Charlize Theron's relationship with Sean Penn is ''sacred.''

The 'Mad Max: Fury Road' star, who is rumoured to be engaged to the 54-year-old actor, admits she struggles not to share too many details about their romance because she is ''really happy'' with him.

The 39-year-old actress said: ''It's great when you find something that's real and it's hard to keep that sacred, to keep that private in this work, but I go home at night and I close my front door and I'm a really, really happy girl.''

The Oscar winner's three-year-old son, Jackson, whom she adopted in 2012, is also a very important part of her life and she wants to make the world a better place for him.

She told E! News: ''My interests in the world are still the same, if not more interested, to be as proactive as I possibly can to make it better for him. I mean, he deserves that. It is strange how I was that person before but now I just have this extra driving force behind me. I want to leave something great behind for him.''

The blonde beauty is looking forward to taking her son to the Cannes Film Festival in the south of France next month.

She said: ''I'm lucky he's at an age where he can travel with me and be part of the adventure.''