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Chapter 2 syntax, grammar, and flow suggestions.

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Eric Winchell 2014-04-18 16:24:42 -07:00
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@ -61,7 +61,7 @@ A description for the payement: "Purchase at Bob's Cafe"
Unlike a QR code that simply contains a destination bitcoin address, a "payment request" is a QR encoded URL that contains a destination address, a payment amount and a generic description such as "Bob's Cafe". This allows a bitcoin wallet application to pre-fill the information to send the payment while showing a human-readable description to the user. See <<payment request URL>>, for more details. You can scan the QR code above with a bitcoin wallet application to see what Alice would see.
====
Bob says "That's one-dollar-fifty, or fifteen milibits".
Bob says "That's one-dollar-fifty, or fifteen millibits".
Alice uses her smartphone to scan the barcode on display. Her smartphone shows a payment of +0.0150 BTC+ to +Bob's Cafe+ and she selects +Send+ to authorize the payment. Within a few seconds (about the same time as a credit card authorization), Bob would see the transaction on the register, completing the transaction.
@ -83,12 +83,12 @@ Transactions are like lines in a double-entry bookkeeping ledger. In simple term
.Transaction As Double-Entry Bookkeeping
image::images/Transaction_Double_Entry.png["Transaction Double-Entry"]
The transaction contains proof of ownership for each amount of bitcoin (inputs) whose value is transfered, in the form of a digital signature from the owner, that can be independently validated by anyone. In bitcoin terms, "spending" is signing the value of a previous transaction for which you have the keys, over to a new owner identified by a bitcoin address.
The transaction contains proof of ownership for each amount of bitcoin (inputs) whose value is transferred, in the form of a digital signature from the owner, that can be independently validated by anyone. In bitcoin terms, "spending" is signing the value of a previous transaction for which you have the keys over to a new owner identified by a bitcoin address.
[TIP]
====
_Transactions_ move value *from* _transaction inputs_ *to* _transaction outputs_. An input is where the coin value is coming from, usually a previous transaction's output. A transaction output assigns a new owner to the value by associating it with a key. The destination key is called an _encumberance_, it imposes a requirement for a signature for the funds to be redeemed in future transactions. Outputs from one transaction can be used as inputs in a new transaction, thus creating a chain of ownership as the value is moved from address to address.
_Transactions_ move value *from* _transaction inputs_ *to* _transaction outputs_. An input is where the coin value is coming from, usually a previous transaction's output. A transaction output assigns a new owner to the value by associating it with a key. The destination key is called an _encumberance_. It imposes a requirement for a signature for the funds to be redeemed in future transactions. Outputs from one transaction can be used as inputs in a new transaction, thus creating a chain of ownership as the value is moved from address to address.
====
@ -106,7 +106,7 @@ The most common form of transaction is a simple payment from one address to anot
.Most Common Transaction
image::images/Bitcoin_Transaction_Structure_Common.png["Common Transaction"]
Another common form of transaction is a transaction that aggregates several inputs into a single output. This represents the real-world equivalent of exchanging a pile of coins and currency notes for a single larger note. Transactions like these are sometimes generated by wallet applications to cleanup lots of smaller amounts that were received as change for payments.
Another common form of transaction is a transaction that aggregates several inputs into a single output. This represents the real-world equivalent of exchanging a pile of coins and currency notes for a single larger note. Transactions like these are sometimes generated by wallet applications to clean up lots of smaller amounts that were received as change for payments.
[[transaction-aggregating]]
.Transaction Aggregating Funds
@ -120,7 +120,7 @@ image::images/Bitcoin_Transaction_Structure_Distribution.png["Distributing Trans
=== Constructing A Transaction
Alice's wallet application contains all the logic for selecting appropriate inputs and outputs to build a transaction to Alice's specification. Alice only needs to specify a destination and an amount and the rest happens in the wallet application without her seeing the details. Importantly, a wallet application can construct transactions even if completely offline. Like writing a cheque at home and later sending it to the bank in an envelope, the transaction does not need to be constructed and signed while connected to the bitcoin network, it only has to be sent to the network eventually for it to be executed.
Alice's wallet application contains all the logic for selecting appropriate inputs and outputs to build a transaction to Alice's specification. Alice only needs to specify a destination and an amount and the rest happens in the wallet application without her seeing the details. Importantly, a wallet application can construct transactions even if completely offline. Like writing a cheque at home and later sending it to the bank in an envelope, the transaction does not need to be constructed and signed while connected to the bitcoin network. It only has to be sent to the network eventually for it to be executed.
