本文是精讀比特幣的白皮書，一次嘗試
Bitcoin: A Peer-to-Peer Electronic Cash System
比特幣：一個點對點的電子現金系統
Abstract
A purely peer-to-peer version (of electronic cash )would allow online payments (to be sent directly)( from one party to another)( without going through a financial institution(機構)).
Digital signatures provide part of the solution, but the main benefits are lost (if a trusted third party is still required )(to prevent double-spending.)
有了數字簽名，P2P交易確實不需要第三方了，但是爲了解決雙重支付（同一筆錢，用了2次），必須要第三方，
這樣最大的好處就損失了。
白皮書主要討論： P2P電子現金系統（無第三方） + 雙花問題（一個電子幣，花2次？）
We propose a solution( to the double-spending problem )(using a peer-to-peer network.)
The network timestamps transactions (by hashing them into an ongoing chain )(of hash-based proof-of-work),( forming a record that cannot be changed )(without redoing the proof-of work.)
網絡給交易打了時間戳，形成了一個不能被篡改的記錄，除非你重來POW
The longest chain not only serves as proof (of the sequence of events witnessed), but proof that it came (from the largest pool of CPU power.)
As long as a majority of CPU power is controlled (by nodes that are not cooperating to attack the network), they'll generate the longest chain and outpace(超過，趕上) attackers.
The network itself requires minimal structure.
不是很複雜，小結構就能處理
Messages are broadcast (on a best effort basis準則,基礎), and nodes can leave and rejoin the network at will, (accepting the longest proof-of-work chain)( as proof of what happened )(while they were gone.)
1. 簡介 (Introduction)
Commerce( on the Internet) has come to rely almost exclusively (on financial institutions)( serving as trusted third parties )(to process electronic payments. )
exclusively:排他的， -- 只依賴於金融機構
While the system works well enough (for most transactions), it still suffers from the inherent(固有的) weaknesses (of the trust based model.)
Completely non-reversible(不可逆轉) transactions are not really possible, since financial institutions cannot avoid mediating disputes(調節 糾紛).
The cost (of mediation )increases transaction costs, limiting the minimum practical transaction size and cutting off the possibility (for small casual transactions), and there is a broader cost in the loss (of ability )(to make non-reversible payments for non-reversible services. )
衝突調節成本高，增加轉賬成本。（小額交易沒有了，轉賬一筆2元手續費，50元封頂，不夠銀行手續費的呢）
With the possibility (of reversal), the need (for trust) spreads.
Merchants must be wary (of their customers), (hassling them )(for more information)( than they would otherwise need.)
wary 警惕 hassle 爭辯，不斷煩擾
A certain percentage of fraud is accepted (as unavoidable.)
一定比例的欺騙，不可避免
These costs and payment uncertainties can be avoided (in person )(by using physical currency), but no mechanism exists (to make payments)( over a communications channel without a trusted party.)
What is needed is an electronic payment system (based on cryptographic proof )(instead of trust), (allowing any two willing parties to transact directly with each other)( without the need for a trusted third party.)
基於 加密驗證，而不是信用 （通過密碼學，讓別人不知道我密碼，但是通過方法，可以知道我確實有密碼，擁有餘額）
Transactions( that are computationally impractical to reverse) would protect sellers (from fraud), and routine escrow mechanisms could easily be implemented to protect buyers.
在計算上 不可逆，保護買賣雙方 ？？？？不可理解啊
In this paper, we propose a solution (to the double-spending problem)( using a peer-to-peer distributed timestamp server)( to generate computational proof )(of the chronological order of transactions.)
提出方案
The system is secure (as long as honest nodes collectively control more CPU power than any cooperating group of attacker nodes.)
總結：
不可逆轉是比特幣的特點，但是也正是 勒索、暗網等罪惡，可以利用。只是回滾 分叉的代價太大了，不可實現。
以太坊被盜走很多，回滾否？硬分叉的是ETH，堅持不分叉的是ETC。
2. 交易 (Transactions)
We define an electronic coin (as a chain of digital signatures.)
Each owner transfers the coin (to the next)( by digitally signing a hash )(of the previous transaction) and the public key (of the next owner and adding these to the end of the coin.)
A payee can verify the signatures (to verify the chain of ownership.)
The problem (of course) is the payee can't verify that one of the owners did not double-spend the coin.
A common solution is to introduce a trusted central authority, or mint(鑄幣廠), that checks every transaction for double spending.
After each transaction, the coin must be returned (to the mint )(to issue a new coin), and only coins issued directly (from the mint )are trusted not to be double-spent.
