Put false front is an expression used to describe someone's attempt to present a false image or pretense. The two antonyms for this phrase are 'authenticity' and 'sincerity'. Authenticity is the quality or state of being genuine and truthful; it is characterized by honesty, transparency, and openness. Sincerity, on the other hand, implies genuineness and honesty in one's actions and intentions, without any hidden agenda or ulterior motives. These antonyms represent the opposite of putting up a false front, which is to be real, honest, and truthful in presenting oneself, actions, and intentions to others.