《精通比特币》英文版批注导读•第6章 比特币交易记录 — ScalersTalk成长会 – 持续行动,刻意学习 – ScalersTalk Wonderland

《精通比特币》英文版批注导读•第6章 比特币交易记录

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这一章讲的比特币的交易记录的细节。通过这一章你可以理解,比特币的交易是采用什么样的组织形式构成的,同时也可以看到,我们从钱包感知到的比特币,和真实网络数据中的比特币,抽象的层次仍然存在区别。文章的技术细节比较多,阅读需要一些耐心。

本章原文地址:

https://github.com/bitcoinbook/bitcoinbook/blob/develop/ch06.asciidoc

相关文章:

《精通比特币》英文版批注导读•第1章

《精通比特币》英文版批注导读•第2章 比特币工作原理

《精通比特币》英文版批注导读•第3-4章 比特币密钥与地址

《精通比特币》英文版批注导读•第4章(2) 比特币地址

《精通比特币》英文版批注导读•第5章 比特币钱包技术

Chapter 6 Transactions

Introduction

Transactions are the most important part of the bitcoin system. Everything else in bitcoin is designed to ensure that transactions can be created, propagated on the network, validated, and finally added to the global ledger of transactions (the blockchain). Transactions are data structures that encode the transfer of value between participants in the bitcoin system. Each transaction is a public entry in bitcoin’s blockchain, the global double-entry bookkeeping ledger.

在比特币中,每一笔交易都是公开在区块链上的。

In this chapter we will examine all the various forms of transactions, what they contain, how to create them, how they are verified, and how they become part of the permanent record of all transactions. When we use the term “wallet” in this chapter, we are referring to the software that constructs transactions, not just the database of keys.

在本章中,钱包更多的是指发起交易的软件,不止是上一章所述存储密钥的数据库。

Transactions in Detail

In [ch02_bitcoin_overview], we looked at the transaction Alice used to pay for coffee at Bob’s coffee shop using a block explorer (Alice’s transaction to Bob’s Cafe).

The block explorer application shows a transaction from Alice’s “address” to Bob’s “address.” This is a much simplified view of what is contained in a transaction. In fact, as we will see in this chapter, much of the information shown is constructed by the block explorer and is not actually in the transaction.

在交易浏览器里看到的信息,很多是由浏览器构造的,并没有存在交易中。实际的交易过程中所交换的数据,与在浏览器上看到的不一样。

Figure 1. Alice’s transaction to Bob’s Cafe

Transactions—Behind the Scenes

Behind the scenes, an actual transaction looks very different from a transaction provided by a typical block explorer. In fact, most of the high-level constructs we see in the various bitcoin application user interfaces do not actually exist in the bitcoin system.

We can use Bitcoin Core’s command-line interface (getrawtransaction and decoderawtransaction) to retrieve Alice’s “raw” transaction, decode it, and see what it contains. The result looks like this:

Alice’s transaction decoded

{

  “version”: 1,

  “locktime”: 0,

  “vin”: [

    {

      “txid”: “7957a35fe64f80d234d76d83a2a8f1a0d8149a41d81de548f0a65a8a999f6f18″,

      “vout”: 0,

      “scriptSig” : “3045022100884d142d86652a3f47ba4746ec719bbfbd040a570b1deccbb6498c75c4ae24cb02204b9f039ff08df09cbe9f6addac960298cad530a863ea8f53982c09db8f6e3813[ALL]0484ecc0d46f1918b30928fa0e4ed99f16a0fb4fde0735e7ade8416ab9fe423cc5412336376789d172787ec3457eee41c04f4938de5cc17b4a10fa336a8d752adf”,

      “sequence”: 4294967295

    }

  ],

  “vout”: [

    {

      “value”: 0.01500000,

      “scriptPubKey”: “OP_DUPOP_HASH160 ab68025513c3dbd2f7b92a94e0581f5d50f654e7 OP_EQUALVERIFYOP_CHECKSIG”

    },

    {

      “value”: 0.08450000,

      “scriptPubKey”: “OP_DUPOP_HASH160 7f9b1a7fb68d60c536c2fd8aeaa53a8f3cc025a8 OP_EQUALVERIFY OP_CHECKSIG”,

    }

  ]

}

 

You may notice a few things about this transaction, mostly the things that are missing! Where is Alice’s address? Where is Bob’s address? Where is the 0.1 input “sent” by Alice? In bitcoin, there are no coins, no senders, no recipients, no balances, no accounts, and no addresses. All those things are constructed at a higher level for the benefit of the user, to make things easier to understand.

在比特币里,没有币,没有收方、没有付方、没有余额表、没有账户、没有地址,所有这些概念只是为了用户理解方便,构建出的更高层的概念。

You may also notice a lot of strange and indecipherable fields and hexadecimal strings. Don’t worry, we will explain each field shown here in detail in this chapter.

Transaction Outputs and Inputs

The fundamental building block of a bitcoin transaction is a transaction output. Transaction outputs are indivisible chunks of bitcoin currency, recorded on the blockchain, and recognized as valid by the entire network. Bitcoin full nodes track all available and spendable outputs, known as unspent transaction outputs, or UTXO. The collection of all UTXO is known as the UTXO set and currently numbers in the millions of UTXO. The UTXO set grows as new UTXO is created and shrinks when UTXO is consumed. Every transaction represents a change (state transition) in the UTXO set.

When we say that a user’s wallet has “received” bitcoin, what we mean is that the wallet has detected an UTXO that can be spent with one of the keys controlled by that wallet. Thus, a user’s bitcoin “balance” is the sum of all UTXO that user’s wallet can spend and which may be scattered among hundreds of transactions and hundreds of blocks. The concept of a balance is created by the wallet application. The wallet calculates the user’s balance by scanning the blockchain and aggregating the value of any UTXO the wallet can spend with the keys it controls. Most wallets maintain a database or use a database service to store a quick reference set of all the UTXO they can spend with the keys they control.

交易的输出是不可再切割的,比特币的全节点维护所有可以花的输出,这些称作UTXO。这个UTXO集合会随着新的未花的钱增加而扩大,也会随着消耗而减小。每一笔交易就代表了UTXO的状态的变化。钱包的目的就是在区块链上检测,自己掌握的密钥能花的钱。你可以理解为,钱包里面掌握了一堆的钥匙,钱包通过这些钥匙在区块链上找到能打开的房间,钱包就记下来自己能打开的房间号,房间里面就对应着钱。

A transaction output can have an arbitrary (integer) value denominated as a multiple of satoshis. Just as dollars can be divided down to two decimal places as cents, bitcoin can be divided down to eight decimal places as satoshis. Although an output can have any arbitrary value, once created it is indivisible. This is an important characteristic of outputs that needs to be emphasized: outputs are discrete and indivisible units of value, denominated in integer satoshis. An unspent output can only be consumed in its entirety by a transaction.

尽管比特币的交易数额是可以变化的,但是一旦创建以后,就不可以再分割了。如果你要花的一笔钱比UTXO 要更大的话,这笔整钱仍然会先花出,然后再得到一笔找零。因此大部分比特币的交易都会涉及到找零的问题。用户无法把一个UTXO分割成两半,就像同样无法把一张固定面额的钱分割成两份。

If an UTXO is larger than the desired value of a transaction, it must still be consumed in its entirety and change must be generated in the transaction. In other words, if you have an UTXO worth 20 bitcoin and want to pay only 1 bitcoin, your transaction must consume the entire 20-bitcoin UTXO and produce two outputs: one paying 1 bitcoin to your desired recipient and another paying 19 bitcoin in change back to your wallet. As a result of the indivisible nature of transaction outputs, most bitcoin transactions will have to generate change.

Imagine a shopper buying a $1.50 beverage, reaching into her wallet and trying to find a combination of coins and bank notes to cover the $1.50 cost. The shopper will choose exact change if available e.g. a dollar bill and two quarters (a quarter is $0.25), or a combination of smaller denominations (six quarters), or if necessary, a larger unit such as a $5 note. If she hands too much money, say $5, to the shop owner, she will expect $3.50 change, which she will return to her wallet and have available for future transactions.

