SSH key authentication for backup spaces and cloud storages

If you use SCP, SFTP, rsync or BorgBackup, you can log in using SSH key authentication without entering a password.

Important note: Depending on the SSH port of the backup account/storage box you use, you may need to use a specific format for the public SSH key. For SSH port 22 (SFTP and SCP only), you are required to use a public SSH key in RFC4716 format. For SSH port 23 (SFTP, SCP, rsync and BorgBackup) you are required to use a common public SSH key in OpenSSH format. If you want to use the services over both ports, then you must store the public SSH key in both formats.

Using an ed25519 key is not supported on SSH port 22.

Please note that each sub-account requires its own authorized_keys file.

Generating SSH keys

You can use ssh-keygen to generate a new pair of SSH keys:

Warning: With the default settings, using ssh-keygen will overwrite an existing SSH key! As an alternative, with the parameter (-f) you can specify a different file path.

Optional: Converting your key to RFC4716 format

This is only necessary if you would like to use SCP or SFTP via SSH port 22.

To convert the public SSH key into the correct format, enter the following command:

server> ssh-keygen -e -f .ssh/ | grep -v "Comment:" > .ssh/

The public SSH key should now look like the one below:

server> cat .ssh/

Creating authorized_keys file

Insert the required public SSH keys into a new local authorized_keys file.

For SSH over port 23 (SCP, SFTP, Rsync and Borg Backup), add the public SSH key in OpenSSH format:

server> cat .ssh/ >> storagebox_authorized_keys

If you converted your public SSH key to RFC4716 format in the previous step, add it as well:

server> cat .ssh/ >> storagebox_authorized_keys

You can also add the public SSH key in both formats.

Uploading authorized_keys

Now you need to upload the generated authorized_keys file to the storage box/backup account. To do this, create the directory .ssh with the file rights 0700 (rwx——) and create the file authorized_keys with the public SSH keys and the file rights 0600 (rw——-).

You can do this with the following command, for example:

server> echo -e "mkdir .ssh \n chmod 700 .ssh \n put storagebox_authorized_keys .ssh/authorized_keys \n chmod 600 .ssh/authorized_keys" | sftp's password:
Connected to'.
sftp> mkdir .ssh
sftp> chmod 700 .ssh
Changing mode on /.ssh
sftp> put storagebox_authorized_keys .ssh/authorized_keys
Uploading storagebox_authorized_keys to /.ssh/authorized_keys
storagebox_authorized_keys 100% 2916 2.0MB/s 00:00
sftp> chmod 600 .ssh/authorized_keys
Changing mode on /.ssh/authorized_keys

Based on which SSH key format you have uploaded you should be able to log in without a password on port 22 and/or 23:

sftp -P <22 or 23>
Connected to
sftp> quit

Warning: The ssh-copy-id command cannot be used to upload the public SSH key.