In this gripping short play, David Mamet combines mercurial intelligence with genuinely Hitchcockian menace. The Cryptogram is a journey back into childhood and the moment of its vanishing—the moment when the sheltering world is suddenly revealed as a place full of dangers.             On a night in 1959 a boy is waiting to go on a camping trip with his father. His mother wants him to go to sleep. A family friend is trying to entertain them—or perhaps distract them. Because in the dark corners of this domestic scene, there are rustlings that none of the players want to hear. And out of things as innocuous as a shattered teapot and a ripped blanket, Mamet re-creates a child terrifying discovery that the grownups are speaking in code, and that that code may never be breakable.