Public-key cryptography is method of make secure connections using a pair of cryptographic keys: one public key and one private key. The private key is a secret (not to be shared with anyone) and the public key is shared with anyone.
The program "putty" comes with a program called "putty-gen". It is a gui client.
When using putty-gen, be sure to use send us the one-line public key. You can find this at the top of the putty-gen dialog box in the "Public key for pasting into authorized_keys file" box.