[preface]
== Preface

=== Writing the Bitcoin Book

I first stumbled upon bitcoin in mid-2011. My immediate reaction was more or less "Pfft! Nerd money!" and I ignored it for another six months, failing to grasp its importance. This is a reaction that I have seen repeated among many of the smartest people I know, which gives me some consolation. The second time I came across bitcoin in a mailing list discussion, I decided to read the white paper written by Satoshi Nakamoto, to study the authoritative source and see what it was all about. I still remember the moment I finished reading those nine pages, when I realized that bitcoin was not simply a digital currency, but a network of trust that could also provide the basis for so much more than just currencies. The realization that "this isn't money, it's a decentralized trust network," started me on a four-month journey to devour every scrap of information about bitcoin I could find. I became obsessed and enthralled, spending twelve or more hours each day glued to a screen, reading, writing, coding, and learning as much as I could. I emerged from this state of fugue, more than 20 lbs lighter from lack of consistent meals, determined to dedicate myself to working on bitcoin. 

Two years later, after creating a number of small startups to explore various bitcoin-related services and products, I decided that it was time to write my first book. Bitcoin was the topic that had driven me into a frenzy of creativity, consumed my thoughts, and was the most exciting technology I had encountered since the Internet. It was now time to share my passion about this amazing technology with a broader audience. 

=== Intended Audience

This book is mostly intended for coders. If you can use a programming language, this book will teach you how cryptographic currencies work, how to use them, and how to develop software that works with them. The first few chapters are also suitable as an in-depth introduction to bitcoin for noncoders—those trying to understand the inner workings of bitcoin and crypto-currencies.

=== Why Are There Bugs on the Cover?

The leafcutter ant is a species that exhibits highly complex behavior in a colony super-organism, but each individual ant operates on a set of simple rules driven by social interaction and the exchange of chemical scents (pheromones). Per Wikipedia: "Next to humans, leafcutter ants form the largest and most complex animal societies on Earth." Leafcutter ants don't actually eat leaves, but rather use them to farm a fungus, which is the central food source for the colony. Get that? These ants are farming! 

Although ants form a caste-based society and have a queen for producing offspring, there is no central authority or leader in an ant colony. The highly intelligent and sophisticated behavior exhibited by a multimillion member colony is an emergent property from the interaction of the individuals in a social network. 

Nature demonstrates that decentralized systems can be resilient and can produce emergent complexity and incredible sophistication without the need for a central authority, hierarchy, or complex parts.

Bitcoin is a highly sophisticated decentralized trust network that can support a myriad of financial processes. Yet, each node in the bitcoin network follows a few simple mathematical rules. The interaction between many nodes is what leads to the emergence of the sophisticated behavior, not any inherent complexity or trust in any single node. Like an ant colony, the bitcoin network is a resilient network of simple nodes following simple rules that together can do amazing things without any central coordination.

=== Conventions Used in This Book

The following typographical conventions are used in this book:

_Italic_:: Indicates new terms, URLs, email addresses, filenames, and file extensions.

+Constant width+:: Used for program listings, as well as within paragraphs to refer to program elements such as variable or function names, databases, data types, environment variables, statements, and keywords.

**`Constant width bold`**:: Shows commands or other text that should be typed literally by the user.

_++Constant width italic++_:: Shows text that should be replaced with user-supplied values or by values determined by context.


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=== GitHub Source Code

This book is available on GitHub, as a repository that contains the text, images, and code: 

https://github.com/aantonop/bitcoinbook

Fork the book code, try the code examples, or submit corrections via GitHub. 

=== Code Examples

The examples are illustrated in Python, C++, and using the command-line of a Unix-like operating system such as Linux or Mac OS X. All code snippets are available in the GitHub repository and can be accessed online at https://github.com/aantonop/bitcoinbook in the _code_ subdirectory of the main repository. 

All the code snippets can be replicated on most operating systems with a minimal installation of compilers and interpreters for the corresponding languages. Where necessary, we provide basic installation instructions and step-by-step examples of the output of those instructions. 

Some of the code snippets and code output have been reformatted for print. In all such cases, the lines have been split by a backslash (\) character, followed by a newline character. When transcribing the examples, remove those two characters and join the lines again and you should see identical results as shown in the example. 

All the code snippets use real values and calculations where possible, so that you can build from example to example and see the same results in any code you write to calculate the same values. For example, the private keys and corresponding public keys and addresses are all real. The sample transactions, blocks, and blockchain references have all been introduced in the actual bitcoin blockchain and are part of the public ledger, so you can review them on any bitcoin system.

=== Using Code Examples

This book is here to help you get your job done. In general, if example code is offered with this book, you may use it in your programs and documentation. You do not need to contact us for permission unless you’re reproducing a significant portion of the code. For example, writing a program that uses several chunks of code from this book does not require permission. Selling or distributing a CD-ROM of examples from O’Reilly books does require permission. Answering a question by citing this book and quoting example code does not require permission. Incorporating a significant amount of example code from this book into your product’s documentation does require permission.