==== Getting the right inputs
@ -128,7 +128,7 @@ Alice's wallet application will first have to find inputs that can pay for the a
If the wallet application does not maintain a copy of unspent transaction outputs, it can query the bitcoin network to retrieve this information, using a variety of APIs available by different providers, or by asking a full-index node using the bitcoin JSON RPC API. Below we see an example of a RESTful API request, constructed as a HTTP GET command to a specific URL. This URL will return all the unspent transaction outputs for an address, giving any application the information it needs to construct transaction inputs for spending. We use the simple command-line HTTP client _cURL_ to retrieve the response:
.Lookup all the unspent outputs for Alice's address 1Cdid9KFAaatwczBwBttQcwXYCpvK8h7FK
.Look up all the unspent outputs for Alice's address 1Cdid9KFAaatwczBwBttQcwXYCpvK8h7FK
----
$ curl https://blockchain.info/unspent?active=1Cdid9KFAaatwczBwBttQcwXYCpvK8h7FK
@ -150,23 +150,23 @@ $ curl https://blockchain.info/unspent?active=1Cdid9KFAaatwczBwBttQcwXYCpvK8h7FK
}
----
The response above shows that the bitcoin network knows of one unspent output (one that has not been redeemed yet) under the ownership of Alice's address _+1Cdid9KFAaatwczBwBttQcwXYCpvK8h7FK+_. The response includes the reference to the transaction in which this unspent output is contained (the payment from Joe) and it's value in Satoshis, at 10 million, equivalent to 0.10 bitcoin. With this information, Alice's wallet application can construct a transaction to transfer that value to new owner addresses.
The response above shows that the bitcoin network knows of one unspent output (one that has not been redeemed yet) under the ownership of Alice's address _+1Cdid9KFAaatwczBwBttQcwXYCpvK8h7FK+_. The response includes the reference to the transaction in which this unspent output is contained (the payment from Joe) and its value in Satoshis, at 10 million, equivalent to 0.10 bitcoin. With this information, Alice's wallet application can construct a transaction to transfer that value to new owner addresses.
[TIP]
====
Lookup the transaction from Joe to Alice, to see the information referenced above, as it is stored in the bitcoin blockchain. Using the blockchain explorer web application, follow the URL below:
Look up the transaction from Joe to Alice to see the information referenced above as it is stored in the bitcoin blockchain. Using the blockchain explorer web application, follow the URL below:
https://blockchain.info/tx/7957a35fe64f80d234d76d83a2a8f1a0d8149a41d81de548f0a65a8a999f6f18
====
As you can see, Alice's wallet contains enough bitcoins in a single unspent output to pay for the cup of coffee. Had this not been the case, Alice's wallet application might have to "rummage" through a pile of smaller unspent outputs, like picking coins from a purse, until it could find enough to pay for coffee. In both cases, there might be a need to get some change back, which we will see in the next section, as the wallet application creates the transaction outputs (payments).
As you can see, Alice's wallet contains enough bitcoins in a single unspent output to pay for the cup of coffee. Had this not been the case, Alice's wallet application might have to "rummage" through a pile of smaller unspent outputs, like picking coins from a purse until it could find enough to pay for coffee. In both cases, there might be a need to get some change back, which we will see in the next section, as the wallet application creates the transaction outputs (payments).
==== Creating the outputs
A transaction output is created in the form of a script, that creates an encumberance on the value and can only be redeemed by the introduction of a solution to the script. In simpler terms, Alice's transaction output will contain a script that says something like "This output is payable to whoever can present a signature from the key corresponding to Bob's public address". Since only Bob has the wallet with the keys corresponding to that address, only Bob's wallet can present such a signature to redeem this output. Alice will therefore "encumber" the output value with a demand for a signature from Bob.
A transaction output is created in the form of a script that creates an encumberance on the value and can only be redeemed by the introduction of a solution to the script. In simpler terms, Alice's transaction output will contain a script that says something like "This output is payable to whoever can present a signature from the key corresponding to Bob's public address". Since only Bob has the wallet with the keys corresponding to that address, only Bob's wallet can present such a signature to redeem this output. Alice will therefore "encumber" the output value with a demand for a signature from Bob.
