Initially, I thought Cipher and Furiosa were one and the same, until I realized that Cipher would be far too old to become Furiosa in the Wastes of Australia. It makes more sense if Cipher had been impregnated by Dom, tried to make Dom own up to it, but eventually gave up for fear of arrest and fled to her secret Outback ranch/HQ.

Of course, this theory will get shot down once it is revealed that Cipher dies in F8, so...