I'm being prompted for a passphrase each time I try to connect with my ssh key.
$ ssh firstname.lastname@example.org -i .ssh/private_key
I was given these credentials (private key) by the server admin, so I cannot regenerate the key, and there is nothing wrong with the credentials. I copied/pasted the key exactly (there is no botching of dashes with hyphens due to a mail client, etc). I verified there are no spaces / line break issues with vim, an the same issue happens. Permissions are set correctly (to 0600).
What did I do wrong?