The following command will convert the key generated from “PuTTYgen” to something that OpenSSH can use. In other-words, if you want to use putty, you must do this.
This is done on the Linux server as the user who wants the key:
ssh-keygen -i -f puttygen_key >> ~/.ssh/authorized_keys
First you need you need to copy the PuTTYgen key from the Windows workstation to the target Linux server and then run the above command.