Similarly, a bitcoin transaction must be created from a user’s UTXO in whatever denominations that user has available. Users cannot cut an UTXO in half any more than they can cut a dollar bill in half and use it as currency. The user’s wallet application will typically select from the user’s available UTXO to compose an amount greater than or equal to the desired transaction amount.

As with real life, the bitcoin application can use several strategies to satisfy the purchase amount: combining several smaller units, finding exact change, or using a single unit larger than the transaction value and making change. All of this complex assembly of spendable UTXO is done by the user’s wallet automatically and is invisible to users. It is only relevant if you are programmatically constructing raw transactions from UTXO.

UTXO 也可以自动合并,小的合并成大的。这些复杂的过程是由钱包自动完成。通过这样的UTXO 的消耗和生成的过程,比特币就在不同的用户之间转移。

A transaction consumes previously recorded unspent transaction outputs and creates new transaction outputs that can be consumed by a future transaction. This way, chunks of bitcoin value move forward from owner to owner in a chain of transactions consuming and creating UTXO.

The exception to the output and input chain is a special type of transaction called the coinbase transaction, which is the first transaction in each block. This transaction is placed there by the “winning” miner and creates brand-new bitcoin payable to that miner as a reward for mining. This special coinbase transaction does not consume UTXO; instead, it has a special type of input called the “coinbase.” This is how bitcoin’s money supply is created during the mining process, as we will see in [mining].

比特币的第一笔钱是只有输出没有输入的,因为是新挖出来的币,出来以后便是直接使用了。

Tip What comes first? Inputs or outputs, the chicken or the egg? Strictly speaking, outputs come first because coinbase transactions, which generate new bitcoin, have no inputs and create outputs from nothing.

Transaction Outputs

Every bitcoin transaction creates outputs, which are recorded on the bitcoin ledger. Almost all of these outputs, with one exception (see [op_return]) create spendable chunks of bitcoin called UTXO, which are then recognized by the whole network and available for the owner to spend in a future transaction.

UTXO are tracked by every full-node bitcoin client in the UTXO set. New transactions consume (spend) one or more of these outputs from the UTXO set.

Transaction outputs consist of two parts:

  • An amount of bitcoin, denominated in satoshis, the smallest bitcoin unit

  • A cryptographic puzzle that determines the conditions required to spend the output

The cryptographic puzzle is also known as a locking script, a witness script, or a scriptPubKey.

交易的输出分成两个部分,一部分是输出的比特币的数量,一部分就是脚本锁。脚本锁的就是一首密码学的题,谁符合条件,这笔钱谁就可以花。

The transaction scripting language, used in the locking script mentioned previously, is discussed in detail in Transaction Scripts and Script Language.

Now, let’s look at Alice’s transaction (shown previously in Transactions—Behind the Scenes) and see if we can identify the outputs. In the JSON encoding, the outputs are in an array (list) named vout:

“vout”: [

  {

    “value”: 0.01500000,

    “scriptPubKey”: “OP_DUPOP_HASH160 ab68025513c3dbd2f7b92a94e0581f5d50f654e7 OP_EQUALVERIFY

    OP_CHECKSIG”

  },

  {

    “value”: 0.08450000,

    “scriptPubKey”: “OP_DUPOP_HASH160 7f9b1a7fb68d60c536c2fd8aeaa53a8f3cc025a8 OP_EQUALVERIFYOP_CHECKSIG”,

  }

]

As you can see, the transaction contains two outputs. Each output is defined by a value and a cryptographic puzzle. In the encoding shown by Bitcoin Core, the value is shown in bitcoin, but in the transaction itself it is recorded as an integer denominated in satoshis. The second part of each output is the cryptographic puzzle that sets the conditions for spending. Bitcoin Core shows this as scriptPubKey and shows us a human-readable representation of the script.

The topic of locking and unlocking UTXO will be discussed later, in Script Construction (Lock + Unlock). The scripting language that is used for the script in scriptPubKey is discussed in Transaction Scripts and Script Language. But before we delve into those topics, we need to understand the overall structure of transaction inputs and outputs.

Transaction serialization—outputs

When transactions are transmitted over the network or exchanged between applications, they are serialized. Serialization is the process of converting the internal representation of a data structure into a format that can be transmitted one byte at a time, also known as a byte stream. Serialization is most commonly used for encoding data structures for transmission over a network or for storage in a file. The serialization format of a transaction output is shown in Transaction output serialization.

交易在网络传播交换,需要序列化。序列化就是把数据带有结构的表示形式,转换成一个字节流,可以依次在网络上传输,一次传一个字节,形成字节流。

Table 1. Transaction output serialization

为了便于处理,比特币本身就会按照一定数据结构存储。序列化与结构化的概念可以这样理解:你出差的时候,要把衣服折叠或者卷起放在行李箱里,这样便于运输;而你在家里或者到达目的地的时候,需要把衣服再展开挂起来,这样才能还原成以前的结构。比特币的交易数据本身就会有自己的结构,但是传输的时候,要转换成一个一个字节排列的流式数据,传输出去。这就是序列化与反序列化了。

The process of converting from the byte-stream representation of a transaction to a library’s internal representation data structure is called deserialization or transaction parsing. The process of converting back to a byte-stream for transmission over the network, for hashing, or for storage on disk is called serialization. Most bitcoin libraries have built-in functions for transaction serialization and deserialization.

See if you can manually decode Alice’s transaction from the serialized hexadecimal form, finding some of the elements we saw previously. The section containing the two outputs is highlighted in Alice’s transaction, serialized and presented in hexadecimal notation to help you:

Example 1. Alice’s transaction, serialized and presented in hexadecimal notation

这个案例就是以二进制形式对交易数据进行分析。

0100000001186f9f998a5aa6f048e51dd8419a14d8a0f1a8a2836dd73 4d2804fe65fa35779000000008b483045022100884d142d86652a3f47 ba4746ec719bbfbd040a570b1deccbb6498c75c4ae24cb02204b9f039 ff08df09cbe9f6addac960298cad530a863ea8f53982c09db8f6e3813 01410484ecc0d46f1918b30928fa0e4ed99f16a0fb4fde0735e7ade84 16ab9fe423cc5412336376789d172787ec3457eee41c04f4938de5cc1 7b4a10fa336a8d752adfffffffff0260e31600000000001976a914ab68025513c3dbd2f7b92a94e0581f5d50f654e788acd0ef8000000000001976a9147f9b1a7fb68d60c536c2fd8aeaa53a8f3cc025a888ac 00000000

 

Here are some hints:

  • There are two outputs in the highlighted section, each serialized as shown in Transaction output serialization.

  • The value of 0.015 bitcoin is 1,500,000 satoshis. That’s 16 e3 60 in hexadecimal.

  • In the serialized transaction, the value 16 e3 60 is encoded in little-endian (least-significant-byte-first) byte order, so it looks like 60 e3 16.

  • The scriptPubKey length is 25 bytes, which is 19 in hexadecimal.

Transaction Inputs

Transaction inputs identify (by reference) which UTXO will be consumed and provide proof of ownership through an unlocking script.

To build a transaction, a wallet selects from the UTXO it controls, UTXO with enough value to make the requested payment. Sometimes one UTXO is enough, other times more than one is needed. For each UTXO that will be consumed to make this payment, the wallet creates one input pointing to the UTXO and unlocks it with an unlocking script.

交易的输入就是每一笔要花的钱的来源。钱包创建一个输入,指向一笔UTXO,用脚本解锁。具体而言,交易输入数据的第一部分是前一笔交易的哈希以及索引,第二部分就是解锁脚本,解锁以后才能花钱。解决脚本主要是数字签名与公钥,用于证明比特币的归属。

Let’s look at the components of an input in greater detail. The first part of an input is a pointer to an UTXO by reference to the transaction hash and an output index, which identifies the specific UTXO in that transaction. The second part is an unlocking script, which the wallet constructs in order to satisfy the spending conditions set in the UTXO. Most often, the unlocking script is a digital signature and public key proving ownership of the bitcoin. However, not all unlocking scripts contain signatures. The third part is a sequence number, which will be discussed later.

