Windows

1. Download and install PuTTYgen from http://www.chiark.greenend.org.uk/~sgtatham/putty/download.html.
2. Start puttygen.
3. Select the Generate button.
4. After the key is created, click on the Save private key button to download the file that contains private key. 
5. Click on the Save public key button to download the file that contains public key. 

Linux

On a Debian distribution (Ubuntu, Mint), apt-get can be used to install the putty-tool package.

apt-get install putty-tools

On a Red Hat distribution (CentOS, Fedora, Red Hat), dnf or yum can be used to install the putty package.

dnf install putty

Create the private key pair.

~]# puttygen -t rsa -b 2048 -o example.ppk

Create the public certificate.

~]# puttygen -L example.ppk > example.pub