笔记 MIT6.824 Lecture 19: Bitcoin

目录

• 前言
• 一、Bitcoin
• 二、the BitCoin block chain
• 三、weak points
• 总结

前言

讲的是比特币，一个去中心化的例子，谈到了相关的技术与问题

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一、Bitcoin

1.1 去中心化

peer-to-peer有很多参与者，且它们中会有malicious，需要有新的解决方法

1.2 digital currency 的challenges

1. outright forgery - 造假
2. double spendig
3. theft

1.3 idea: signed sequency of transactions

1. 有bunch of coins, 有owners
2. 每个coin都是有完整的交易记录
3. 每个coin最近一次的交易indicates its owners

1.4 transaction record

pub(user1) : public key of new owner
hash(prev): hash of this coin’s previous transaction record
sig(user2): signature over transaction by previous onwer’s private key

  Y owns a coin, previously given to it by X:
    T6: pub(X), ...
    T7: pub(Y), hash(T6), sig(X)
  Y buys a hamburger from Z and pays with this coin
    Z sends public key to Y
    Y creates a new transaction and signs it
    T8: pub(Z), hash(T7), sig(Y)
  Y sends transaction record to Z
  Z verifies:
    T8's sig(Y) corresponds to T7's pub(Y)
  Z gives hamburger to Y

1.5 prevent double-spending

publish a log of all transactions
ensure everyone sees the same log (in same order!)
ensure Y can’t un-publish a transaction

result:
  Z will see Y->Z came before Y->Q, and will accept Y->Z
  Q will see Y->Z came before Y->Q, and will reject Y->Q
  a "public ledger"

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二、the BitCoin block chain

the goal: agreement on transaction log to prevent double-spending
the block chain contains transactions on all coins
有很多peers
– 有完整的chain
– TCP 链接
– 新的block以及transactions会by TPC通知给各个peers

new block every 10 minutes containing xactions since prev block payee doesn’t accept transaction until it’s in the block chain

为什么要create each new block

2.1 how is a fork resolved?

会有Fork的可能性，但是会有一个会在后面接更多的blocks，然后多的那个就会变成主要的，之后的mining都会在这个上进行，这个叫做winning fork。

在没有决定谁是winning fork的时候，transaction是不可靠的，所以需要等待后面有多个Bolcks才能确保交易被成功记录在winning fork上

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三、weak points

– too bad it’s a new currency as well as a payment system
– transaction confirmation takes at least 10 minutes, or 60 for high confidence
– flooding limits performance, may be a point of attack
– maximum block size plus 10 minutes limits max transactions per second
– users have trouble securing private keys

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总结

通过ledger账本去中心化是一个greate idea，但是block以及transactions速度的限制不利于频繁大规模地交易，也就有很多不适用的场景