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.
A bitcoin address looks like 1DSrfJdB2AnWaFNgSbv3MZC2m74996JafV. It consists of a string of letters and numbers. It’s really an encoded base58check version of a public key 160-bit hash. Just like you ask others to send an email to your email address, you would ask others to send you bitcoin to one of your bitcoin addresses.
Bitcoin Improvement Proposals. A set of proposals that members of the bitcoin community have submitted to improve bitcoin. For example, BIP-21 is a proposal to improve the bitcoin uniform resource identifier (URI) scheme.
The name of the currency unit (the coin), the network, and the software.
A grouping of transactions, marked with a timestamp, and a fingerprint of the previous block. The block header is hashed to produce a proof of work, thereby validating the transactions. Valid blocks are added to the main blockchain by net‐ work consensus.
A list of validated blocks, each linking to its predecessor all the way to the genesis block.
Byzantine Generals Problem
A reliable computer system must be able to cope with the failure of one or more of its components. A failed component may exhibit a type of behavior that is often overlooked—namely, sending conflicting information to different parts of the system. The problem of coping with this type of failure is expressed abstractly as the Byzantine Generals Problem.
A special field used as the sole input for coinbase transactions. The coinbase allows claiming the block reward and provides up to 100 bytes for arbitrary data. Not to be confused with Coinbase transaction.
The first transaction in a block. Always created by a miner, it includes a single coinbase. Not to be confused with Coinbase.
Refers to keeping a reserve of bitcoin offline. Cold storage is achieved when Bitcoin private keys are created and stored in a secure offline environment. Cold storage is important for anyone with bitcoin holdings. Online computers are vulnerable to hackers and should not be used to store a significant amount of bitcoin.
An open source Bitcoin 2.0 protocol that enables developers to create digital assets on top of bitcoin blockchain utilizing its functionalities beyond currency.
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, and so on. Six or more confirmations is considered sufficient proof that a transaction cannot be reversed.
When several nodes, usually most nodes on the network, all have the same blocks in their locally-validated best block chain. Not to be confused with consensus rules.
The block validation rules that full nodes follow to stay in consensus with other nodes. Not to be confused with consensus.
A network-wide setting that controls how much computation is required to produce a proof of work.
A network-wide recalculation of the difficulty that occurs once every 2,016 blocks and considers the hashing power of the previous 2,016 blocks.
A difficulty at which all the computation in the network will find blocks approximately every 10 minutes.
Double spending is the result of successfully spending some money more than once. Bitcoin protects against double spending by verifying each transaction added to the block chain to ensure that the inputs for the transaction had not previously already been spent.
Elliptic Curve Digital Signature Algorithm or ECDSA is a cryptographic algo‐ rithm used by Bitcoin to ensure that funds can only be spent by their rightful owners.
As difficulty increased, miners often cycled through all 4 billion values of the nonce without finding a block. Because the coinbase script can store between 2 and 100 bytes of data, miners started using that space as extra nonce space, allowing them to explore a much larger range of block header values to find valid blocks.
The sender of a transaction often includes a fee to the network for processing the requested transaction. Most transactions require a minimum fee of 0.5 mBTC.
Fork, also known as accidental fork, occurs when two or more blocks have the same block height, forking the block chain. Typically occurs when two or more miners find blocks at nearly the same time. Can also happen as part of an attack.
The first block in the blockchain, used to initialize the cryptocurrency.
Hard fork, also known as Hard-Forking Change, is a permanent divergence in the blockchain, commonly occurs when non-upgraded nodes can’t validate blocks created by upgraded nodes that follow newer consensus rules. Not to be confused with fork, soft fork, software fork or Git fork.
A hardware wallet is a special type of bitcoin wallet which stores the user’s private keys in a secure hardware device.
A digital fingerprint of some binary input.
A hashlock is a type of encumbrance that restricts the spending of an output until a specified piece of data is publicly revealed. Hashlocks have the useful property that once any hashlock is opened publicly, any other hashlock secured using the same key can also be opened. This makes it possible to create multiple outputs that are all encumbered by the same hashlock and which all become spendable at the same time.
The Hierarchical Deterministic (HD) key creation and transfer protocol (BIP32), which allows creating child keys from parent keys in a hierarchy.
Wallets using the Hierarchical Deterministic (HD Protocol) key creation and transfer protocol (BIP32).
HD wallet seed
HD wallet seed or root seed is a potentially-short value used as a seed to generate the master private key and master chain code for an HD wallet.