Consider our example in Transactions—Behind the Scenes. The transaction inputs are an array (list) called vin:

The transaction inputs in Alice’s transaction

“vin”: [

  {

    “txid”: “7957a35fe64f80d234d76d83a2a8f1a0d8149a41d81de548f0a65a8a999f6f18″,

    “vout”: 0,

    “scriptSig” : “3045022100884d142d86652a3f47ba4746ec719bbfbd040a570b1deccbb6498c75c4ae24cb02204b9f039ff08df09cbe9f6addac960298cad530a863ea8f53982c09db8f6e3813[ALL]0484ecc0d46f1918b30928fa0e4ed99f16a0fb4fde0735e7ade8416ab9fe423cc5412336376789d172787ec3457eee41c04f4938de5cc17b4a10fa336a8d752adf”,

    “sequence”: 4294967295

  }

]

As you can see, there is only one input in the list (because one UTXO contained sufficient value to make this payment). The input contains four elements:

 

  • A transaction ID, referencing the transaction that contains the UTXO being spent

  • An output index (vout), identifying which UTXO from that transaction is referenced (first one is zero)

  • A scriptSig, which satisfies the conditions placed on the UTXO, unlocking it for spending

  • A sequence number (to be discussed later)

In Alice’s transaction, the input points to the transaction ID:

7957a35fe64f80d234d76d83a2a8f1a0d8149a41d81de548f0a65a8a999f6f18

and output index 0 (i.e., the first UTXO created by that transaction). The unlocking script is constructed by Alice’s wallet by first retrieving the referenced UTXO, examining its locking script, and then using it to build the necessary unlocking script to satisfy it.

如果只有一个哈希值,你是无法从中知道关于交易的信息的,既没有数额,也没有脚本。于是要通过引用的UTXO查到下一笔交易的情况。

Looking just at the input you may have noticed that we don’t know anything about this UTXO, other than a reference to the transaction containing it. We don’t know its value (amount in satoshi), and we don’t know the locking script that sets the conditions for spending it. To find this information, we must retrieve the referenced UTXO by retrieving the underlying transaction. Notice that because the value of the input is not explicitly stated, we must also use the referenced UTXO in order to calculate the fees that will be paid in this transaction (see Transaction Fees).

It’s not just Alice’s wallet that needs to retrieve UTXO referenced in the inputs. Once this transaction is broadcast to the network, every validating node will also need to retrieve the UTXO referenced in the transaction inputs in order to validate the transaction.

Transactions on their own seem incomplete because they lack context. They reference UTXO in their inputs but without retrieving that UTXO we cannot know the value of the inputs or their locking conditions. When writing bitcoin software, anytime you decode a transaction with the intent of validating it or counting the fees or checking the unlocking script, your code will first have to retrieve the referenced UTXO from the blockchain in order to build the context implied but not present in the UTXO references of the inputs. For example, to calculate the amount paid in fees, you must know the sum of the values of inputs and outputs. But without retrieving the UTXO referenced in the inputs, you do not know their value. So a seemingly simple operation like counting fees in a single transaction in fact involves multiple steps and data from multiple transactions.

如果是实现比特币的软件,在解码交易信息进行验证、计算费用或者检查解锁脚本的时候,首先要从区块上找出引用的UTXO的信息,这样才能看到上下文的情况。这有点像我们在论文里引用参考文献,我们往往是列出一个文献的名字,如果需要了解更多的信息,需要自行到数据库中查找。

We can use the same sequence of commands with Bitcoin Core as we used when retrieving Alice’s transaction (getrawtransaction and decoderawtransaction). With that we can get the UTXO referenced in the preceding input and take a look:

Alice’s UTXO from the previous transaction, referenced in the input

“vout”: [

   {

     “value”: 0.10000000,

     “scriptPubKey”: “OP_DUPOP_HASH160 7f9b1a7fb68d60c536c2fd8aeaa53a8f3cc025a8 OP_EQUALVERIFYOP_CHECKSIG”

   }

 ]

We see that this UTXO has a value of 0.1 BTC and that it has a locking script (scriptPubKey) that contains “OP_DUP OP_HASH160…”.

一般在比特币的不同实现中,都存在类似专门的函数,帮你查找前一笔交易的情况。

Tip To fully understand Alice’s transaction we had to retrieve the previous transaction(s) referenced as inputs. A function that retrieves previous transactions and unspent transaction outputs is very common and exists in almost every bitcoin library and API.

Transaction serialization—inputs

When transactions are serialized for transmission on the network, their inputs are encoded into a byte stream as shown in Transaction input serialization.

Table 2. Transaction input serialization

 

As with the outputs, let’s see if we can find the inputs from Alice’s transaction in the serialized format. First, the inputs decoded:

“vin”: [

  {

    “txid”: “7957a35fe64f80d234d76d83a2a8f1a0d8149a41d81de548f0a65a8a999f6f18″,

    “vout”: 0,

    “scriptSig” : “3045022100884d142d86652a3f47ba4746ec719bbfbd040a570b1deccbb6498c75c4ae24cb02204b9f039ff08df09cbe9f6addac960298cad530a863ea8f53982c09db8f6e3813[ALL]0484ecc0d46f1918b30928fa0e4ed99f16a0fb4fde0735e7ade8416ab9fe423cc5412336376789d172787ec3457eee41c04f4938de5cc17b4a10fa336a8d752adf”,

    “sequence”: 4294967295

  }

],

Now, let’s see if we can identify these fields in the serialized hex encoding in Alice’s transaction, serialized and presented in hexadecimal notation:

Example 2. Alice’s transaction, serialized and presented in hexadecimal notation

0100000001186f9f998a5aa6f048e51dd8419a14d8a0f1a8a2836dd734d2804fe65fa35779000000008b483045022100884d142d86652a3f47ba4746ec719bbfbd040a570b1deccbb6498c75c4ae24cb02204b9f039ff08df09cbe9f6addac960298cad530a863ea8f53982c09db8f6e381301410484ecc0d46f1918b30928fa0e4ed99f16a0fb4fde0735e7ade8416ab9fe423cc5412336376789d172787ec3457eee41c04f4938de5cc17b4a10fa336a8d752adfffffffff0260e31600000000001976a914ab6 8025513c3dbd2f7b92a94e0581f5d50f654e788acd0ef800000000000 1976a9147f9b1a7fb68d60c536c2fd8aeaa53a8f3cc025a888ac00000 000

Hints:

  • The transaction ID is serialized in reversed byte order, so it starts with (hex) 18 and ends with 79

  • The output index is a 4-byte group of zeros, easy to identify

  • The length of the scriptSig is 139 bytes, or 8b in hex

  • The sequence number is set to FFFFFFFF, again easy to identify

Transaction Fees

Most transactions include transaction fees, which compensate the bitcoin miners for securing the network. Fees also serve as a security mechanism themselves, by making it economically infeasible for attackers to flood the network with transactions. Mining and the fees and rewards collected by miners are discussed in more detail in [mining].

比特币的交易引用是一种安全机制,对每笔交易收费,可以防止攻击者采用泛洪的方式攻击网络。这就像寄快递要收钱,你就不会没事胡乱寄快递出去。

This section examines how transaction fees are included in a typical transaction. Most wallets calculate and include transaction fees automatically. However, if you are constructing transactions programmatically, or using a command-line interface, you must manually account for and include these fees.

Transaction fees serve as an incentive to include (mine) a transaction into the next block and also as a disincentive against abuse of the system by imposing a small cost on every transaction. Transaction fees are collected by the miner who mines the block that records the transaction on the blockchain.