This transaction will also include a second output, because Alice's funds are in a the form of a 0.10 BTC output, too much money for the 0.015 BTC cup of coffee. Alice will need 0.085 BTC in change. Alice's change payment is created _by Alice's wallet_ in the very same transaction as the payment to Bob. Essentially, Alice's wallet breaks her funds into two payments, one to Bob, one back to herself. She can then use the change output in a subsequent transaction, thus spending it later.
This transaction will also include a second output, because Alice's funds are in a the form of a 0.10 BTC output, too much money for the 0.015 BTC cup of coffee. Alice will need 0.085 BTC in change. Alice's change payment is created _by Alice's wallet_ in the very same transaction as the payment to Bob. Essentially, Alice's wallet breaks her funds into two payments: one to Bob, and one back to herself. She can then use the change output in a subsequent transaction, thus spending it later.
Finally, for the transaction to be processed by the network in a timely fashion, Alice's wallet application will add a small fee. This is not explicit in the transaction, it is implied by the difference between inputs and outputs. If instead of taking 0.085 in change, Alice creates only 0.0845 as the second output, there will be 0.0005 BTC (half a millibitcoin) left over. The input's 0.10 BTC is not fully spent with the two outputs, as they will add up to less than 0.10. The resulting difference is the _transaction fee_ which is collected by the miner as a fee for including the transaction in a block and putting it on the blockchain ledger.
@ -194,31 +194,31 @@ Since the transaction contains all the information necessary to process, it does
===== How it propagates
Alice's wallet application can send the new transaction to any of the other bitcoin clients it is connected to, over WiFi or mobile data, or any Internet connection. Her bitcoin wallet does not have to be connected to Bob's bitcoin wallet directly and she does not have to use the Internet connection offered by the cafe, though both those options are possible too. Any bitcoin network node (other client) that receives a valid transaction it has not seen before, will immediately forward it to other nodes it is connected to. Thus, the transaction rapidly propagates out across the peer-to-peer network, reaching a large percentage of the nodes within a few seconds.
Alice's wallet application can send the new transaction to any of the other bitcoin clients it is connected to over any Internet connection: wired, WiFi, or mobile. Her bitcoin wallet does not have to be connected to Bob's bitcoin wallet directly and she does not have to use the Internet connection offered by the cafe, though both those options are possible too. Any bitcoin network node (other client) that receives a valid transaction it has not seen before, will immediately forward it to other nodes it is connected to. Thus, the transaction rapidly propagates out across the peer-to-peer network, reaching a large percentage of the nodes within a few seconds.
===== Bob's view
If Bob's bitcoin wallet application is directly connected to Alice's wallet application, it may be the first node to receive the transaction. However, even if Alice's wallet sends it through other nodes, the transaction will reach Bob's wallet within a few seconds. Bob's wallet will immediately identify Alice's transaction as an incoming payment because it contains outputs redeemable by Bob's keys. Bob's wallet application can also independently verify that the transaction is well-formed, uses previously-unspent inputs and contains sufficient transaction fees to be included in the next block. At this point, Bob can assume, with little risk, that the transaction will shortly be included in a block and confirmed.
If Bob's bitcoin wallet application is directly connected to Alice's wallet application, it may be the first node to receive the transaction. However, even if Alice's wallet sends it through other nodes, the transaction will reach Bob's wallet within a few seconds. Bob's wallet will immediately identify Alice's transaction as an incoming payment because it contains outputs redeemable by Bob's keys. Bob's wallet application can also independently verify that the transaction is well-formed, uses previously-unspent inputs and contains sufficient transaction fees to be included in the next block. At this point Bob can assume, with little risk, that the transaction will shortly be included in a block and confirmed.
[TIP]
====
A common misconception about bitcoin transactions is that they must be "confirmed" by waiting 10 minutes for a new block, or up to sixty minutes for a full six confirmations. While confirmations ensure the transaction has been accepted by the whole network, for small value items like a cup of coffee, such a delay is unecessary. A merchant may accept a valid small-value transaction with no confirmations, with no more risk than a credit card payment made without ID or a signature, as many do today
A common misconception about bitcoin transactions is that they must be "confirmed" by waiting 10 minutes for a new block, or up to sixty minutes for a full six confirmations. While confirmations ensure the transaction has been accepted by the whole network, such a delay is unecessary for small value items like a cup of coffee. A merchant may accept a valid small-value transaction with no confirmations, with no more risk than a credit card payment made without ID or a signature, as many do today.
====
=== Bitcoin Mining
The transaction is now propagated on the bitcoin network. It does not become part of the shared ledger (the _blockchain_) until it is verified and included in a block, in a process called _mining_. See <<mining>> for a detailed explanation.
