Flipping a coin over the phone: Solution

1. Alice chooses a string that ends with either heads or tails

2. She checksums this string

3. She sends the checksum to Bob

        7b93c6b134ce346d211cefb4a473061f

4. Now Bob chooses heads or tails and tells his choice to Alice

5. Alice divulges the original string

        sezvvamredfkpkpvsgccidzgvtxfolzlobivdenz:tails

6. Bob calculates the checksum for this string

7. Bob verifies that the checksum matches what Alice sent before

• Alice can't change her choice after Bob guesses

• Because she can't construct a second string with the same checksum

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Copyright © 2005 M. J. Dominus