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ashoff2-BitCommitment - Bit Commitment Fair Coin Flips and...

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Bit Commitment, Fair Coin Flips, and One-Way Accumulators Matt Ashoff 11/9/2004 Cryptographic Protocols
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Outline Bit Commitment Definition Properties Applications Implementations Fair Coin Flips Definition Implementations One-Way Accumulators Definition Example Motivation Applications References
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Definition Bit Commitment Goal is to ensure bit commitment. Simplest example: Decide who goes first in a game If Bob guesses correctly, he goes Alice picks a bit (0 or 1) and locks it in a box Bob guesses a bit The box is opened to see if he is right Two parts: Commitment Unveiling Must ensure that: Alice cannot change her bit after Bob guesses Bob cannot know what Alice’s bit is until she unveils it Assume no trusted third-party
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Properties Bit Commitment Ideally, bit commitment has two interesting properties: It is unconditionally secure if implemented correctly As opposed to computationally secure, which is a requirement for most algorithms It requires only a noisy channel However, implementing the algorithm ideally is the key
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Applications Bit Commitment Zero-Knowledge Protocols Identification Schemes Multi-party Computation Fair Coin Flips Electronic Voting
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Implementations Bit Commitment Symmetric Cryptography Alice encrypts her bit with a random key Sends to Bob At a later time, she sends Bob the key He can then verify the bit Disadvantage: Alice may be able to generate another key so that the bit is changed once she knows the result.
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