Commitment scheme lighting talk

Wojciech Langiewicz @ DRUG 62

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How we play games?

Trust between playersTrusted third party to settle disputesGame master to settle disputesetc...

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For example coin flipping

1. Alice makes a decision and announces it: heads or tails

2. Bob flips the coin and announces the result3. If Alice is correct, she wins, otherwise Bob

wins

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How to play games without trust?

Commitment scheme!

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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 result4. Bob opens the envelope and checks who

won5

Coin flipping over the internet

1. Alice:a. SHA256(“Alice calls heads”) ->

70d41676e5cf4f8a6f23452ee6797da3897139abe3c20523b889acb1b0b025b7

2. Alice announces the hash, so Bob can see it3. Bob flips a coin and shows the result4. Alice reveals her commitment: “Alice calls

heads”5. Bob calculates SHA256 and checks if it

matches first hash 6

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

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Other applications

● zero knowledge proofs● online voting● secret sharing

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Reading morehttp://en.wikipedia.org/wiki/Commitment_scheme“Cyberdice: peer-to-peer gambling in the presence of cheaters” Frank Stajano and Richard Clayton

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