ssh: Exited: String too long | when do ssh -i .ssh/id_rsa user@host

This can be a problems about compatibility from key convert from other format.

But If you are dealing with small device Like camera or small NAS like Dlink-320 (ALT-F) you are highly likely to encounter a variation of SSH call Dropbear ssh.

Dropbare ssh is ssh system for small device like ip camera or any embedded system.

To generate the key use

dropbearkey -t rsa -f ~/.ssh/identity  # or ~/.ssh/id_rsa depend on your need. 

The command will show you public key.

you can also create the public key from this command.

dropbearkey -y -f ~/.ssh/identity | grep "^ssh-rsa " >> authorized_keys

Thank you kzo81 from linuxquestion.org for the idea.