A Hashed TimeLock Contract or HTLC is a class of payments that use hashlocks and timelocks to require that the receiver of a payment either acknowledge receiving the payment prior to a deadline by generating cryptographic proof of payment or forfeit the ability to claim the payment, returning it to the payer.
Know your customer (KYC) is the process of a business, identifying and verify‐ ing the identity of its clients. The term is also used to refer to the bank regulation which governs these activities.
LevelDB is an open source on-disk key-value store. LevelDB is a light-weight, single-purpose library for persistence with bindings to many platforms.
Lightning Network is a proposed implementation of Hashed Timelock Contracts (HTLCs) with bi-directional payment channels which allows payments to be securely routed across multiple peer-to-peer payment channels. This allows the formation of a network where any peer on the network can pay any other peer even if they don’t directly have a channel open between each other.
Locktime, or more technically nLockTime, is the part of a transaction which indicates the earliest time or earliest block when that transaction may be added to the block chain.
The bitcoin Mempool (memory pool) is a collection of all transaction data in a block that have been verified by bitcoin nodes, but are not yet confirmed.
The root node of a merkle tree, a descendant of all the hashed pairs in the tree. Block headers must include a valid merkle root descended from all transactions in that block.
A tree constructed by hashing paired data (the leaves), then pairing and hashing the results until a single hash remains, the merkle root. In Bitcoin, the leaves are almost always transactions from a single block.
A network node that finds valid proof of work for new blocks, by repeated hashing.
Multisignature (multisig) refers to requiring more than one key to authorize a bitcoin transaction.
A peer-to-peer network that propagates transactions and blocks to every bitcoin node on the network.
The “nonce” in a bitcoin block is a 32-bit (4-byte) field whose value is set so that the hash of the block will contain a run of leading zeros. The rest of the fields may not be changed, as they have a defined meaning.
An off-chain transaction is the movement of value outside of the block chain. While an on-chain transaction—usually referred to as simply a transaction— modifies the blockchain and depends on the blockchain to determine its validity an off-chain transaction relies on other methods to record and validate the transaction.
Operation codes from the Bitcoin Script language which push data or perform functions within a pubkey script or signature script.
Open Assets protocol
The Open Assets Protocol is a simple and powerful protocol built on top of the bitcoin blockchain. It allows issuance and transfer of user-created assets. The Open Assets protocol is an evolution of the concept of colored coins.
An opcode used in one of the outputs in an OP_RETURN transaction. Not to be confused with OP_RETURN transaction.
A transaction type relayed and mined by default in Bitcoin Core 0.9.0 and later that adds arbitrary data to a provably unspendable pubkey script that full nodes don’t have to store in their UTXO database. Not to be confused with OP_RETURN opcode.
Blocks whose parent block has not been processed by the local node, so they can’t be fully validated yet.
Transactions that can’t go into the pool due to one or more missing input transactions.
Output, transaction output, or TxOut is an output in a transaction which contains two fields: a value field for transferring zero or more satoshis and a pubkey script for indicating what conditions must be fulfilled for those satoshis to be further spent.
Transactions that pay a bitcoin address contain P2PKH or Pay To PubKey Hash scripts. An output locked by a P2PKH script can be unlocked (spent) by present‐ ing a public key and a digital signature created by the corresponding private key.
P2SH or Pay-to-Script-Hash is a powerful new type of transaction that greatly simplifies the use of complex transaction scripts. With P2SH the complex script that details the conditions for spending the output (redeem script) is not presented in the locking script. Instead, only a hash of it is in the locking script.
P2SH addresses are Base58Check encodings of the 20-byte hash of a script, P2SH addresses use the version prefix “5”, which results in Base58Check-encoded addresses that start with a “3”. P2SH addresses hide all of the complexity, so that the person making a payment does not see the script.
The signature of a P2WPKH (Pay-to-Witness-Public-Key-Hash) contains the same information as a P2PKH spending, but is located in the witness field instead of the scriptSig field. The scriptPubKey is also modified.
The difference between P2SH and P2WSH (Pay-to-Witness-Script-Hash) is about the cryptographic proof location change from the scriptSig field to the wit‐ ness field and the scriptPubKey that is also modified.
In the most specific sense, a paper wallet is a document containing all of the data necessary to generate any number of Bitcoin private keys, forming a wallet of keys. However, people often use the term to mean any way of storing bitcoin offline as a physical document. This second definition also includes paper keys and redeemable codes.
A micropayment channel or payment channel is class of techniques designed to allow users to make multiple bitcoin transactions without committing all of the transactions to the bitcoin blockchain. In a typical payment channel, only two transactions are added to the block chain but an unlimited or nearly unlimited number of payments can be made between the participants.
