The pre-installed SSH daemon (sshd) is insecure in the basic configuration. To ensure greater protection, it is necessary to integrate a to integrate key authentication.

Firstly, we create a key pair under Linux:

$ ssh-keygen -t rsa -b 4096
Generating public/private rsa key pair.
Enter file in which to save the key (/home/user/.ssh/id_rsa): /home/user/.ssh/id_rsa
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/user/.ssh/id_rsa.
Your public key has been saved in /home/user/.ssh/id_rsa.pub.
The key fingerprint is:
35:9f:6e:c2:46:62:09:2d:dc:dd:1e:79:cc:56:d9:2b root@v05-s42

Be sure to enter a password, otherwise you can access the server simply by possessing the private key!

We rename id_rsa.pub to authorised_keys and download id_rsa locally to the computer. It is important to delete id_rsa securely afterwards! (If necessary, install wipe with apt-get install wipe)

$ wipe id_rsa
Okay to WIPE 1 regular file ? (Yes/No) yes
Operation finished.
1 file wiped and 0 special files ignored in 0 directories, 0 symlinks removed but not followed, 0 errors occured.

We repeat the process with all users who should have access to the sshd.

Set modes:

chmod 0700 .ssh
chmod 0600 .ssh/authorized_keys

Now we download puttygen.exe down. We open puttygen.exe and navigate in the menu to Conversion / Import Key. At this point, we select the generated private key that we have downloaded to our computer. Now add a suitable comment and we can click on save private key button.

We will need this key with the .ppk extension to log in to putty later. Never upload this key to the server!

Now we test whether the connection is established with the generated key. To do this, we open putty.exe and enter the host name as usual. Before we now click on open we switch to the left in the tree view to Connection / SSH / Auth and under private key file for authentication and enter the path to our ppk file. Now click on open button. If the login was successful, and without error message, we can completely deactivate the password authentication in our sshd.

Now we edit the sshd configuration file

/etc/ssh/sshd_config

# Hier ist es sinnvoll einen Port oberhalb von 1024 zu nehmen
Port 22
 
# Unbedingt Protokoll 2 verwenden!
Protocol 2
 
# RSAAuthentication deaktivieren
RSAAuthentication no
 
# PubkeyAuthentication aktivieren
PubkeyAuthentication yes
 
# Don't read the user's ~/.rhosts and ~/.shosts files
IgnoreRhosts yes
 
# Wollen wir nicht
RhostsRSAAuthentication no
HostbasedAuthentication no
PermitEmptyPasswords no
 
# Wollen wir erst recht nicht
PasswordAuthentication no #UNBEDINGT AUSKOMMENTIEREN UND AUF NO SETZEN!!!!
ChallengeResponseAuthentication no

/etc/init.d/ssh restart

The current SSH session is not closed. To correct configuration errors, the current session should remain open until everything is working correctly!