Install mkpasswd in Debian or any other distro containing APT by command:
sudo apt-get install -y mkpasswd
For some reason Ubuntu‘s repository contains makepasswd that was written by Marco d’Itri and has similar functionality. To install it run
sudo apt-get install makepasswd.
Generate strong password by executing mkpasswd or makepasswd in command line.
mkpasswd -s "salt"
Another way is to get strong password online with www.goodpassword.com that provides you with service free of charge.