To generate random and secure passwords offline I recommend oldie mkpasswd. It was written by Don Libes from National Institute of Standards and Technology very long time ago but works perfectly.
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.