交易费的计算是根据交易数据的大小(以KB来衡量)而确定,不是根据转账金额的数目来决定。交易费用不是强制的,但是如果没有交易费用,矿工最终也可能处理,但是如果有交易费用,会优先处理。但是2016年以后,不带交易费用的、交易费用少的可能就不会被矿工处理,甚至都无法在网络上传播了。

Transaction fees are calculated based on the size of the transaction in kilobytes, not the value of the transaction in bitcoin. Overall, transaction fees are set based on market forces within the bitcoin network. Miners prioritize transactions based on many different criteria, including fees, and might even process transactions for free under certain circumstances. Transaction fees affect the processing priority, meaning that a transaction with sufficient fees is likely to be included in the next block mined, whereas a transaction with insufficient or no fees might be delayed, processed on a best-effort basis after a few blocks, or not processed at all. Transaction fees are not mandatory, and transactions without fees might be processed eventually; however, including transaction fees encourages priority processing.

Over time, the way transaction fees are calculated and the effect they have on transaction prioritization has evolved. At first, transaction fees were fixed and constant across the network. Gradually, the fee structure relaxed and may be influenced by market forces, based on network capacity and transaction volume. Since at least the beginning of 2016, capacity limits in bitcoin have created competition between transactions, resulting in higher fees and effectively making free transactions a thing of the past. Zero fee or very low fee transactions rarely get mined and sometimes will not even be propagated across the network.

In Bitcoin Core, fee relay policies are set by the minrelaytxfee option. The current default minrelaytxfee is 0.00001 bitcoin or a hundredth of a millibitcoin per kilobyte. Therefore, by default, transactions with a fee less than 0.00001 bitcoin are treated as free and are only relayed if there is space in the mempool; otherwise, they are dropped. Bitcoin nodes can override the default fee relay policy by adjusting the value of minrelaytxfee.

Any bitcoin service that creates transactions, including wallets, exchanges, retail applications, etc., must implement dynamic fees. Dynamic fees can be implemented through a third-party fee estimation service or with a built-in fee estimation algorithm. If you’re unsure, begin with a third-party service and as you gain experience design and implement your own algorithm if you wish to remove the third-party dependency.

Fee estimation algorithms calculate the appropriate fee, based on capacity and the fees offered by “competing” transactions. These algorithms range from simplistic (average or median fee in the last block) to sophisticated (statistical analysis). They estimate the necessary fee (in satoshis per byte) that will give a transaction a high probability of being selected and included within a certain number of blocks. Most services offer users the option of choosing high, medium, or low priority fees. High priority means users pay higher fees but the transaction is likely to be included in the next block. Medium and low priority means users pay lower transaction fees but the transactions may take much longer to confirm.

有许多服务给用户高、中、低手续费的选项。高优先代表用户多付交易费,有可以会立即打包到下一区块中;中低优先级可能需要更长的确认时间。如果钱包采用固定的静态交易费用,用户体验不会太好,因为交易经常会卡住一直得不到确认。

Many wallet applications use third-party services for fee calculations. One popular service is http://bitcoinfees.21.co, which provides an API and a visual chart showing the fee in satoshi/byte for different priorities.

TipStatic fees are no longer viable on the bitcoin network. Wallets that set static fees will produce a poor user experience as transactions will often get “stuck” and remain unconfirmed. Users who don’t understand bitcoin transactions and fees are dismayed by “stuck” transactions because they think they’ve lost their money.

The chart in Fee estimation service bitcoinfees.21.co shows the real-time estimate of fees in 10 satoshi/byte increments and the expected confirmation time (in minutes and number of blocks) for transactions with fees in each range. For each fee range (e.g., 61–70 satoshi/byte), two horizontal bars show the number of unconfirmed transactions (1405) and total number of transactions in the past 24 hours (102,975), with fees in that range. Based on the graph, the recommended high-priority fee at this time was 80 satoshi/byte, a fee likely to result in the transaction being mined in the very next block (zero block delay). For perspective, the median transaction size is 226 bytes, so the recommended fee for a transaction size would be 18,080 satoshis (0.00018080 BTC).

根据下图中的信息,推荐的高优先交易费是每字节80聪。网络上也有专门用于估测交易引用的API。

The fee estimation data can be retrieved via a simple HTTP REST API, at https://bitcoinfees.21.co/api/v1/fees/recommended. For example, on the command line using the curl command:

Using the fee estimation API

$ curl https://bitcoinfees.21.co/api/v1/fees/recommended

{“fastestFee”:80,”halfHourFee”:80,”hourFee”:60}

The API returns a JSON object with the current fee estimate for fastest confirmation (fastestFee), confirmation within three blocks (halfHourFee) and six blocks (hourFee), in satoshi per byte.

Figure 2. Fee estimation service bitcoinfees.21.co

Adding Fees to Transactions

The data structure of transactions does not have a field for fees. Instead, fees are implied as the difference between the sum of inputs and the sum of outputs. Any excess amount that remains after all outputs have been deducted from all inputs is the fee that is collected by the miners:

交易费用在比特币中没有专门的显示字段,计算方式就是把输入的值求和,减去输出的值。这就意味着你必须算清楚你的账户下所有的输入值,否则如果你会花一大笔手续费用给矿工。另外一个情况是,如果你手动创建一笔交易,却忘记了设置零钱的输出,那你相当于把零钱全部给了矿工,“不用找了!”

Transaction fees are implied, as the excess of inputs minus outputs:

Fees = Sum(Inputs) – Sum(Outputs)

This is a somewhat confusing element of transactions and an important point to understand, because if you are constructing your own transactions you must ensure you do not inadvertently include a very large fee by underspending the inputs. That means that you must account for all inputs, if necessary by creating change, or you will end up giving the miners a very big tip!

For example, if you consume a 20-bitcoin UTXO to make a 1-bitcoin payment, you must include a 19-bitcoin change output back to your wallet. Otherwise, the 19-bitcoin “leftover” will be counted as a transaction fee and will be collected by the miner who mines your transaction in a block. Although you will receive priority processing and make a miner very happy, this is probably not what you intended.

Warning If you forget to add a change output in a manually constructed transaction, you will be paying the change as a transaction fee. “Keep the change!” might not be what you intended.

Let’s see how this works in practice, by looking at Alice’s coffee purchase again. Alice wants to spend 0.015 bitcoin to pay for coffee. To ensure this transaction is processed promptly, she will want to include a transaction fee, say 0.001. That will mean that the total cost of the transaction will be 0.016. Her wallet must therefore source a set of UTXO that adds up to 0.016 bitcoin or more and, if necessary, create change. Let’s say her wallet has a 0.2-bitcoin UTXO available. It will therefore need to consume this UTXO, create one output to Bob’s Cafe for 0.015, and a second output with 0.184 bitcoin in change back to her own wallet, leaving 0.001 bitcoin unallocated, as an implicit fee for the transaction.

Now let’s look at a different scenario. Eugenia, our children’s charity director in the Philippines, has completed a fundraiser to purchase schoolbooks for the children. She received several thousand small donations from people all around the world, totaling 50 bitcoin, so her wallet is full of very small payments (UTXO). Now she wants to purchase hundreds of schoolbooks from a local publisher, paying in bitcoin.

As Eugenia’s wallet application tries to construct a single larger payment transaction, it must source from the available UTXO set, which is composed of many smaller amounts. That means that the resulting transaction will source from more than a hundred small-value UTXO as inputs and only one output, paying the book publisher. A transaction with that many inputs will be larger than one kilobyte, perhaps several kilobytes in size. As a result, it will require a much higher fee than the median-sized transaction.

最后强调一遍,有许多钱包会超付交易费用,以获得处理优先级。交易费与交易的额度无关。

Eugenia’s wallet application will calculate the appropriate fee by measuring the size of the transaction and multiplying that by the per-kilobyte fee. Many wallets will overpay fees for larger transactions to ensure the transaction is processed promptly. The higher fee is not because Eugenia is spending more money, but because her transaction is more complex and larger in size—the fee is independent of the transaction’s bitcoin value.

