A private key is necessary in ssh to authenticate the connection securely.
Generally ssh will look for a file in the
~/.ssh/ dir with a name of
(or if not using rsa a file beginning with
id_ and the encryption algo name). That file should contain the private key.
You can, however, specify a different primary key at the command line with the
-i flag like so:
ssh -i ~/.ssh/my_other_key.pem
Additionally, you may use the
IdentityFile option the the
ssh_config file to permanently configure the private key for a specific connection.