Note: Adding Whonix Onion Repository (this) should be preferred over adding Whonix Clearnet Repository (below).

Complete the follow steps to obtain the Whonix Signing Key.

sudo apt-key --keyring /etc/apt/trusted.gpg.d/whonix.gpg adv --keyserver hkp:// --recv-keys 916B8D99C38EAF5E8ADC7A2A8D66066A2EEACCDA

Install apt-transport-tor from the Debian repository.

sudo apt-get install apt-transport-tor

Add Whonix's APT repository for default Whonix using Debian stable. At the time of writing this was stretch.

echo "tor+http://deb.dds6qkxpwdeubwucdiaord2xgbbeyds25rbsgr73tbfpqpt4a6vjwsyd.onion stretch main contrib non-free" | sudo tee /etc/apt/sources.list.d/whonix.list