Pooled mining is a mining approach where multiple generating clients contribute to the generation of a block, and then split the block reward according the contributed processing power.
Proof-of-Stake (PoS) is a method by which a cryptocurrency blockchain network aims to achieve distributed consensus. Proof-of-Stake asks users to prove owner‐ ship of a certain amount of currency (their “stake” in the currency).
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.
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 12.5 BTC per block.
RIPEMD-160 is a 160-bit cryptographic hash function. RIPEMD-160 is a strengthened version of RIPEMD with a 160-bit hash result, and is expected to be secure for the next ten years or more.
A satoshi is the smallest denomination of bitcoin that can be recorded on the blockchain. It is the equivalent of 0.00000001 bitcoin and is named after the crea‐ tor of Bitcoin, Satoshi Nakamoto.
Satoshi Nakamoto is the name used by the person or people who designed Bit‐ coin and created its original reference implementation, Bitcoin Core. As a part of the implementation, they also devised the first blockchain database. In the pro‐ cess they were the first to solve the double spending problem for digital currency. Their real identity remains unknown.
Bitcoin uses a scripting system for transactions. Forth-like, Script is simple, stack-based, and processed from left to right. It is purposefully not Turing- complete, with no loops.
ScriptPubKey (aka pubkey script)
ScriptPubKey or pubkey script, is a script included in outputs which sets the con‐ ditions that must be fulfilled for those satoshis to be spent. Data for fulfilling the conditions can be provided in a signature script.
ScriptSig (aka signature script)
ScriptSig or signature script, is the data generated by a spender which is almost always used as variables to satisfy a pubkey script.
secret key (aka private key)
The secret number that unlocks bitcoin sent to the corresponding address. A secret key looks like the following:
Segregated Witness is a proposed upgrade to the Bitcoin protocol which techno‐ logical innovation separates signature data from bitcoin transactions. Segregated Witness is a proposed soft fork; a change that technically makes Bitcoin’s protocol rules more restrictive.
The Secure Hash Algorithm or SHA is a family of cryptographic hash functions published by the National Institute of Standards and Technology (NIST).
simpliied payment veriication (SPV)
SPV or simplified payment verification is a method for verifying particular trans‐ actions were included in a block without downloading the entire block. The method is used by some lightweight Bitcoin clients.
soft fork or Soft-Forking Change is a temporary fork in the blockchain which commonly occurs when miners using non-upgraded nodes don’t follow a new consensus rule their nodes don’t know about. Not to be confused with fork, hard fork, software fork or Git fork.
Block which were successfully mined but which isn’t included on the current best block chain, likely because some other block at the same height had its chain extended first.
A timelock is a type of encumbrance that restricts the spending of some bitcoin until a specified future time or block height. Timelocks feature prominently in many Bitcoin contracts, including payment channels and hashed timelock contracts.
In simple terms, a transfer of bitcoin 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.
An unordered collection of transactions that are not in blocks in the main chain, but for which we have input transactions.
A program language is called “Turing complete,” if that it can run any program that a Turing machine can run given enough time and memory.
unspent transaction output (UTXO)
UTXO is an unspent transaction output that can be spent as an input in a new transaction.
Software that holds all your bitcoin addresses and secret keys. Use it to send, receive, and store your bitcoin.
Wallet Import Format (WIF)
WIF or Wallet Import Format is a data interchange format designed to allow exporting and importing a single private key with a flag indicating whether or not it uses a compressed public key.
Some contributed definitions have been sourced under a CC-BY license from the bit‐ coin Wiki or from other open source documentation sources.
What Is Bitcoin?
Bitcoin is a collection of concepts and technologies that form the basis of a digital money ecosystem. Units of currency called bitcoin are used to store and transmit value among participants in the bitcoin network. Bitcoin users communicate with each other using the bitcoin protocol primarily via the internet, although other trans‐ port networks can also be used.
The bitcoin protocol stack, available as open source software, can be run on a wide range of computing devices, including laptops and smartphones, making the technology easily accessible.
Users can transfer bitcoin over the network to do just about anything that can be done with conventional currencies, including buy and sell goods, send money to people or organizations, or extend credit. Bitcoin can be purchased, sold, and exchanged for other currencies at specialized currency exchanges. Bitcoin in a sense is the per‐ fect form of money for the internet because it is fast, secure, and borderless.
Unlike traditional currencies, bitcoin are entirely virtual. There are no physical coins or even digital coins per se. The coins are implied in transactions that transfer value from sender to recipient. Users of bitcoin own keys that allow them to prove owner‐ ship of bitcoin in the bitcoin network.
