PuTTYgen is a key generator. It generates pairs of public and private keys to be used with PuTTY, PSCP, and Plink, as well as the PuTTY authentication agent, Pageant (see chapter 9). PuTTYgen generates RSA and DSA keys.
When you run PuTTYgen you will see a window where you have two choices: ‘Generate’, to generate a new public/private key pair, or ‘Load’ to load in an existing private key.