The problem (with this solution) is that the fate( of the entire money system )depends on the company (running the mint), with every transaction having to go through them, just like a bank.
We need a way for the payee to know that the previous owners did not sign any earlier transactions.
For our purposes, the earliest transaction is the one (that counts), so we don't care about later attempts to double-spend.
The only way to confirm the absence (of a transaction) is to be aware of all transactions.
遍歷所有哇？
In the mint based model, the mint was aware (of all transactions )and decided which arrived first.
鑄幣廠模式，鑄幣廠知道哪個算數
To accomplish this (without a trusted party), transactions must be publicly announced [1], and we need a system (for participants to agree on a single history)( of the order )(in which they were received.)
The payee needs proof (that at the time of each transaction, the majority of nodes agreed it was the first received.)
3. Timestamp Server(時間戳 服務器)
The solution (we propose) begins with a timestamp server.
A timestamp server works (by taking a hash)( of a block of items)( to be timestamped and widely publishing the hash),( such as in a newspaper or Usenet post) [2-5].
The timestamp proves that the data must have existed at the time, obviously, in order to get into the hash.
Each timestamp includes the previous timestamp in its hash, forming a chain, with each additional timestamp reinforcing the ones before it.
所以具體時間點不重要，重要的是先後關係，有上一個區塊的hash
4. Proof-of-Work
To implement a distributed timestamp server (on a peer-to-peer basis), we will need to use a proof of-work system (similar to Adam Back's Hashcash) [6], rather than newspaper or Usenet posts.
費勁九牛二虎之力，撞到隨機數hash之後，開頭8個0， 而 驗證人只要短暫驗證一下，就知道滿足要求。
The proof-of-work involves scanning for a value that when hashed, such as with SHA-256, the hash begins (with a number of zero bits).
The average work required is exponential 指數的(in the number of zero bits required )and can be verified by executing a single hash.
For our timestamp network, we implement the proof-of-work (by incrementing a nonce in the block )(until a value is found that gives the block's hash the required zero bits).
Once the CPU effort has been expended to make it satisfy the proof-of-work, the block cannot be changed without redoing the work. As later blocks are chained after it, the work to change the block would include redoing all the blocks after it.
The proof-of-work also solves the problem (of determining representation)( in majority decision making).
一個ip投一票，他想的是 CPU算力，但是 現在這個情況 算力也是壟斷集中
If the majority were based on one-IP-address-one-vote, it could be subverted （顛覆、破壞）by anyone able to allocate （分配）many IPs.
Proof-of-work is essentially one-CPU-one-vote.
The majority decision is represented by the longest chain, which has the greatest proof of-work effort invested in it.
If a majority of CPU power is controlled by honest nodes, the honest chain will grow the fastest and outpace any competing chains.
To modify a past block, an attacker would have to redo the proof-of work of the block and all blocks after it and then catch up with and surpass the work of the honest nodes.
We will show later that the probability of a slower attacker catching up diminishes exponentially as subsequent blocks are added.
To compensate補償 (for increasing hardware speed and varying interest)( in running nodes over time), the proof-of-work difficulty is determined by a moving average targeting an average number of blocks per hour.
If they're generated too fast, the difficulty increases.
5. Network
The steps to run the network are as follows:
1) New transactions are broadcast to all nodes.
2) Each node collects new transactions into a block.
3) Each node works on finding a difficult proof-of-work (for its block.)
4) When a node finds a proof-of-work, it broadcasts the block( to all nodes.)
5) Nodes accept the block only if all transactions in it are valid and not already spent.
6) Nodes express their acceptance of the block by working on creating the next block in the chain, using the hash of the accepted block as the previous hash.
Nodes always consider the longest chain to be the correct one and will keep working on extending it.
If two nodes broadcast different versions of the next block simultaneously, some nodes may receive one or the other first.
In that case, they work on the first one they received, but save the other branch in case it becomes longer.
The tie will be broken when the next proofof-work is found and one branch becomes longer;
the nodes that were working on the other branch will then switch to the longer one.
New transaction broadcasts do not necessarily need to reach all nodes.
As long as they reach many nodes, they will get into a block before long.
Block broadcasts are also tolerant of dropped messages.
If a node does not receive a block, it will request it when it receives the next block and realizes it missed one.
6. Incentive
By convention, the first transaction in a block is a special transaction that starts a new coin owned by the creator of the block.
屬於礦工，沒有輸入，收款方是自己
This adds an incentive for nodes to support the network, and provides a way to initially distribute coins into circulation, since there is no central authority to issue them.