Transaction Scripts and Script Language

The bitcoin transaction script language, called Script, is a Forth-like reverse-polish notation stack-based execution language. If that sounds like gibberish, you probably haven’t studied 1960s programming languages, but that’s ok—we will explain it all in this chapter. Both the locking script placed on an UTXO and the unlocking script are written in this scripting language. When a transaction is validated, the unlocking script in each input is executed alongside the corresponding locking script to see if it satisfies the spending condition.

比特币采用了一种“前向逆波兰堆栈式执行语言”,其实可以理解为一种脚本语言,并且用这个脚本语言对UTXO加锁或者解锁。

Script is a very simple language that was designed to be limited in scope and executable on a range of hardware, perhaps as simple as an embedded device. It requires minimal processing and cannot do many of the fancy things modern programming languages can do. For its use in validating programmable money, this is a deliberate security feature.

Today, most transactions processed through the bitcoin network have the form “Payment to Bob’s bitcoin address” and are based on a script called a Pay-to-Public-Key-Hash script. However, bitcoin transactions are not limited to the “Payment to Bob’s bitcoin address” script. In fact, locking scripts can be written to express a vast variety of complex conditions. In order to understand these more complex scripts, we must first understand the basics of transaction scripts and script language.

目前大部分比特币交易采用的都是“会给公钥哈希”脚本。比特币的交易验证不是基于静态的特征(例如脚本长成啥样),而是通过执行脚本语言来判断的。

In this section, we will demonstrate the basic components of the bitcoin transaction scripting language and show how it can be used to express simple conditions for spending and how those conditions can be satisfied by unlocking scripts.

Tip Bitcoin transaction validation is not based on a static pattern, but instead is achieved through the execution of a scripting language. This language allows for a nearly infinite variety of conditions to be expressed. This is how bitcoin gets the power of “programmable money.”

Turing Incompleteness

这样的脚本语言,没有循环与复杂的流程控制能力(只有条件判断),因此被认为是图灵非完备语言。而这个不完备的属性可以防止用来生成“死循环”逻辑炸弹,这样会引发拒绝服务攻击。语言的功能削弱,语言的缺陷也会减少。

The bitcoin transaction script language contains many operators, but is deliberately limited in one important way—there are no loops or complex flow control capabilities other than conditional flow control. This ensures that the language is not Turing Complete, meaning that scripts have limited complexity and predictable execution times. Script is not a general-purpose language. These limitations ensure that the language cannot be used to create an infinite loop or other form of “logic bomb” that could be embedded in a transaction in a way that causes a denial-of-service attack against the bitcoin network. Remember, every transaction is validated by every full node on the bitcoin network. A limited language prevents the transaction validation mechanism from being used as a vulnerability.

 

Stateless Verification

The bitcoin transaction script language is stateless, in that there is no state prior to execution of the script, or state saved after execution of the script. Therefore, all the information needed to execute a script is contained within the script. A script will predictably execute the same way on any system. If your system verifies a script, you can be sure that every other system in the bitcoin network will also verify the script, meaning that a valid transaction is valid for everyone and everyone knows this. This predictability of outcomes is an essential benefit of the bitcoin system.

比特币脚本语言是无状态的,执行的时候不需要保存前置的状态信息。这样,不管是谁进行验证,得到的结果都是一致的。

Script Construction (Lock + Unlock)

Bitcoin’s transaction validation engine relies on two types of scripts to validate transactions: a locking script and an unlocking script.

 

比特币有两种脚本验证交易:锁定脚本与解锁脚本。锁定脚本是放在输出上,规定了什么条件满足以后,这笔钱才能花。

A locking script is a spending condition placed on an output: it specifies the conditions that must be met to spend the output in the future. Historically, the locking script was called a scriptPubKey, because it usually contained a public key or bitcoin address (public key hash). In this book we refer to it as a “locking script” to acknowledge the much broader range of possibilities of this scripting technology. In most bitcoin applications, what we refer to as a locking script will appear in the source code as scriptPubKey. You will also see the locking script referred to as a witness script (see [segwit]) or more generally as a cryptographic puzzle. These terms all mean the same thing, at different levels of abstraction.

解锁脚本是交易输入的一部分。大部分情况来自用户钱包里私钥产生的数字签名。但是未必所有的解锁脚本都需要包括签名。

An unlocking script is a script that “solves,” or satisfies, the conditions placed on an output by a locking script and allows the output to be spent. Unlocking scripts are part of every transaction input. Most of the time they contain a digital signature produced by the user’s wallet from his or her private key. Historically, the unlocking script was called scriptSig, because it usually contained a digital signature. In most bitcoin applications, the source code refers to the unlocking script as scriptSig. You will also see the unlocking script referred to as a witness (see [segwit]). In this book, we refer to it as an “unlocking script” to acknowledge the much broader range of locking script requirements, because not all unlocking scripts must contain signatures.

在验证中,先复制解锁脚本,取出输入引用的UTXO,找到对应的锁定脚本,然后先执行解锁脚本再执行加锁脚本。只有有效成功解锁的UTXO才能被花掉,并且从交易输出里移出。

Every bitcoin validating node will validate transactions by executing the locking and unlocking scripts together. Each input contains an unlocking script and refers to a previously existing UTXO. The validation software will copy the unlocking script, retrieve the UTXO referenced by the input, and copy the locking script from that UTXO. The unlocking and locking script are then executed in sequence. The input is valid if the unlocking script satisfies the locking script conditions (see Separate execution of unlocking and locking scripts). All the inputs are validated independently, as part of the overall validation of the transaction.

Note that the UTXO is permanently recorded in the blockchain, and therefore is invariable and is unaffected by failed attempts to spend it by reference in a new transaction. Only a valid transaction that correctly satisfies the conditions of the output results in the output being considered as “spent” and removed from the set of unspent transaction outputs (UTXO set).

Combining scriptSig and scriptPubKey to evaluate a transaction script is an example of the unlocking and locking scripts for the most common type of bitcoin transaction (a payment to a public key hash), showing the combined script resulting from the concatenation of the unlocking and locking scripts prior to script validation.

Figure 3. Combining scriptSig and scriptPubKey to evaluate a transaction script

The script execution stack

脚本的执行采用了栈的结构,栈的特点就是后进先出。例如,假如有一个加的操作,先把两个数入栈,然后弹出求和后,把结果再入栈。

Bitcoin’s scripting language is called a stack-based language because it uses a data structure called a stack. A stack is a very simple data structure that can be visualized as a stack of cards. A stack allows two operations: push and pop. Push adds an item on top of the stack. Pop removes the top item from the stack. Operations on a stack can only act on the topmost item on the stack. A stack data structure is also called a Last-In-First-Out, or “LIFO” queue.

The scripting language executes the script by processing each item from left to right. Numbers (data constants) are pushed onto the stack. Operators push or pop one or more parameters from the stack, act on them, and might push a result onto the stack. For example, OP_ADD will pop two items from the stack, add them, and push the resulting sum onto the stack.

Conditional operators evaluate a condition, producing a boolean result of TRUE or FALSE. For example, OP_EQUAL pops two items from the stack and pushes TRUE (TRUE is represented by the number 1) if they are equal or FALSE (represented by zero) if they are not equal. Bitcoin transaction scripts usually contain a conditional operator, so that they can produce the TRUE result that signifies a valid transaction.

A simple script

Now let’s apply what we’ve learned about scripts and stacks to some simple examples.

In Bitcoin’s script validation doing simple math, the script 2 3 OP_ADD 5 OP_EQUAL demonstrates the arithmetic addition operator OP_ADD, adding two numbers and putting the result on the stack, followed by the conditional operator OP_EQUAL, which checks that the resulting sum is equal to 5. For brevity, the OP_ prefix is omitted in the step-by-step example. For more details on the available script operators and functions, see [tx_script_ops].

在比特币的脚本中,解锁与加锁脚本执行最后的结果如果为“真”,那验证就通过了。验证的过程其实就是把加锁脚本与解锁脚本组合在一起运算。

Although most locking scripts refer to a public key hash (essentially, a bitcoin address), thereby requiring proof of ownership to spend the funds, the script does not have to be that complex. Any combination of locking and unlocking scripts that results in a TRUE value is valid. The simple arithmetic we used as an example of the scripting language is also a valid locking script that can be used to lock a transaction output.

