How can I assign Ctrl+Alt+Delete to shutdown dialog?

This can be achieved by adding a new custom keyboard shortcut.

Disable the current Ctrl+Alt+Delete shortcut

Select the keyboard app from the dash

On the shortcut tab change the current shortcut of logout to something else, say Alt+Delete.

Then click on custom shortcut and add a new shortcut.

Enter Shutdown in the Name field and /usr/lib/indicator-session/gtk-logout-helper --shutdown in the Command field.

Click apply and assign the shortcut (Ctrl+Alt+Delete)