貨幣激勵的方式，也是貨幣發行的方式
The steady addition (of a constant of amount of new coins )is analogous類比 to gold miners (expending resources)( to add gold to circulation).
In our case, it is CPU time and electricity that is expended.(花費)
The incentive can also be funded (with transaction fees).
If the output value (of a transaction) is less( than its input value), the difference is a transaction fee (that is added to the incentive value)( of the block containing the transaction).
Once a predetermined number (of coins) have entered circulation, the incentive can transition entirely (to transaction fees and be completely inflation free.)
不擔心通貨膨脹？ 想多了吧
The incentive may help encourage nodes to stay honest.
If a greedy attacker is able to assemble（集合 裝配） more CPU power (than all the honest nodes), he would have to choose (between using it )(to defraud欺騙 people)( by stealing back his payments), or using it to generate new coins.
He ought to find it more profitable (to play by the rules), such rules that favour him (with more new coins)( than everyone else combined), than to undermine the system and the validity有效性 of his own wealth.
7. Reclaiming Disk Space
Once the latest transaction (in a coin) is buried埋藏 (under enough blocks), the spent transactions (before it can be discarded )(to save disk space. )
To facilitate(促進 助長 使容易) this (without breaking the block's hash), transactions are hashed in a Merkle Tree [7][2][5],( with only the root) included in the block's hash.
莫克爾樹 不懂啊！
Old blocks can then be compacted（壓緊） (by stubbing off branches of the tree).(剪掉枝葉)
The interior(內部) hashes do not need to be stored.
A block header (with no transactions) would be about 80 bytes.
區塊頭？
If we suppose blocks are generated every 10 minutes, 80 bytes * 6 * 24 * 365 = 4.2MB per year.
With computer systems typically selling with 2GB of RAM as of 2008, and Moore's Law predicting current growth of 1.2GB per year, storage should not be a problem (even if the block headers must be kept in memory.)
狹義上說，比特幣錢包是指能生成和管理私鑰和收發幣地址(類似賬號),並且能顯示你的比特幣餘額的這樣一個軟件。
而從廣義上說，私鑰就是錢包。只要我有私鑰，我就能消費掉餘額。
通過seed 種子，創建私鑰。
助記詞 完全能輕易 轉化成私鑰。一定要 手動 物理保存助記詞 不要連接網絡，不要複製粘貼！
拋硬幣256個，得到私鑰，得到公鑰，得到BTC收款地址
8. Simplified Payment Verification
It is possible to verify payments( without running a full network node).
不運行全節點，也能驗證支付
A user only needs to keep a copy (of the block headers )(of the longest proof-of-work chain), (which he can get )(by querying network nodes)( until he's convinced he has the longest chain), and obtain the Merkle branch (linking the transaction )(to the block it's timestamped in. )
He can't check the transaction (for himself), but (by linking it to a place )(in the chain), he can see that a network node has accepted it, and blocks added after it further confirm the network has accepted it.
As such, the verification is reliable (as long as honest nodes control the network), but is more vulnerable (if the network is overpowered by an attacker).
While network nodes can verify transactions (for themselves), the simplified method can be fooled (by an attacker's fabricated transactions )(for as long as the attacker can continue to overpower the network).
One strategy (to protect against this )would be to accept alerts (from network nodes)( when they detect an invalid block), (prompting the user's software to download the full block and alerted transactions to confirm the inconsistency.)
Businesses (that receive frequent payments) will probably still want to run their own nodes (for more independent security and quicker verification.)
9. Combining and Splitting Value
Although it would be possible to handle coins individually, it would be unwieldy to make a separate transaction for every cent in a transfer. To allow value to be split and combined, transactions contain multiple inputs and outputs. Normally there will be either a single input from a larger previous transaction or multiple inputs combining smaller amounts, and at most two outputs: one for the payment, and one returning the change, if any, back to the sender.
It should be noted that fan-out, where a transaction depends on several transactions, and those transactions depend on many more, is not a problem here. There is never the need to extract a complete standalone copy of a transaction's history.
10. Privacy
The traditional banking model achieves a level of privacy by limiting access to information to the parties involved and the trusted third party. The necessity to announce all transactions publicly precludes this method, but privacy can still be maintained by breaking the flow of information in another place: by keeping public keys anonymous. The public can see that someone is sending an amount to someone else, but without information linking the transaction to anyone. This is similar to the level of information released by stock exchanges, where the time and size of individual trades, the "tape", is made public, but without telling who the parties were.
As an additional firewall, a new key pair should be used for each transaction to keep them from being linked to a common owner. Some linking is still unavoidable with multi-input transactions, which necessarily reveal that their inputs were owned by the same owner. The risk is that if the owner of a key is revealed, linking could reveal other transactions that belonged to the same owner.
