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[[ch06_intro]]
=== Introduction
((("transactions", "defined")))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.
((("transactions", "defined")))((("warnings and cautions", see="also security")))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.
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.
[[tx_structure]]
=== Transactions in Detail
((("use cases", "buying coffee")))((("transactions", "use cases", "buying coffee")))In <<ch02_bitcoin_overview>>, we looked at the transaction Alice used to pay for coffee at Bob's coffee shop, using a block explorer (<<alices_transactions_to_bobs_cafe>>):
((("use cases", "buying coffee")))In <<ch02_bitcoin_overview>>, we looked at the transaction Alice used to pay for coffee at Bob's coffee shop, using a block explorer (<<alices_transactions_to_bobs_cafe>>):
[[alices_transactions_to_bobs_cafe]]
.Alice's transaction to Bob's Cafe
@ -282,7 +282,7 @@ Hints:
[[tx_fees]]
==== Transaction Fees
((("transactions", "outputs and inputs", "transaction fees")))((("fees", "transaction fees")))((("mining and consensus", "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 <<bitcoin_network_ch08>>.
((("transactions", "outputs and inputs", "transaction fees")))((("fees", "transaction fees")))((("mining and consensus", "rewards and 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 <<bitcoin_network_ch08>>.
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.
@ -292,17 +292,17 @@ Transaction fees are calculated based on the size of the transaction in kilobyte
Over time, the way transaction fees are calculated and the effect they have on transaction prioritization has been evolving. At first, transaction fees were fixed and constant across the network. Gradually, the fee structure has been relaxed so that it 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.
((("fee relay policies")))((("minrelaytxfee option")))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 milli-bitcoin per kilobyte. Therefore by default, transactions with a fee less than 0.0001 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+.
((("fees", "fee relay policies")))((("minrelaytxfee option")))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 milli-bitcoin per kilobyte. Therefore by default, transactions with a fee less than 0.0001 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+.
((("dynamic fees")))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.
((("dynamic fees")))((("fees", "dynamic fees")))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.
((("bitcoinfees")))Many wallet applications use third-party services for fee calculations. One popular service is http://bitcoinfees.21.co/[http://bitcoinfees.21.co], which provides an API and a visual chart showing the fee in satoshi/byte for different priorities.
((("bitcoinfees (third-party service)")))Many wallet applications use third-party services for fee calculations. One popular service is http://bitcoinfees.21.co/[http://bitcoinfees.21.co], which provides an API and a visual chart showing the fee in satoshi/byte for different priorities.
[TIP]
====
((("static fees")))Static 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.
((("static fees")))((("fees", "static fees")))Static 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 <<bitcoinfees21co>> 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&#x2013;70 satoshi/byte), two horizontal bars show the number of unconfirmed transactions (1405) and total number of transactions in the past 24 hours (102975), 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).
@ -341,9 +341,9 @@ For example, if you consume a 20-bitcoin UTXO to make a 1-bitcoin payment, you m
((("warnings and cautions", "change outputs")))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.
====
((("use cases", "buying coffee")))((("transactions", "use cases", "buying coffee")))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.
((("use cases", "buying coffee")))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.
((("transactions", "use cases", "charitable donations")))((("use cases", "charitable donations")))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.
((("use cases", "charitable donations")))((("charitable donations")))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 a kilobyte or several kilobytes in size. As a result, it will require a much higher fee than the median-sized transaction.
@ -368,16 +368,16 @@ In this section, we will demonstrate the basic components of the bitcoin transac
==== Turing Incompleteness
((("transactions", "scripts and Script language", "Turing incompleteness")))((("Turing incompleteness")))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. ((("denial-of-service attacks")))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.
((("Turing incompleteness")))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. ((("denial-of-service attacks")))((("denial-of-service attacks", see="also security")))((("security", "denial-of-service attacks")))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
((("transactions", "scripts and Script language", "stateless verification")))((("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.
((("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.
[[tx_lock_unlock]]
==== Script Construction (Lock + Unlock)
((("transactions", "scripts and Script language", "lock and unlock script construction")))Bitcoin's transaction validation engine relies on two types of scripts to validate transactions: a locking script and an unlocking script.
Bitcoin's transaction validation engine relies on two types of scripts to validate transactions: a locking script and an unlocking script.
((("locking scripts")))((("unlocking scripts")))((("scripting", "locking scripts")))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. ((("scriptPubKey")))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+. ((("witnesses")))((("cryptographic puzzles")))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.
@ -458,7 +458,7 @@ First, the unlocking script is executed, using the stack execution engine. If th
[[p2pkh]]
==== Pay-to-Public-Key-Hash (P2PKH)
((("transactions", "scripts and Script language", "Pay-to-Public-Key-Hash")))((("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_sigs>>).
((("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_sigs>>).
((("use cases", "buying coffee")))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:
@ -616,7 +616,7 @@ In the example of Alice's transaction (see the list in <<seralization_of_signatu
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&#x201d;-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.
+ALL|ANYONECANPAY+ :: ((("charitable donations")))((("use cases", "charitable donations")))This construction can be used to make a "crowdfunding&#x201d;-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.
+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.