Use part of the arithmetic example script as the locking script:

3 OP_ADD 5 OP_EQUAL

which can be satisfied by a transaction containing an input with the unlocking script:

2

The validation software combines the locking and unlocking scripts and the resulting script is:

2 3 OP_ADD 5 OP_EQUAL

As we saw in the step-by-step example in Bitcoin’s script validation doing simple math, when this script is executed, the result is OP_TRUE, making the transaction valid. Not only is this a valid transaction output locking script, but the resulting UTXO could be spent by anyone with the arithmetic skills to know that the number 2 satisfies the script.

Tip Transactions are valid if the top result on the stack is TRUE (noted as {0x01}), any other nonzero value, or if the stack is empty after script execution. Transactions are invalid if the top value on the stack is FALSE (a zero-length empty value, noted as {}) or if script execution is halted explicitly by an operator, such as OP_VERIFY, OP_RETURN, or a conditional terminator such as OP_ENDIF. See [tx_script_ops] for details.

Figure 4. Bitcoin’s script validation doing simple math

The following is a slightly more complex script, which calculates 2 + 7 — 3 + 1. Notice that when the script contains several operators in a row, the stack allows the results of one operator to be acted upon by the next operator:

2 7 OP_ADD 3 OP_SUB 1 OP_ADD 7 OP_EQUAL

Try validating the preceding script yourself using pencil and paper. When the script execution ends, you should be left with the value TRUE on the stack.

 

Separate execution of unlocking and locking scripts

在早期的比特币客户端,解锁与加锁脚本是连接在一起执行的,但是这样可以作弊,存在安全风险。之后就把加锁与解锁的脚本通过栈分开执行。先执行解锁脚本,如果执行正常,复制栈,再执行加锁脚本。

In the original bitcoin client, the unlocking and locking scripts were concatenated and executed in sequence. For security reasons, this was changed in 2010, because of a vulnerability that allowed a malformed unlocking script to push data onto the stack and corrupt the locking script. In the current implementation, the scripts are executed separately with the stack transferred between the two executions, as described next.

First, the unlocking script is executed, using the stack execution engine. If the unlocking script is executed without errors (e.g., it has no “dangling” operators left over), the main stack is copied and the locking script is executed. If the result of executing the locking script with the stack data copied from the unlocking script is “TRUE,” the unlocking script has succeeded in resolving the conditions imposed by the locking script and, therefore, the input is a valid authorization to spend the UTXO. If any result other than “TRUE” remains after execution of the combined script, the input is invalid because it has failed to satisfy the spending conditions placed on the UTXO.

Pay-to-Public-Key-Hash (P2PKH)

The vast majority of transactions processed on the bitcoin network spend outputs locked with a Pay-to-Public-Key-Hash or “P2PKH” script. These outputs contain a locking script that locks the output to a public key hash, more commonly known as a bitcoin address. An output locked by a P2PKH script can be unlocked (spent) by presenting a public key and a digital signature created by the corresponding private key (see Digital Signatures (ECDSA)).

采用P2PKH的方式,锁定脚本把输出锁定到公钥哈希值上,即比特币地址上;通过公钥以及私钥签名可以实现解锁。

 

For example, let’s look at Alice’s payment to Bob’s Cafe again. Alice made a payment of 0.015 bitcoin to the cafe’s bitcoin address. That transaction output would have a locking script of the form:

OP_DUP OP_HASH160 <Cafe Public Key Hash> OP_EQUALVERIFY OP_CHECKSIG

The Cafe Public Key Hash is equivalent to the bitcoin address of the cafe, without the Base58Check encoding. Most applications would show the public key hash in hexadecimal encoding and not the familiar bitcoin address Base58Check format that begins with a “1.”

The preceding locking script can be satisfied with an unlocking script of the form:

解锁的形式如下:

<Cafe Signature> <Cafe Public Key>

The two scripts together would form the following combined validation script:

<Cafe Signature> <Cafe Public Key> OP_DUP OP_HASH160

<Cafe Public Key Hash> OP_EQUALVERIFY OP_CHECKSIG

When executed, this combined script will evaluate to TRUE if, and only if, the unlocking script matches the conditions set by the locking script. In other words, the result will be TRUE if the unlocking script has a valid signature from the cafe’s private key that corresponds to the public key hash set as an encumbrance.

如果解锁脚本的签名有效,签名与公钥哈希对应,验证即可通过。

Figures #P2PubKHash1 and #P2PubKHash2 show (in two parts) a step-by-step execution of the combined script, which will prove this is a valid transaction.

Figure 5. Evaluating a script for a P2PKH transaction (part 1 of 2)

Figure 6. Evaluating a script for a P2PKH transaction (part 2 of 2)

Digital Signatures (ECDSA)

So far, we have not delved into any detail about “digital signatures.” In this section we look at how digital signatures work and how they can present proof of ownership of a private key without revealing that private key.

The digital signature algorithm used in bitcoin is the Elliptic Curve Digital Signature Algorithm, or ECDSA. ECDSA is the algorithm used for digital signatures based on elliptic curve private/public key pairs, as described in [elliptic_curve]. ECDSA is used by the script functions OP_CHECKSIG, OP_CHECKSIGVERIFY, OP_CHECKMULTISIG, and OP_CHECKMULTISIGVERIFY. Any time you see those in a locking script, the unlocking script must contain an ECDSA signature.

ECDSA是比特币用于验证的签名机制,这是构建在椭圆曲线上的。

A digital signature serves three purposes in bitcoin (see the following sidebar). First, the signature proves that the owner of the private key, who is by implication the owner of the funds, has authorized the spending of those funds. Secondly, the proof of authorization is undeniable (nonrepudiation). Thirdly, the signature proves that the transaction (or specific parts of the transaction) have not and cannot be modified by anyone after it has been signed.

签名作用有三:一是证明私钥所有;二是证明认证不可抵赖;三是证明交易无篡改。

Note that each transaction input is signed independently. This is critical, as neither the signatures nor the inputs have to belong to or be applied by the same “owners.” In fact, a specific transaction scheme called “CoinJoin” uses this fact to create multi-party transactions for privacy.

Note Each transaction input and any signature it may contain is completely independent of any other input or signature. Multiple parties can collaborate to construct transactions and sign only one input each.

Wikipedia’s Definition of a “Digital Signature”

A digital signature is a mathematical scheme for demonstrating the authenticity of a digital message or documents. A valid digital signature gives a recipient reason to believe that the message was created by a known sender (authentication), that the sender cannot deny having sent the message (nonrepudiation), and that the message was not altered in transit (integrity).

Source: https://en.wikipedia.org/wiki/Digital_signature

How Digital Signatures Work

A digital signature is a mathematical scheme that consists of two parts. The first part is an algorithm for creating a signature, using a private key (the signing key), from a message (the transaction). The second part is an algorithm that allows anyone to verify the signature, given also the message and a public key.

Creating a digital signature

In bitcoin’s implementation of the ECDSA algorithm, the “message” being signed is the transaction, or more accurately a hash of a specific subset of the data in the transaction (see Signature Hash Types (SIGHASH)). The signing key is the user’s private key. The result is the signature:

比特币采用ECDSA进行签名,未必是对完整的交易数据进行签名,也可以仅仅签名一部分。

\(\(Sig = F_{sig}(F_{hash}(m), dA)\)\)

  • dA is the signing private key
  • m is the transaction (or parts of it)
  • Fhash is the hashing function
  • Fsig is the signing algorithm
  • Sig is the resulting signature
More details on the mathematics of ECDSA can be found in ECDSA Math.

The function Fsig produces a signature Sig that is composed of two values, commonly referred to as R and S:

Sig = (R, S)

Now that the two values R and S have been calculated, they are serialized into a byte-stream using an international standard encoding scheme called the Distinguished Encoding Rules, or DER.

