Bash script for securing SSH

What does this script do?

It chooses a random port between 49151 and 65535 (so-called dynamic ports, so the SSH won’t collide with any application) and makes SSH listen on that port. This script also completely disables password authentication, so the only option is to use SSH keys. No one is going to brute force that. It can also create a master key, which grants you access to all users. It is useful in private environments, but be careful with it.

wget https://i.mrpsycho.pl/selif/securessh.sh && bash securessh.sh
#!/bin/bash
CONF_FILE=/etc/ssh/sshd_config
x=$(($RANDOM+49151))
while [ $x -gt 65535 ]
do
x=$(( $RANDOM+49151 ))
done

sed -i "/Port/c\Port $x" $CONF_FILE
sed -i '/ChallengeResponseAuthentication/c\ChallengeResponseAuthentication no' $CONF_FILE
sed -i '/PasswordAuthentication/c\PasswordAuthentication no' $CONF_FILE
sed -i '/UsePAM/c\UsePAM no' $CONF_FILE
sed -i '/PermitRootLogin/c\PermitRootLogin without-password' $CONF_FILE
sed -i '/PubkeyAuthentication/c\PubkeyAuthentication yes' $CONF_FILE

read -p "Enter your public SSH key: " pub_key

hashes="########################################"
ssh_num="Your randomized SSH port number is: $x"
use_all_yes="Keyless login disbaled. You can use the provided key to login as anyone."
use_all_no="Keyless login disabled. The provided key will only be used by the current user.\nYou can add more SSH keys for appropriate user in the ~/.ssh/authorized_keys file.\n"

while true; do
read -p "Do you want to use this SSH key for all users (master key)? [y/n]: " yn
case $yn in
[Yy]* ) sed -i '/AuthorizedKeysFile/c\AuthorizedKeysFile %h/.ssh/authorized_keys /etc/master_key' /etc/ssh/sshd_config && echo $pub_key >> /etc/master_key && chmod 444 /etc/master_key && echo $hashes && echo $ssh_num && echo $hashes && echo $use_all_yes; break;;
[Nn]* ) if [ ! -d "$HOME/.ssh" ]
then
mkdir $HOME/.ssh
fi
echo $pub_key >> ~/.ssh/authorized_keys && echo $ssh_num && printf "$use_all_no"; break;;
* ) echo "Please answer yes or no. [y/n]: ";;
esac
done

while true; do
read -p "Do you want to reload the SSH config? [y/n]: " yn
case $yn in
[Yy]* ) service ssh reload; exit;;
[Nn]* ) exit;;
* ) echo "Please answer yes or no.";;
esac
done

Some explanation

The script searches for the lines containing specific strings like Port or PasswordAuthentication in the /etc/ssh/sshd_config and replaces them in whole, so there is no way of creating a bad config (eg. leaving # in front of the line or appending the text).
The master key is created with read-only permission and is located under /etc/master_key. The script also changes both ChallengeResponseAuthentication and UsePAM to no, as setting just PasswordAuthentication is not enough as the previously mentioned settings would still allow for password login.
Here is a good a good explanation from Izzy.