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"Commitment scheme" lighting talk


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Commitment scheme lighting talk given at DRUG #62 2015.04.20

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"Commitment scheme" lighting talk

  1. 1. Commitment scheme lighting talk Wojciech Langiewicz @ DRUG 62 1
  2. 2. How we play games? Trust between players Trusted third party to settle disputes Game master to settle disputes etc... 2
  3. 3. For example coin flipping 1. Alice makes a decision and announces it: heads or tails 2. Bob flips the coin and announces the result 3. If Alice is correct, she wins, otherwise Bob wins 3
  4. 4. How to play games without trust? Commitment scheme! 4
  5. 5. Coin flipping in real live with commitment scheme 1. Alice makes a decision and writes it down on a piece of paper: heads or tails 2. Alice puts a piece of paper into an envelope and gives it to Bob 3. Bob flips the coin and announces the result 4. Bob opens the envelope and checks who won 5
  6. 6. Coin flipping over the internet 1. Alice: a. SHA256(“Alice calls heads”) -> 70d41676e5cf4f8a6f23452ee6797da3897139abe3c20523b889acb1b0b025b7 2. Alice announces the hash, so Bob can see it 3. Bob flips a coin and shows the result 4. Alice reveals her commitment: “Alice calls heads” 5. Bob calculates SHA256 and checks if it matches first hash 6
  7. 7. Results ● Bob cannot use fake coin to skew the results ● Bob still has to give you your winnings ● But if he doesn’t you have a proof that he cheated ● Alice might not reveal her commitment after the coin is flipped 7
  8. 8. Other applications ● zero knowledge proofs ● online voting ● secret sharing 8
  9. 9. Reading more “Cyberdice: peer-to-peer gambling in the presence of cheaters” Frank Stajano and Richard Clayton 9