Introduction to Crypto and Cryptocurrencies week three 笔记和个人理解

Lecture 3.1

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Bitcoin Transactions

• Append-only ledger
• Decentralized consensus
• Miners to validate transactions

Lecture 3.2

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Bitcoin scripts

Output "addresses" are really scripts

把output中的地址转换为合约

the secret is that the input address is also a script

实际上   input scripts  called scriptsig

            output scripts called scriptPublickey

forth language.    is a stack-based language    no loops

一种类似forth的语言   ：简单，但是support for pretty sophistcated cryptography

there are special purpose instructions to do compute hash functions and to compute signatures and verigy signatures

Bitcoin script instructions 

256 opcodes total (15disabled,75 reserved)

         可以做的运算： Arithmetic    if/then      logic/date handling     Crypto

Lecture 3.3

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Applications of Bitcoin scripts

Example 1:Escrow transactions(第三方交易)

a third party and do ascrow transactions

MULTIGIS  多重签名技术

Example 2：Green addresses

有个bank的存在

现实中有两家公司：Instawaller and mount Gox 

Example 3：Efficient micro-payments

problem : Alice wants to pay Bob for each minue of phone service . She doesn't want to incur a transaction fee every minute.

solution: low value transaction every minute

multi-palyer lotteries

Lecture 3.4

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Bitcoin blocks

why bundle transactions together?

• single unit of work for miners
• limit length of hash-chaina of blocks
• • faster to verify history

Lecture 3.5

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The bitcoin network

let us talk about how Bitcoin blocks get put together.

Bitcoin P2P network

• ad-hoc protocol(runs on TCP port 8333)
• Ad-hoc network with random topology
• all nodes are equal
• new nodes can join at any time
• Forget non-responding nodes after 3 hr

should I relay a proposed transaction?

•      Transaction valid with current block chain
•      script matches a whitelist 
• •      avoid unusual scripts
•      Haven't seen beford
• •      avoid infinite loops
•      doesn;t conflict with others I've relayed
• •      avoid double-spengs

Race conditions

• Default behavior:accept what you hear first
• Network position matters
• miners may implement other logic!

How big is the network?

•      impossible to measure exactly
•      estimates-up to 1M 1P addresser/month
•      Only about 5-10k "full nodes"
• • Permannently connected
  • Fully-validate
•      this number may be dropping!

Lecture 3.6

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Limitations & improvements

10min.average creation time per block

1M bytes in a block

Througput limits in Bitcoin

7 transactions/sec

compare to :

     VISA,PayPal

Cryptographic limits in Bitcoin

     only 1 signature algorithm(ECDSA/P256)

     Hard-coded hash functions

Hard-forking changes to Bitcoin

Next week, we will talk about  Human beings aren't work for people, not nodes

• How do people interact with the network?
• How do people exchange bitcoins for cash?
• How do people securely store bitcoins?

Currency needs to work for people, not nodes.