Coin flipping by telephone

A protocol for two distant parties to agree on a random bit without trusting either party to announce an unverified coin toss. In the Rabin-Williams scheme, a publicly committed square hides which of two half-interval roots the sender selected. The other party guesses a digit distinguishing those roots; subsequent root and factor revelations verify the completed exchange. The protocol's fairness relies on prescribed randomness and the difficulty of finding a second root; it does not force a party to complete communication.