We appreciate, but do not require, attribution. An attribution usually includes the title, author, publisher, and ISBN. For example: “_Mastering Bitcoin_ by Andreas M. Antonopoulos (O’Reilly). Copyright 2014 Andreas M. Antonopoulos, 978-1449374044.”

Some editions of this books are offered under an open source license, such as CC-BY-NC (creativecommons.org) in which case the terms of that license apply.

If you feel your use of code examples falls outside fair use or the permission given above, feel free to contact us at pass:[<email>permissions@oreilly.com</email>].

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=== Acknowledgments

This book represents the efforts and contributions of many people. I am grateful for all the help I received from friends, colleagues, and even complete strangers, who joined me in this effort to write the definitive technical book on crypto-currencies and bitcoin. 

It is impossible to make a distinction between the bitcoin technology and the bitcoin community, and this book is as much a product of that community as it is a book on the technology. My work on this book was encouraged, cheered on, supported, and rewarded by the entire bitcoin community from the very beginning until the very end. More than anything, this book has allowed me to be part of a wonderful community for two years and I can't thank you enough for accepting me in this community. There are far too many people to mention by name—people I've met at conferences, events, seminars, meetups, pizza gatherings, and small private gatherings, as well as many who communicated with me by Twitter, on reddit, on bitcointalk.org, and on GitHub who have had an impact on this book. Every idea, analogy, question, answer, and explanation you find in this book was at some point inspired, tested, or improved through my interactions with the community. Thank you all for your support; without you this book would not have happened. I am forever grateful.

The journey to becoming an author starts long before the first book, of course. My first language (and schooling) was Greek, so I had to take a remedial English Writing course in my first year of university. I owe thanks to Diana Kordas, my English Writing teacher, who helped me build confidence and skills that year. Later, as a professional, I developed my technical writing skills on the topic of data centers, writing for _Network World_ magazine. I owe thanks to John Dix and John Gallant who gave me my first writing job as a columnist at _Network World_ and to my editor Michael Cooney and my colleague Johna Till Johnson who edited my columns and made them fit for publication. Writing 500 words a week for four years gave me enough experience to eventually consider becoming an author. Thanks to Jean de Vera for her early encouragement to become an author and for always believing and insisting that I had a book in me.

Thanks also to those who supported me when I submitted my book proposal to O'Reilly, by providing references and reviewing the proposal. Specifically, thanks to John Gallant, Gregory Ness, Richard Stiennon, Joel Snyder, Adam B. Levine, Sandra Gittlen, John Dix, Johna Till Johnson, Roger Ver, and Jon Matonis. Special thanks to Richard Kagan and Tymon Mattoszko who reviewed early versions of the proposal and Matthew Owain Taylor who copyedited the proposal.

Thanks to Cricket Liu, author of the O'Reilly title _DNS and BIND_, who introduced me to O'Reilly. Thanks also to Michael Loukides and Allyson MacDonald at O'Reilly who worked for months to help make this book happen. Allyson was especially patient when deadlines were missed and deliverables delayed as life intervened in our planned schedule. 

The first few drafts of the first few chapters were the hardest, because bitcoin is a difficult subject to unravel. Every time I pulled on one thread of the bitcoin technology, I had to pull in the whole thing. I repeatedly got stuck and a bit despondent as I struggled to make the topic easy to understand and create a narrative around such a dense technical subject. Eventually, I decided to tell the story of bitcoin through the stories of the people using bitcoin and the whole book became a lot easier to write. I owe thanks to my friend and mentor, Richard Kagan, who helped me unravel the story and get past the moments of writer's block, and Pamela Morgan who reviewed early drafts of each chapter and asked the hard questions to make them better. Also, thanks to the developers of the San Francisco Bitcoin Developers Meetup group and Taariq Lewis, the group's co-founder, for helping to test the early material.

During the development of the book, I made early drafts available on GitHub and invited public comments. More than a hundred comments, suggestions, corrections, and contributions were submitted in response. Those contributions are explicitly acknowledged, with my thanks, in <<github_contrib>>. Special thanks to Minh T. Nguyen who volunteered to manage the GitHub contributions and added many significant contributions himself. Thanks also to Andrew Naugler for infographic design. 

Once the book was drafted, it went through several rounds of technical review. Thanks to Cricket Liu and Lorne Lantz for their thorough review, comments, and support.

Several bitcoin developers contributed code samples, reviews, comments, and encouragement. Thanks to Amir Taaki for example code snippets and many great comments, Vitalik Buterin and Richard Kiss for help with elliptic curve math and code contributions, Gavin Andresen for corrections, comments, and encouragement, and Michalis Kargakis for comments, contributions, and btcd writeup. 

