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Lecture 9: Coin Flipping by Telephone

Lecture 9: Coin Flipping by Telephone. Wayne Patterson SYCS 654 Spring 2010. Coin Flipping by Telephone. Originally presented by Manuel Blum, then at UC Berkeley (now at Carnegie-Mellon):

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Lecture 9: Coin Flipping by Telephone

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  1. Lecture 9:Coin Flipping by Telephone Wayne Patterson SYCS 654 Spring 2010

  2. Coin Flipping by Telephone • Originally presented by Manuel Blum, then at UC Berkeley (now at Carnegie-Mellon): • M. Blum. Coin flipping by telephone. In Proceedings of IEEE Spring Computer Conference, pages 133--137. IEEE, 1982. • Blum was Rene Peralta’s thesis advisor

  3. As Rene Posed the Problem • Alice and Bob have decided to divorce. They are in different cities, want to decide who gets the (house, car, dog, …) • They decide to flip a coin • Naturally they don’t trust each other …

  4. Alice and Bob Agree on a Secure One-Way Function • For example, the function could be ax (mod p), for an agreed-upon large prime number p and an agreed-upon primitive root a.

  5. Alice Will Flip the Coin • The act of flipping is the choice of a secret x, 1 < x < p-1. • x is either odd or even (i.e. heads or tails) • Alice chooses x (the flip) and announces to Bob the computation y = ax mod p.

  6. What Does Bob Do? • Bob receives y, and guesses that x is odd or even (heads or tails). • Bob sends his guess to Alice. • Alice verifies the result, whether Bob guessed right or wrong, and proves her decision by sending x to Bob. • Bob can verify that Alice did not lie by performing the same computation y = ax mod p. • If Alice lied about the value of x, Bob can prove the lie by the fact that the y he computes will not be the same y he had received.

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