Explanation/Reference:
Before your DBaaS database was created, you or a coworker generated a private and public SSH key pair, perhaps using PuTTY Key Generator.
The public key was specified when your database instance was created; a copy of that public key was stored in the VM hosting your database.
When you define in PuTTY an SSH connection to the VM hosting your database, you will specify a copy of the private key stored on your local PC. When you initiate a PuTTY connection, the VM compares the private key to the matching public key stored in the VM. The VM permits the connection when the private and public key match as a valid key pair.
References: http://www.oracle.com/webfolder/technetwork/tutorials/obe/cloud/dbaas/ obe_dbaas_connecting_to_an_instance/obe_dbaas_connecting_to_an_instance.html