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Thread on developments exactly one month since Fidelity Bonds were added to JoinMarket

JoinMarket used to work by the user's wallet randomly choosing market makers out there to do a coinjoin with, but what if all those makers were actually controlled by the same person? That attacker could unmix all the coinjoins and spy on users, this is called a sybil attack

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Fidelity bonds are a mechanism for resistance against such sybil attacks. Honest makers in JoinMarket are now incentivized to lock up bitcoins using the OP_CHECKLOCKTIMEVERIFY opcode. For a sybil attacker to be successful they have to lock up a huge amount of bitcoins

Currently 264 BTC are locked up, including this one maker who locked up 124 coins for one year. Several other makers have locked up tens of bitcoins each some for multiple years.

In terms of sybil resistance, if a taker does a coinjoin with 10 other makers (the default value), then in order to successfully attack a sybil attacker would have to lock up over 128k BTC (6.1 billion USD) for one year, or destroy 160 BTC forever

What if top fidelity bonds in the orderbook are actually controlled by the same person? They would be foregoing about 8.39 BTC² in sacrificed value, nearly twice as much as the total value right now which is 4.51 BTC². And their success rate would only be about 6% per coinjoin

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