 

Serialization of signatures (DER)

Let’s look at the transaction Alice created again. In the transaction input there is an unlocking script that contains the following DER-encoded signature from Alice’s wallet:

3045022100884d142d86652a3f47ba4746ec719bbfbd040a570b1deccbb6498c75c4ae24cb02204b9f039ff08df09cbe9f6addac960298cad530a863ea8f53982c09db8f6e381301

That signature is a serialized byte-stream of the R and S values produced by Alice’s wallet to prove she owns the private key authorized to spend that output. The serialization format consists of nine elements as follows:

  • 0x30—indicating the start of a DER sequence
  • 0x45—the length of the sequence (69 bytes)
  • 0x02—an integer value follows
  • 0x21—the length of the integer (33 bytes)
  • R—00884d142d86652a3f47ba4746ec719bbfbd040a570b1deccbb6498c75c4ae24cb
  • 0x02—another integer follows
  • 0x20—the length of the integer (32 bytes)
  • S—4b9f039ff08df09cbe9f6addac960298cad530a863ea8f53982c09db8f6e3813
  • A suffix (0x01) indicating the type of hash used (SIGHASH_ALL)

See if you can decode Alice’s serialized (DER-encoded) signature using this list. The important numbers are R and S; the rest of the data is part of the DER encoding scheme.

签名由R, S两部分组成,其他数据部分采用DER编码的格式。

Verifying the Signature

To verify the signature, one must have the signature (R and S), the serialized transaction, and the public key (that corresponds to the private key used to create the signature). Essentially, verification of a signature means “Only the owner of the private key that generated this public key could have produced this signature on this transaction.”

而要验证签名,需要知道R、S、序列化的交易以及公钥。

The signature verification algorithm takes the message (a hash of the transaction or parts of it), the signer’s public key and the signature (R and S values), and returns TRUE if the signature is valid for this message and public key.

Signature Hash Types (SIGHASH)

Digital signatures are applied to messages, which in the case of bitcoin, are the transactions themselves. The signature implies a commitment by the signer to specific transaction data. In the simplest form, the signature applies to the entire transaction, thereby committing all the inputs, outputs, and other transaction fields. However, a signature can commit to only a subset of the data in a transaction, which is useful for a number of scenarios as we will see in this section.

签名其实就是对交易数据的承诺,当然签名可以只对数据的一部分进行签名,SIGHASH这个标志位就是在标明到底对哪一部分进步签名。

Bitcoin signatures have a way of indicating which part of a transaction’s data is included in the hash signed by the private key using a SIGHASH flag. The SIGHASH flag is a single byte that is appended to the signature. Every signature has a SIGHASH flag and the flag can be different from input to input. A transaction with three signed inputs may have three signatures with different SIGHASH flags, each signature signing (committing) different parts of the transaction.

Remember, each input may contain a signature in its unlocking script. As a result, a transaction that contains several inputs may have signatures with different SIGHASH flags that commit different parts of the transaction in each of the inputs. Note also that bitcoin transactions may contain inputs from different “owners,” who may sign only one input in a partially constructed (and invalid) transaction, collaborating with others to gather all the necessary signatures to make a valid transaction. Many of the SIGHASH flag types only make sense if you think of multiple participants collaborating outside the bitcoin network and updating a partially signed transaction.

There are three SIGHASH flags: ALL, NONE, and SINGLE, as shown in SIGHASH types and their meanings.

Table 3. SIGHASH types and their meanings

 

In addition, there is a modifier flag SIGHASH_ANYONECANPAY, which can be combined with each of the preceding flags. When ANYONECANPAY is set, only one input is signed, leaving the rest (and their sequence numbers) open for modification. The ANYONECANPAY has the value 0x80 and is applied by bitwise OR, resulting in the combined flags as shown in SIGHASH types with modifiers and their meanings.

 

Table 4. SIGHASH types with modifiers and their meanings

The way SIGHASH flags are applied during signing and verification is that a copy of the transaction is made and certain fields within are truncated (set to zero length and emptied). The resulting transaction is serialized. The SIGHASH flag is added to the end of the serialized transaction and the result is hashed. The hash itself is the “message” that is signed. Depending on which SIGHASH flag is used, different parts of the transaction are truncated. The resulting hash depends on different subsets of the data in the transaction. By including the SIGHASH as the last step before hashing, the signature commits the SIGHASH type as well, so it can’t be changed (e.g., by a miner).

Note All SIGHASH types sign the transaction nLocktime field (see [transaction_locktime_nlocktime]). In addition, the SIGHASH type itself is appended to the transaction before it is signed, so that it can’t be modified once signed.

In the example of Alice’s transaction (see the list in Serialization of signatures (DER)), we saw that the last part of the DER-encoded signature was 01, which is the SIGHASH_ALL flag. This locks the transaction data, so Alice’s signature is committing the state of all inputs and outputs. This is the most common signature form.

Let’s look at some of the other SIGHASH types and how they can be used in practice:

ALL|ANYONECANPAY

This construction can be used to make a “crowdfunding”-style transaction. Someone attempting to raise funds can construct a transaction with a single output. The single output pays the “goal” amount to the fundraiser. Such a transaction is obviously not valid, as it has no inputs. However, others can now amend it by adding an input of their own, as a donation. They sign their own input with ALL|ANYONECANPAY. Unless enough inputs are gathered to reach the value of the output, the transaction is invalid. Each donation is a “pledge,” which cannot be collected by the fundraiser until the entire goal amount is raised.

对于仅对一个输入和所有输出签名的情况,适用于众筹的情况,因为收钱的地址通过签名固定住以后,付钱的人可以修改并增加地址。

NONE

This construction can be used to create a “bearer check” or “blank check” of a specific amount. It commits to the input, but allows the output locking script to be changed. Anyone can write their own bitcoin address into the output locking script and redeem the transaction. However, the output value itself is locked by the signature.

如果采用不签名的方式,就是一个不记名支票。

NONE|ANYONECANPAY

This construction can be used to build a “dust collector.” Users who have tiny UTXO in their wallets can’t spend these without the cost in fees exceeding the value of the dust. With this type of signature, the dust UTXO can be donated for anyone to aggregate and spend whenever they want.

There are some proposals to modify or expand the SIGHASH system. One such proposal is Bitmask Sighash Modes by Blockstream’s Glenn Willen, as part of the Elements project. This aims to create a flexible replacement for SIGHASH types that allows “arbitrary, miner-rewritable bitmasks of inputs and outputs” that can express “more complex contractual precommitment schemes, such as signed offers with change in a distributed asset exchange.”

NoteYou will not see SIGHASH flags presented as an option in a user’s wallet application. With few exceptions, wallets construct P2PKH scripts and sign with SIGHASH_ALL flags. To use a different SIGHASH flag, you would have to write software to construct and sign transactions. More importantly, SIGHASH flags can be used by special-purpose bitcoin applications that enable novel uses.

ECDSA Math

As mentioned previously, signatures are created by a mathematical function Fsig that produces a signature composed of two values R and S. In this section we look at the function Fsig in more detail.

The signature algorithm first generates an ephemeral (temporary) private public key pair. This temporary key pair is used in the calculation of the R and S values, after a transformation involving the signing private key and the transaction hash.

The temporary key pair is based on a random number k, which is used as the temporary private key. From k, we generate the corresponding temporary public key P (calculated as P = k*G, in the same way bitcoin public keys are derived; see [pubkey]). The value of the digital signature is then the x coordinate of the ephemeral public key P.

签名的结果R其实是临时公钥的x坐标值。

From there, the algorithm calculates the S value of the signature, such that:

S = k-1 (Hash(m) + dA * Rmodp

where:

  • is the ephemeral private key
  • R is the x coordinate of the ephemeral public key
  • dA is the signing private key
  • m is the transaction data
  • p is the prime order of the elliptic curve
Verification is the inverse of the signature generation function, using the R, S values and the public key to calculate a value P, which is a point on the elliptic curve (the ephemeral public key used in signature creation):

P = S-1 * Hash(m)* G + S-1 * R * Qa

where:

  • and S are the signature values
  • Qa is Alice’s public key
  • is the transaction data that was signed
  • G is the elliptic curve generator point

If the x coordinate of the calculated point P is equal to R, then the verifier can conclude that the signature is valid.

