I wanted to SSH into a server of mine earlier today, but I was met with this error when I tried to SSH in using the key:

$ ssh -i private-key username@host
Load key "private-key": Invalid key length
username@host's password:

There is some information on this about the exact SSH version that I was using, but one thing that was not clear was if this was a server error or a client error.

As it turns out, this is a client error, the server itself is still fine. There’s a lot of information on the internet, but suffice to say that as of OpenSSH 7.9, you need a key at least 2048 bits long. So to solve this issue, you just need to use an older version of ssh.

If you don’t have an older version of the SSH client available, you could probably get around the issue by installing a virtual machine with an older version of the SSH client. Debian 9(stretch) comes with a version of openssh-client that will work.