11. Calculations
We consider the scenario of an attacker trying to generate an alternate chain faster than the honest chain. Even if this is accomplished, it does not throw the system open to arbitrary changes, such as creating value out of thin air or taking money that never belonged to the attacker. Nodes are not going to accept an invalid transaction as payment, and honest nodes will never accept a block containing them. An attacker can only try to change one of his own transactions to take back money he recently spent.
The race between the honest chain and an attacker chain can be characterized as a Binomial Random Walk. The success event is the honest chain being extended by one block, increasing its lead by +1, and the failure event is the attacker's chain being extended by one block, reducing the gap by -1.
The probability of an attacker catching up from a given deficit is analogous to a Gambler's Ruin problem. Suppose a gambler with unlimited credit starts at a deficit and plays potentially an infinite number of trials to try to reach breakeven. We can calculate the probability he ever reaches breakeven, or that an attacker ever catches up with the honest chain, as follows [8]:
Given our assumption that p > q, the probability drops exponentially as the number of blocks the attacker has to catch up with increases. With the odds against him, if he doesn't make a lucky lunge forward early on, his chances become vanishingly small as he falls further behind.
We now consider how long the recipient of a new transaction needs to wait before being sufficiently certain the sender can't change the transaction. We assume the sender is an attacker who wants to make the recipient believe he paid him for a while, then switch it to pay back to himself after some time has passed. The receiver will be alerted when that happens, but the sender hopes it will be too late.
The receiver generates a new key pair and gives the public key to the sender shortly before signing. This prevents the sender from preparing a chain of blocks ahead of time by working on it continuously until he is lucky enough to get far enough ahead, then executing the transaction at that moment. Once the transaction is sent, the dishonest sender starts working in secret on a parallel chain containing an alternate version of his transaction.
The recipient waits until the transaction has been added to a block and z blocks have been linked after it. He doesn't know the exact amount of progress the attacker has made, but assuming the honest blocks took the average expected time per block, the attacker's potential progress will be a Poisson distribution with expected value:
To get the probability the attacker could still catch up now, we multiply the Poisson density for each amount of progress he could have made by the probability he could catch up from that point:
Rearranging to avoid summing the infinite tail of the distribution...
Converting to C code...
#include <math.h>
double AttackerSuccessProbability(double q, int z){
double p = 1.0 - q;
double lambda = z * (q / p);
double sum = 1.0;
int i, k;
for (k = 0; k <= z; k++){
double poisson = exp(-lambda);
for (i = 1; i <= k; i++){
poisson *= lambda / i;
}
sum -= poisson * (1 - pow(q / p, z - k));
}
return sum;
}
Running some results, we can see the probability drop off exponentially with z.
q=0.1
z=0 P=1.0000000
z=1 P=0.2045873
z=2 P=0.0509779
z=3 P=0.0131722
z=4 P=0.0034552
z=5 P=0.0009137
z=6 P=0.0002428
z=7 P=0.0000647
z=8 P=0.0000173
z=9 P=0.0000046
z=10 P=0.0000012
q=0.3
z=0 P=1.0000000
z=5 P=0.1773523
z=10 P=0.0416605
z=15 P=0.0101008
z=20 P=0.0024804
z=25 P=0.0006132
z=30 P=0.0001522
z=35 P=0.0000379
z=40 P=0.0000095
z=45 P=0.0000024
z=50 P=0.0000006
Solving for P less than 0.1%...
P < 0.001
q=0.10 z=5
q=0.15 z=8
q=0.20 z=11
q=0.25 z=15
q=0.30 z=24
q=0.35 z=41
q=0.40 z=89
q=0.45 z=340
12. Conclusion
We have proposed a system for electronic transactions without relying on trust. We started with the usual framework of coins made from digital signatures, which provides strong control of ownership, but is incomplete without a way to prevent double-spending. To solve this, we proposed a peer-to-peer network using proof-of-work to record a public history of transactions that quickly becomes computationally impractical for an attacker to change if honest nodes control a majority of CPU power. The network is robust in its unstructured simplicity. Nodes work all at once with little coordination. They do not need to be identified, since messages are not routed to any particular place and only need to be delivered on a best effort basis. Nodes can leave and rejoin the network at will, accepting the proof-of work chain as proof of what happened while they were gone. They vote with their CPU power, expressing their acceptance of valid blocks by working on extending them and rejecting invalid blocks by refusing to work on them. Any needed rules and incentives can be enforced with this consensus mechanism.