 

上述的签名过程具有一定的复杂性,如果基础不扎实可以跳过。但是如果要验证通过,计算出的点P的x坐标值,需要与R相等,验算才算通过。

Note that in verifying the signature, the private key is neither known nor revealed.

Tip ECDSA is necessarily a fairly complicated piece of math; a full explanation is beyond the scope of this book. A number of great guides online take you through it step by step: search for “ECDSA explained” or try this one: http://bit.ly/2r0HhGB.

The Importance of Randomness in Signatures

As we saw in ECDSA Math, the signature generation algorithm uses a random key k, as the basis for an ephemeral private/public key pair. The value of k is not important, as long as it is random. If the same value k is used to produce two signatures on different messages (transactions), then the signing private key can be calculated by anyone. Reuse of the same value for k in a signature algorithm leads to exposure of the private key!

Warning If the same value k is used in the signing algorithm on two different transactions, the private key can be calculated and exposed to the world!

在签名中有一个临时的随机密钥k,k可以任意取值,但是必须是随机的。如果k重复出现,那就会暴露私钥。在比特币中就出现过不正确使用k造成的失窃问题。从这个意义上采用确定性随机过程生成k,要比采用随机熵更好。

This is not just a theoretical possibility. We have seen this issue lead to exposure of private keys in a few different implementations of transaction-signing algorithms in bitcoin. People have had funds stolen because of inadvertent reuse of a k value. The most common reason for reuse of a k value is an improperly initialized random-number generator.

To avoid this vulnerability, the industry best practice is to not generate k with a random-number generator seeded with entropy, but instead to use a deterministic-random process seeded with the transaction data itself. This ensures that each transaction produces a different k. The industry-standard algorithm for deterministic initialization of k is defined in RFC 6979, published by the Internet Engineering Task Force.

If you are implementing an algorithm to sign transactions in bitcoin, you must use RFC 6979 or a similarly deterministic-random algorithm to ensure you generate a different k for each transaction.

Bitcoin Addresses, Balances, and Other Abstractions

We began this chapter with the discovery that transactions look very different “behind the scenes” than how they are presented in wallets, blockchain explorers, and other user-facing applications. Many of the simplistic and familiar concepts from the earlier chapters, such as bitcoin addresses and balances, seem to be absent from the transaction structure. We saw that transactions don’t contain bitcoin addresses, per se, but instead operate through scripts that lock and unlock discrete values of bitcoin. Balances are not present anywhere in this system and yet every wallet application prominently displays the balance of the user’s wallet.

从本章来看, 比特币交易中并不包括比特币地址,相反含有脚本用于对比特币进行加锁与解锁。

Now that we have explored what is actually included in a bitcoin transaction, we can examine how the higher-level abstractions are derived from the seemingly primitive components of the transaction.

Let’s look again at how Alice’s transaction was presented on a popular block explorer (Alice’s transaction to Bob’s Cafe).

Figure 7. Alice’s transaction to Bob’s Cafe

On the left side of the transaction, the blockchain explorer shows Alice’s bitcoin address as the “sender.” In fact, this information is not in the transaction itself. When the blockchain explorer retrieved the transaction it also retrieved the previous transaction referenced in the input and extracted the first output from that older transaction. Within that output is a locking script that locks the UTXO to Alice’s public key hash (a P2PKH script). The blockchain explorer extracted the public key hash and encoded it using Base58Check encoding to produce and display the bitcoin address that represents that public key.

区块链浏览器从交易中取出前一个交易的输入,同时把输出中的第一项取出。这样就可以把公钥从脚本里取出来。然后浏览器把公钥进行Base58Check编码,展示成比特币地址的形式。

Similarly, on the right side, the blockchain explorer shows the two outputs; the first to Bob’s bitcoin address and the second to Alice’s bitcoin address (as change). Once again, to create these bitcoin addresses, the blockchain explorer extracted the locking script from each output, recognized it as a P2PKH script, and extracted the public-key-hash from within. Finally, the blockchain explorer reencoded that public key hash with Base58Check to produce and display the bitcoin addresses.

同样地,对于收方而言,也是从公钥的哈希值中,把地址取出以后做转换然后显示出来。

If you were to click on Bob’s bitcoin address, the blockchain explorer would show you the view in The balance of Bob’s bitcoin address.

Figure 8. The balance of Bob’s bitcoin address

 

The blockchain explorer displays the balance of Bob’s bitcoin address. But nowhere in the bitcoin system is there a concept of a “balance.” Rather, the values displayed here are constructed by the blockchain explorer as follows.

To construct the “Total Received” amount, the blockchain explorer first will decode the Base58Check encoding of the bitcoin address to retrieve the 160-bit hash of Bob’s public key that is encoded within the address. Then, the blockchain explorer will search through the database of transactions, looking for outputs with P2PKH locking scripts that contain Bob’s public key hash. By summing up the value of all the outputs, the blockchain explorer can produce the total value received.

当要显示一个账户的余额的时候,先要把比特币地址解码成公钥哈希的形式,然后在交易中搜索脚本中包含公钥的内容,匹配上了以后,把对应交易的金额加起来就可以了。

Constructing the current balance (displayed as “Final Balance”) requires a bit more work. The blockchain explorer keeps a separate database of the outputs that are currently unspent, the UTXO set. To maintain this database, the blockchain explorer must monitor the bitcoin network, add newly created UTXO, and remove spent UTXO, in real time, as they appear in unconfirmed transactions. This is a complicated process that depends on keeping track of transactions as they propagate, as well as maintaining consensus with the bitcoin network to ensure that the correct chain is followed. Sometimes, the blockchain explorer goes out of sync and its perspective of the UTXO set is incomplete or incorrect.

维护UTXO的过程是一个复杂的过程,包括追踪传播的交易,与比特币网络维护一致,确保与主链同步更新等。

From the UTXO set, the blockchain explorer sums up the value of all unspent outputs referencing Bob’s public key hash and produces the “Final Balance” number shown to the user.

In order to produce this one image, with these two “balances,” the blockchain explorer has to index and search through dozens, hundreds, or even hundreds of thousands of transactions.

In summary, the information presented to users through wallet applications, blockchain explorers, and other bitcoin user interfaces is often composed of higher-level abstractions that are derived by searching many different transactions, inspecting their content, and manipulating the data contained within them. By presenting this simplistic view of bitcoin transactions that resemble bank checks from one sender to one recipient, these applications have to abstract a lot of underlying detail. They mostly focus on the common types of transactions: P2PKH with SIGHASH_ALL signatures on every input. Thus, while bitcoin applications can present more than 80% of all transactions in an easy-to-read manner, they are sometimes stumped by transactions that deviate from the norm. Transactions that contain more complex locking scripts, or different SIGHASH flags, or many inputs and outputs, demonstrate the simplicity and weakness of these abstractions.

总体而言,我们在区块链浏览器看到的交易情况,其实是来自于在交易中查找,对内容分析,操作数据内容等过程。如果交易包括更复杂的锁脚本,或者不同的签名标记,或者有过多的输入或者输出,那在显示的时候可能会出现异常。其实在比特币的交易中,的确存在比P2KH更多形式的脚本。

Every day, hundreds of transactions that do not contain P2PKH outputs are confirmed on the blockchain. The blockchain explorers often present these with red warning messages saying they cannot decode an address. The following link contains the most recent “strange transactions” that were not fully decoded: https://blockchain.info/strange-transactions.

As we will see in the next chapter, these are not necessarily strange transactions. They are transactions that contain more complex locking scripts than the common P2PKH. We will learn how to decode and understand more complex scripts and the applications they support next.

总结:本章主要分析了交易的细节,输入、输出以及签名的情况。其实利用脚本进行验证来确保钱正确合理地花出去,是比特币金钱管控的重要手段。

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