ppk

http://www.ualberta.ca/CNS/RESEARCH/LinuxClusters/pka-putty.html

You will get a:

Authenticating with public key "rsa-key-20120424"
Passphrase for key "rsa-key-20120424":


and you should provide the password entered during the creation of the private key.
If you didn't provide an password, you will be logged in automatically.


Linux administrator might have disabled public key authentication

cat /etc/ssh/sshd_config

(see http://docs.oseems.com/application/ssh/disable-public-key-authentication )