The transaction is now propagated on the bitcoin network. It does not become part of the shared ledger (the _blockchain_) until it is verified and included in a block by a process called _mining_. See <<mining>> for a detailed explanation.
The bitcoin system of trust is based on computation. Transactions are bundled into _blocks_ which require an enormous amount of computation to prove, but only a small amount of computation to verify as proven, in a process called _mining_. Mining serves two purposes in bitcoin:
The bitcoin system of trust is based on computation. Transactions are bundled into _blocks_ which require an enormous amount of computation to prove, but only a small amount of computation to verify as proven. Mining serves two purposes in bitcoin:
* Mining creates new bitcoins in each block, almost like a central bank printing new money. The amount of bitcoin created is fixed and diminishes with time
* Mining creates new bitcoins in each block, almost like a central bank printing new money. The amount of bitcoin created is fixed and diminishes with time.
* Mining creates trust by ensuring that transactions are only confirmed if enough computational power was devoted to the block that contains them. More blocks mean more computation which means more trust.
A good way to describe mining is like a giant competitive game of sudoku that resets every time someone finds a solution and whose difficulty automatically adjusts so that it takes approximately 10 minutes to find a solution. Imagine a giant sudoku puzzle, several thousand rows and columns in size. If I show you a completed puzzle you can verify it quite quickly. If it is empty, however, it takes a lot of work to solve! The difficulty of the sudoku can be adjusted by changing its size (more or fewer rows and columns), but it can still be verified quite easily even if it is very large. The "puzzle" used in bitcoin is based on a cryptographic hash and exhibits similar characteristics: it is assymetrically hard to solve, but easy to verify and its difficulty can be adjusted.
A good way to describe mining is like a giant competitive game of sudoku that resets every time someone finds a solution and whose difficulty automatically adjusts so that it takes approximately 10 minutes to find a solution. Imagine a giant sudoku puzzle, several thousand rows and columns in size. If I show you a completed puzzle you can verify it quite quickly. If it is empty, however, it takes a lot of work to solve! The difficulty of the sudoku can be adjusted by changing its size (more or fewer rows and columns), but it can still be verified quite easily even if it is very large. The "puzzle" used in bitcoin is based on a cryptographic hash and exhibits similar characteristics: it is asymetrically hard to solve, but easy to verify and its difficulty can be adjusted.
In <<user-stories>> we introduced Jing, a computer engineering student in Shanghai. Jing is participating in the bitcoin network as a miner. Every 10 minutes or so, Jing joins thousands of other miners in a global race to find a solution to a block of transactions. Finding such a solution, the so-called "Proof-of-Work" requires quadrillions of hashing operations per second, across the entire bitcoin network. The algorithm for "Proof-of-Work" involves repeatedly hashing the header of the block and a random number with the SHA256 cryptographic algorithm, until a solution matching a pre-determined pattern emerges. The first miner to find such a solution wins the round of competition and publishes that block into the blockchain.
In <<user-stories>> we introduced Jing, a computer engineering student in Shanghai. Jing is participating in the bitcoin network as a miner. Every 10 minutes or so, Jing joins thousands of other miners in a global race to find a solution to a block of transactions. Finding such a solution, the so-called "Proof-of-Work", requires quadrillions of hashing operations per second across the entire bitcoin network. The algorithm for "Proof-of-Work" involves repeatedly hashing the header of the block and a random number with the SHA256 cryptographic algorithm until a solution matching a pre-determined pattern emerges. The first miner to find such a solution wins the round of competition and publishes that block into the blockchain.
Jing started mining in 2010 using a very fast desktop computer to find a suitable Proof-of-Work for new blocks. As more miners started joining the bitcoin network, the difficulty of the problem increased rapidly. Soon, Jing and other miners upgraded to more specialized hardware, such as Graphical Processing Units (GPU), as used in gaming desktops or consoles. As this book is written, by 2014, the difficulty is so high that it is only profitable to mine with Application Specific Integrated Circuits, essentially hundreds of mining algorithms printed in hardware, running in parallel on a single silicone chip. Jing also joined a "mining pool", which much like a lottery-pool allows several participants to share their efforts and the rewards. Jing now runs two ASIC machines, which are USB connected devices, to mine for bitcoin 24 hours a day. He pays his electricity costs by selling the bitcoin he is able to generate from mining, creating some income from the profits. His computer runs a copy of bitcoind, the reference bitcoin client, as a back-end to his specialized mining software.
