The bitcoin system, unlike traditional banking and payment systems, is based on de-centralized trust. Instead of a central trusted authority, in bitcoin, trust is achieved as an emergent property from the interactions of different participants in the bitcoin system. In this chapter we will examine bitcoin from a high-level by tracking a single transaction through the bitcoin system and watch as it becomes "trusted" and accepted by the bitcoin mechanism of distributed consensus and is finally recorded on the blockchain, the distributed ledger of all transactions.
Each example below is based upon an actual transaction made on the bitcoin network, simulating the interactions between the users (Joe, Alice and Bob) by sending funds from one wallet to another. While tracking a transaction through the bitcoin network and blockchain, we will use a _blockchain explorer_ site to visualize each step. A blockchain explorer is a web application that operates as a bitcoin search engine, in that it allows you to search for addresses, transactions and blocks and see the relationships and flows between them.
Popular blockchain explorers include:
* blockchain.info
* blockexplorer.com
* biteasy.com
Each of these has a search function that can take an address, transaction hash or block number and find the equivalent data on the bitcoin network and blockchain. With each example, we will provide a URL that takes you directly to the relevant entry, so you can study it in detail.
In the overview diagram below, we see that the bitcoin system consists of users with wallets containing keys, transactions which are propagated across the network and miners who produce (through competitive computation) the consensus blockchain, the authoritative ledger of all transactions. In this chapter, we will trace a single transaction as it travels across the network and examine the interactions between each part of the bitcoin system, at a high level. Subsequent chapters will delve deeper into the technology behind wallets, mining and merchant systems.
Alice, who we introduced in the previous chapter, is a new user who has just acquired her first bitcoin. In <<getting_first_bitcoin>>, Alice met with her frined Joe to exchange some cash for bitcoin. The transaction created by Joe, funded Alice's wallet with 0.10 BTC. Now Alice will make her first retail transaction, buying a cup of coffee at Bob's coffee shop in Palo Alto, California. Bob's coffee shop recently started accepting bitcoin payments, by adding a bitcoin option to his point-of-sale system (see <<bitcoin_for_merchants>> for information on using bitcoin for merchants/retail). The prices at Bob's Cafe are listed in the local currency (US dollars) but at the register, customers have the option of paying in either dollars or bitcoin. Alice places her order for a cup of coffee and Bob enters the transaction at the register. The point-of-sale system will convert the total price from US dollars to bitcoins at the prevailing market rate and displays the prices in both currencies, as well as showing a QR code containing a _payment request_ for this transaction:
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"_.
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.
In the following sections we will examine this transaction in more detail, see how Alice's wallet constructed it, how it was propagated across the network, how it was verified and finally how Bob, the owner of the cafe, can spend that amount in subsequent transactions
The bitcoin network can transact in fractional values, e.g. from millibitcoins (1/1000th of a bitcoin) down to 1/100,000,000th of a bitcoin, which is known as a Satoshi. Throughout this book we’ll use the term “bitcoins” to refer to any quantity of bitcoin currency, from the smallest unit (1 Satoshi) to the total number (21,000,000) of all bitcoins that will ever be mined.
In simple terms, a transaction tells the network that the owner of a number bitcoins has authorized the transfer of some of those bitcoins to another owner. The new owner can now spend these bitcoins by creating another transaction that authorizes transfer to another owner, and so on, in a chain of ownership.
Transactions are like lines in a double-entry bookkeeping ledger. In simple terms, each transaction contains one or more "inputs", which are debits against a bitcoin account. On the other side of the transaction, there are one or more "outputs", which are credits added to a bitcoin account. The inputs and outputs (debits and credits) do not necessarily add up to the same amount. Instead, outputs add up to slightly less than inputs and the difference represents an implied "transaction fee", a small payment collected by the miner who includes the transaction in the ledger.
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.
_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.
Alice's payment to Bob's Cafe utilizes a previous transaction as its input. In the previous chapter Alice received bitcoin from her friend Joe in return for cash. That transaction has a number of bitcoins locked (encumbered) against Alice's key. Her new transaction to Bob's Cafe references the previous transaction as an input and creates new outputs to pay for the cup of coffee and receive change. The transactions form a chain, where the inputs from the latest transaction correspond to outputs from previous transactions. Alice's key provides the signature which unlocks those previous transaction outputs, thereby proving to the bitcoin network that she owns the funds. She attaches the payment for coffee to Bob's address, thereby "encumbering" that output with the requirement that Bob produces a signature in order to spend that amount. This represents a transfer of value between Alice and Bob.
The most common form of transaction is a simple payment from one address to another, which often includes some "change" returned to the original owner. This type of transaction has one input and two outputs and is shown below:
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.
Finally, another transaction form that is seen often on the bitcoin ledger is a transaction that distributes one input to multiple outputs representing multiple recipients. This type of transaction is sometimes used by commercial entities to distribute funds, such as when processing payroll payments to multiple employees.
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
Alice's wallet application will first have to find inputs that can pay for the amount she wants to send to Bob. Most wallet applications keep a small database of "unspent transaction outputs" that are locked (encumbered) with the wallet's own keys. Therefore, Alice's wallet would contain a copy of the transaction output from Joe's transaction which was created in exchange for cash (see <<getting bitcoin>>). A bitcoin wallet application that runs as a full-index client actually contains a copy of *every unspent output* from every transaction in the blockchain. This allows a wallet to construct transaction inputs as well as to quickly verify incoming transactions as having correct inputs.
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:
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.
[TIP]
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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:
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).
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.
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.
