.. with -P switch you can set passphrase for extra security, this is recommended and easy to do:

 puttygen key.pem -o key.ppk -P -C "My server key"  

It is also recommended to set comment for your key using -C switch, because this string will be prompted to you when you are entering your passphrase.

Note that you can also change passphrase afterwards by using -P switch

 m@box:~/Downloads$ puttygen -P key.ppk   
 Enter passphrase to load key:   
 Enter passphrase to save key:   
 Re-enter passphrase to verify:   

And you are done!