引介

OneThousand - 去中心化的抽奖

jan | 3. Oct, 2015 | 2398 次阅读

引介 dapp

OneThousand是一个Solidity写的抽奖游戏。参与者调用合约投注，每注1ETH (量大优惠...)，每1000注开一次奖，随机抽中的幸运儿获得1000ETH的奖励。

随机数由server seed, client seed和投注完毕后一个block的hash决定，保证公平。游戏本身的代码也放在Github和区块链上，完全透明。

编者按：本文于 2018 年 5 月 29 日经过重新编辑。

tomlion
How do we decide who wins?
The winner is decided using three key elements:
1. A server secret of which the hash is revealed beforehand. We have 24 hours upon the termination of a round to reveal the server secret; otherwise everyone gets their money back.
2. A client secret which is updated each time a transaction is sent within a round. Once the round ends, this is set in stone.
3. The blockhash of the block following the round. This is also set in stone upon the end of a round.
We can't cheat you because we can't easily modify (2 and 3). You can't cheat us because you don't know (1). It's to our advantage to pay you to keep the contract running.
```
  //Update client seed
  clientSeed = sha3(clientSeed, msg.sender, msg.value);
```
庄家只要参与最后一笔投注，就可以指定client secret，所以上面说的2很难修改不成立
```
  //Fetch the non-server sources of entropy
  bytes32 serverClientHash = sha3(curSecret, lastClientSeed);

  //Calculate winner and pay out
  uint winnerIdx =  lastProcessed + uint(serverClientHash ^ lastBlockHash) % TICKETSPERROUND;
```
所以庄家可以提前知道serverClientHash。
庄家如果和矿池合作，就可以大幅提高胜率！

randao就快好了， be patient :D
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jan

我觉得没那么容易，第一是要经过两次sha3: 首先你要穷举msg.sender&msg.value的可能组合来碰撞你想要的clientSeed, 然后要穷举server secret来碰撞serverClientHash.

另外一个难点是如何参与最后一笔投注。在只剩两注开奖的时候对手可以一下买两注，庄就没有买最后一注的机会，除非他也下两注，但这时候对手在剩三注开奖的时候一次买三注，以此类推，有一个博弈在里面。

理论上确实有可能做到作弊，实际可能性应该不大?

randao预计什么时候发布?

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gulu

1）庄家会努力争取最后一注。
2）庄家会设法与矿池串谋。
结论：庄家的数学期望大于1。
该方案不完美。期待Randao。

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