Jing started mining in 2010 using a very fast desktop computer to find a suitable Proof-of-Work for new blocks. As more miners started joining the bitcoin network, the difficulty of the problem increased rapidly. Soon, Jing and other miners upgraded to more specialized hardware, such as Graphical Processing Units (GPU), as used in gaming desktops or consoles. As this book is written, by 2014, the difficulty is so high that it is only profitable to mine with Application Specific Integrated Circuits, essentially hundreds of mining algorithms printed in hardware running in parallel on a single silicone chip. Jing also joined a "mining pool", which much like a lottery-pool allows several participants to share their efforts and the rewards. Jing now runs two USB-connected ASIC machines to mine for bitcoin 24 hours a day. He pays his electricity costs by selling the bitcoin he is able to generate from mining, creating some income from the profits. His computer runs a copy of bitcoind, the reference bitcoin client, as a back-end to his specialized mining software.
=== Mining transactions in blocks
@ -231,9 +231,9 @@ Alice's transaction was picked up by the network and included in the pool of unv
You can see the block that includes Alice's transaction here:
https://blockchain.info/block-height/277316
A few minutes later, a new block, #277317 is mined by another miner. As this new block is based on the previous block (#277316) that contained Alice's transaction, it added even more computation on top of that block, thereby strengthening the trust in those transactions. One block mined on top of the one containing the transaction, is called "one confirmation" for that transaction. As the blocks pile on top of each other, it becomes exponentially harder to reverse the transaction, thereby making it more and more trusted by the network.
A few minutes later, a new block, #277317 is mined by another miner. As this new block is based on the previous block (#277316) that contained Alice's transaction, it added even more computation on top of that block, thereby strengthening the trust in those transactions. One block mined on top of the one containing the transaction is called "one confirmation" for that transaction. As the blocks pile on top of each other, it becomes exponentially harder to reverse the transaction, thereby making it more and more trusted by the network.
In the diagram below, we can see block #277316, the one which contains Alice's transaction. Below it are 277,315 blocks, linked to each other in a chain of blocks (blockchain) all the way back to block #0, the genesis block. Over time, as the "height" in blocks increases, so does the computation difficulty for each block and the chain as a whole. The blocks mined after the one that contains Alice's transaction act as further assurance, as they pile on more computation in a longer and longer chain. The blocks above count as "confirmations". By convention, any block with more than 6 confirmation is considered irrevocable, as it would require an immense amount of computation to invalidate and re-calculate six blocks. We will examine the process of mining and the way it builds trust in more detail in <<mining>>.
In the diagram below we can see block #277316, which contains Alice's transaction. Below it are 277,315 blocks, linked to each other in a chain of blocks (blockchain) all the way back to block #0, the genesis block. Over time, as the "height" in blocks increases, so does the computation difficulty for each block and the chain as a whole. The blocks mined after the one that contains Alice's transaction act as further assurance, as they pile on more computation in a longer and longer chain. The blocks above count as "confirmations". By convention, any block with more than 6 confirmations is considered irrevocable, as it would require an immense amount of computation to invalidate and re-calculate six blocks. We will examine the process of mining and the way it builds trust in more detail in <<mining>>.
[[block-alice]]
.Alice's transaction included in block #277,317
@ -245,7 +245,7 @@ Now that Alice's transaction has been embedded in the blockchain as part of a bl
Bob can now spend the output from this and other transactions, by creating his own transactions that reference these outputs as their inputs and assign them new ownership. For example, Bob can pay a contractor or supplier by transferring value from Alice's coffee cup payment to these new owners. Most likely, Bob's bitcoin software will aggregate many small payments into a larger payment, perhaps concentrating all the day's bitcoin revenue into a single transaction. This would move the various payments into a single address, utilized as the store's general "checking" account. For a diagram of an aggregating transaction, see <<transaction-aggregating>>.
As Bob spends the payments received from Alice and other customers, he exends the chain of transactions, which in turn are added to the global blochcain ledger for all to see and trust. Let's assume that Bob pays his web designer Gopesh in Bangalore for a new web site page. Now the chain of transactions will look like this:
As Bob spends the payments received from Alice and other customers, he extends the chain of transactions which in turn are added to the global blochcain ledger for all to see and trust. Let's assume that Bob pays his web designer Gopesh in Bangalore for a new web site page. Now the chain of transactions will look like this:
[[block-alice]]
.Alice's transaction as part of a transaction chain from Joe to Gopesh