I owe my love of words and books to my mother, Theresa, who raised me in a house with books lining every wall. My mother also bought me my first computer in 1982, despite being a self-described technophobe. My father, Menelaos, a civil engineer who just published his first book at 80 years old, was the one who taught me logical and analytical thinking and a love of science and engineering. 

Thank you all for supporting me throughout this journey. 

[[github_contrib]]
==== Early Release Draft (GitHub Contributions)

Many contributors offered comments, corrections, and additions to the early-release draft on GitHub. Thank you all for your contributions to this book. Notable contributors included the following:

===== Name: GitHub ID

* *Minh T. Nguyen, GitHub contribution editor: enderminh*
* Ed Eykholt: edeykholt
* Michalis Kargakis: kargakis
* Erik Wahlström: erikwam
* Richard Kiss: richardkiss
* Eric Winchell: winchell
* Sergej Kotliar: ziggamon
* Nagaraj Hubli: nagarajhubli
* ethers
* Alex Waters: alexwaters
* Mihail Russu: MihailRussu
* Ish Ot Jr.: ishotjr
* James Addison: jayaddison
* Nekomata: nekomata-3
* Simon de la Rouviere: simondlr
* Chapman Shoop: belovachap
* Holger Schinzel: schinzelh
* effectsToCause: vericoin
* Stephan Oeste: Emzy
* Joe Bauers: joebauers
* Jason Bisterfeldt: jbisterfeldt
* Ed Leafe: EdLeafe

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=== Quick Glossary

This quick glossary contains many of the terms used in relation to bitcoin. These terms are used throughout the book, so bookmark this for a quick reference and clarification.

address::
((("bitcoin address")))
((("address", see="bitcoin address")))
((("public key", see="bitcoin address")))
    A bitcoin address looks like +1DSrfJdB2AnWaFNgSbv3MZC2m74996JafV+. It consists of a string of letters and numbers starting with a "1" (number one). Just like you ask others to send an email to your email address, you would ask others to send you bitcoin to your bitcoin address.

bip::
((("bip"))) 
    Bitcoin Improvement Proposals.  A set of proposals that members of the bitcoin community have submitted to improve bitcoin.  For example, BIP0021 is a proposal to improve the bitcoin URI scheme.

bitcoin::
((("bitcoin"))) 
    The name of the currency unit (the coin), the network, and the software.

block::
((("block")))
	A grouping of transactions, marked with a timestamp, and a fingerprint of the previous block. The block header is hashed to find a Proof-Of-Work, thereby validating the transactions. Valid blocks are added to the main blockchain by network consensus.

blockchain::
((("blockchain")))
		A list of validated blocks, each linking to its predecessor all the way to the genesis block.
	
confirmations::
((("confirmations")))
		Once a transaction is included in a block, it has "one confirmation." As soon as _another_ block is mined on the same blockchain, the transaction has two confirmations, etc. Six or more confirmations is considered sufficient proof that a transaction cannot be reversed.

difficulty::
((("difficulty")))
	A network-wide setting that controls how much computation is required to find a Proof-Of-Work.

difficulty target::
((("target difficulty")))
 	A difficulty at which all the computation in the network will find blocks approximately every 10 minutes.

difficulty re-targeting::
((("difficulty re-targeting")))
	A network-wide recalculation of the difficulty that occurs once every 2,106 blocks and considers the hashing power of the previous 2,106 blocks.
	
fees::
((("fees")))
	The sender of a transaction often includes a fee to the network for processing the requested transaction.  Most transactions require a minimum fee of 0.5mBTC.

hash::
((("hash")))
	A digital fingerprint of some binary input.

genesis block::
((("genesis block")))
	The first block in the blockchain, used to initialize the crypto-currency.
	
miner::
((("miner")))
	A network node that finds valid Proof-Of-Work for new blocks, by repeated hashing.

network::
((("network")))
	A peer-to-peer network that propagates transactions and blocks to every bitcoin node on the network. 
	
Proof-Of-Work::
((("proof-of-work")))
	A piece of data that requires significant computation to find. In bitcoin, miners must find a numeric solution to the SHA256 algorithm that meets a network-wide target, the difficulty target. 

reward::
((("reward")))
	An amount included in each new block as a reward by the network to the miner who found the Proof-Of-Work solution. It is currently 25BTC per block.

secret key (aka private key)::
((("secret key")))
((("private key", see="secret key")))
	The secret number that unlocks bitcoins sent to the corresponding address.  A secret key looks like +5J76sF8L5jTtzE96r66Sf8cka9y44wdpJjMwCxR3tzLh3ibVPxh+.
	
transaction::
((("transaction")))
	In simple terms, a transfer of bitcoins from one address to another. More precisely, a transaction is a signed data structure expressing a transfer of value. Transactions are transmitted over the bitcoin network, collected by miners, and included into blocks, made permanent on the blockchain.

wallet::
((("wallet"))) 
	Software that holds all your bitcoin addresses and secret keys. Use it to send, receive, and store your bitcoin.