With these keys they can sign transactions to unlock the value and spend it by transferring it to a new owner. Keys are often stored in a digital wallet on each user’s computer or smartphone. Possession of the key that can sign a transaction is the only prerequisite to spending bitcoin, putting the control entirely in the hands of each user.
Bitcoin is a distributed, peer-to-peer system. As such there is no “central” server or point of control. Bitcoin are created through a process called “mining,” which involves competing to find solutions to a mathematical problem while processing bit‐ coin transactions.
Any participant in the bitcoin network (i.e., anyone using a device running the full bitcoin protocol stack) may operate as a miner, using their computer’s processing power to verify and record transactions. Every 10 minutes, on average, a bitcoin miner is able to validate the transactions of the past 10 minutes and is rewarded with brand new bitcoin.
Essentially, bitcoin mining decentralizes the currency-issuance and clearing functions of a central bank and replaces the need for any central bank.
The bitcoin protocol includes built-in algorithms that regulate the mining function across the network. The difficulty of the processing task that miners must perform is adjusted dynamically so that, on average, someone succeeds every 10 minutes regard‐ less of how many miners (and how much processing) are competing at any moment.
The protocol also halves the rate at which new bitcoin are created every 4 years, and limits the total number of bitcoin that will be created to a fixed total just below 21 million coins. The result is that the number of bitcoin in circulation closely follows an easily predictable curve that approaches 21 million by the year 2140.
Due to bitcoin’s diminishing rate of issuance, over the long term, the bitcoin currency is deflationary. Furthermore, bitcoin cannot be inflated by “printing” new money above and beyond the expected issuance rate.
Behind the scenes, bitcoin is also the name of the protocol, a peer-to-peer network, and a distributed computing innovation. The bitcoin currency is really only the first application of this invention.
Bitcoin represents the culmination of decades of research in cryptography and distributed systems and includes four key innovations brought together in a unique and powerful combination. Bitcoin consists of:
- A decentralized peer-to-peer network (the bitcoin protocol)
- A public transaction ledger (the blockchain)
- A set of rules for independent transaction validation and currency issuance (consensus rules)
- A mechanism for reaching global decentralized consensus on the valid blockchain (Proof-of-Work algorithm)
As a developer, I see bitcoin as akin to the internet of money, a network for propagating value and securing the ownership of digital assets via distributed computation.
There’s a lot more to bitcoin than first meets the eye.
In this chapter we’ll get started by explaining some of the main concepts and terms, getting the necessary software, and using bitcoin for simple transactions. In following chapters we’ll start unwrapping the layers of technology that make bitcoin possible and examine the inner workings of the bitcoin network and protocol.
Digital Currencies Before Bitcoin
The emergence of viable digital money is closely linked to developments in cryptography. This is not surprising when one considers the fundamental challenges involved with using bits to represent value that can be exchanged for goods and services.
Three basic questions for anyone accepting digital money are:
- Can I trust that the money is authentic and not counterfeit?
- Can I trust that the digital money can only be spent once (known as the “double- spend” problem)?
- Can I be sure that no one else can claim this money belongs to them and not me?
Issuers of paper money are constantly battling the counterfeiting problem by using increasingly sophisticated papers and printing technology. Physical money addresses the double-spend issue easily because the same paper note cannot be in two places at once.
Of course, conventional money is also often stored and transmitted digitally. In these cases, the counterfeiting and double-spend issues are handled by clearing all electronic transactions through central authorities that have a global view of the currency in circulation. For digital money, which cannot take advantage of esoteric inks or holographic strips, cryptography provides the basis for trusting the legitimacy of a user’s claim to value.
Specifically, cryptographic digital signatures enable a user to sign a digital asset or transaction proving the ownership of that asset. With the appropriate architecture, digital signatures also can be used to address the double-spend issue.
When cryptography started becoming more broadly available and understood in the late 1980s, many researchers began trying to use cryptography to build digital currencies. These early digital currency projects issued digital money, usually backed by a national currency or precious metal such as gold.
Although these earlier digital currencies worked, they were centralized and, as a result, were easy to attack by governments and hackers. Early digital currencies used a central clearinghouse to settle all transactions at regular intervals, just like a traditional banking system.
Unfortunately, in most cases these nascent digital currencies were targeted by worried governments and eventually litigated out of existence. Some failed in spectacular crashes when the parent company liquidated abruptly.
To be robust against intervention by antagonists, whether legitimate governments or criminal elements, a decentralized digital currency was needed to avoid a single point of attack. Bitcoin is such a system, decentralized by design, and free of any central authority or point of control that can be attacked or corrupted.