Copying private identity key to .ssh/rsa_id causes permission denied error during SSH login | Joachim De Zutter
After copying only a private key to ~/.ssh/rsa_id:

$ ssh -l username hostname
Permission denied (publickey,keyboard-interactive).
$ chmod 600 ~/.ssh/id_rsa
$ ssh -l username hostname
Permission denied (publickey,keyboard-interactive).
$ mv ~/.ssh/id_rsa.pub ~/.ssh/id_rsa.pub.bak
Enter passphrase for key '[...]/.ssh/id_rsa':
Last login: [...] from [...]
username@hostname:~$
$ mv ~/.ssh/id_rsa.pub.bak ~/.ssh/id_rsa.pub
Permission denied (publickey,keyboard-interactive).

After also copying the public key corresponding with ~/.ssh/rsa_id to ~/.ssh/rsa_id.pub:

Enter passphrase for key '[...]/.ssh/id_rsa':
Last login: [...] from [...]
username@hostname:~$