How Bitcoin Works

Digital payments existed before Bitcoin. People could send money through banks, card networks, and online payment companies. These systems made commerce easier, but each depended on a trusted organization to keep the ledger, approve accounts, settle transfers, and resolve disputes.

This dependence creates power and risk. An intermediary can delay a payment, charge a fee, collect and misuse personal data, restrict an account, reverse a transfer, or fail. The issuer of the currency can also change its supply.

For many years, computer scientists tried to create digital money that did not need a central issuer or payment company. The challenge was difficult. Digital information is easy to copy. If a digital coin could be copied and spent twice, it could not function as money. If one company prevented copying, the company became the point of control and failure.

Bitcoin was created to solve this problem with a peer-to-peer network. Its design allows participants to agree on one transaction history without a central ledger keeper. It combines a public ledger (blockchain), digital signatures, a network of validating nodes, proof of work, and economic incentives.

Bitcoin does not remove trust from every part of life. It reduces the need to trust a central authority for the issuance and settlement of its native money.

In this book, Bitcoin with a capital B usually means the network, protocol, or wider system. And bitcoin with a lowercase b means the monetary unit or asset. BTC is the common abbreviation for an amount of bitcoin, especially when writing prices or specific quantities. A wallet or exchange is a service that may connect to Bitcoin; it is not the Bitcoin network itself.

The growth of personal computers and the internet created new opportunities for communication. It also created new forms of surveillance and control. In the late twentieth century, a group of programmers, mathematicians, and activists known as Cypherpunks explored how cryptography could protect privacy and individual freedom in digital systems.

Cryptography

Cryptography uses mathematical methods to protect information and prove that an action is authentic. Encryption can keep a message private. Digital signatures can prove that a message or instruction came from the holder of a secret key without revealing that key.

The Cypherpunks understood that private communication was not enough. People also needed a way to exchange value online. If every digital payment required a bank or company, that intermediary could observe, permit, or prevent the exchange.

Their goal was not simply digital money. Bank deposits were already digital. The harder goal was decentralized digital money: a system with no single issuer, server, or administrator that participants had to trust.

Bitcoin was built upon decades of work by many people. Its design joined earlier ideas in a new way and provided incentives for an open network to keep operating.

Bitcoin was not the first attempt at digital money. It was built on four decades of experimentation, drawing on ideas from many earlier projects that helped shape its successful design.

• DigiCash used cryptography to support private digital payments. It depended on a central company and banking relationships. When the company failed, the system could not continue as an independent money network.
• Hashcash used proof of work to make unwanted email costly. A sender's computer had to perform work before sending a message. The idea showed how computational cost could discourage abuse.
• Bit Gold described a proposed system in which computational work could create scarce digital records. It was not launched as a complete decentralized money system, but it developed important ideas about digital scarcity and proof.
• B-money and reusable proof of work (RPoW) explored ways for participants to create or transfer scarce digital value without relying only on one central database. They were incomplete, but they helped clarify what a working system would need.
• Although BitTorrent was not money, it showed that a peer-to-peer network could let many independent computers share information without one central server controlling the whole system.

Together, these experiments showed useful pieces of the puzzle. They also clarified the remaining problem.

Double spending

Double spending is an attempt to send the same digital unit to two recipients. Digital data can be copied perfectly, so a digital money system needs a way to establish which transaction is valid.

Centralized systems prevent this by letting one trusted database decide which payment came first. On 31 October 2008, Satoshi Nakamoto's whitepaper, Bitcoin: A Peer-to-Peer Electronic Cash System, explained a different approach: a public system that could establish a shared payment history without a bank, company, or other central authority.

The whitepaper brought together the features needed for that approach:

1. A peer-to-peer network: transactions are announced directly to a network of participants.
2. Digital signatures: a private key is secret information that creates a signature to authorize a spend; only its holder can do this.
3. Blocks and mining: miners collect transactions into blocks and use proof of work to compete to add the next block.
4. Independent verification: nodes check transactions and blocks against Bitcoin's rules.
5. A shared history with accumulated proof of work: participants recognize the valid chain of transactions with the most accumulated work as the shared record.

This module explains these concepts in more detail. Together, they gave a practical answer to the double-spend problem: the network can reject a second attempt to spend the same bitcoin without relying on a central authority. Bitcoin offered the missing combination that decades of digital-money experiments had been seeking.

Digital information is normally easy to copy.

Digital scarcity

Digital scarcity means that a system can distinguish a valid unit from an invalid copy and prevent the same unit from being spent twice.

Bitcoin creates digital scarcity by combining signatures, a public transaction history, proof of work, and independent rule verification. A bitcoin can be transferred, but the network rejects an attempt to spend the same bitcoin again.

Scarcity is independently verifiable when people can check the public rules for themselves and no single issuer can quietly change the supply for everyone. This is why Bitcoin's supply limit must be studied together with decentralization and node verification.

A centralized system has a controlling point. A bank controls its customer ledger. A payment company controls its network. A government controls the issuance rules of its currency.

Decentralization

A decentralized system distributes roles and decision enforcement among many independent participants. Bitcoin has no central office that can create accounts, change balances, or approve membership.

Bitcoin's decentralization comes from several features:

• anyone can download and inspect the open-source software
• users can transact without opening an account with the Bitcoin network
• independent nodes check transactions and blocks
• miners compete to propose blocks
• no miner can make an invalid block acceptable to honest nodes
• changes to the rules require voluntary adoption by participants
• the ledger is copied across many computers and locations

Decentralization is not the same as equal influence. Mining equipment, technical knowledge, money, internet access, and software development are not distributed equally. Large service providers can become points of dependence for their own users. Bitcoin's claim is narrower: no single participant has the authority to unilaterally change the ledger or monetary rules for everyone.

Decentralization also creates costs. Coordination can be slower. Users must verify information and take more responsibility. There is no central support desk for the Bitcoin network. These trade-offs are part of the design, not accidental defects that can be removed without changing the system.

Digital signatures are one of the key parts of Bitcoin's design described in the whitepaper. In Bitcoin, a wallet uses a private key to create a digital signature that authorizes a transaction. A wallet is usually an application that manages the keys and creates these signatures for the user. Only the holder of the required private key can authorize the spend; the network can verify the signature without learning the private key itself.

This allows a person to control bitcoin without asking a bank or other central operator for permission. It also means that control of the private key is critical: anyone who obtains it can usually authorize a spend, while a lost key may make the bitcoin irretrievable.

This is the basis of self-custody: controlling the keys yourself rather than relying on a service to control them. Module 3 explains the practical safety responsibilities of keys, backups, and recovery.

Peer-to-peer means that participants interact through a network without requiring a central coordinator. A Bitcoin user can create and sign a transaction, then broadcast it to the network. The recipient does not need an account at the sender's bank because Bitcoin itself provides the common settlement system.

Peer-to-peer does not always mean that only two computers communicate directly. A transaction may pass through wallets, internet providers, and many network nodes. The important point is that no central Bitcoin operator must approve the sender or recipient.

Users can still choose intermediaries. An exchange may hold bitcoin for customers. A payment service may simplify an invoice. A hosted wallet may manage keys. These services can be convenient, but the user then depends on the service's rules and security.

Bitcoin makes direct ownership and transfer possible; it does not force every user to avoid every service. The custody choice determines who has practical control.

Peer-to-peer transfer can be useful when people are in different places or use different banking systems. It also creates responsibility. A valid transaction sent to the wrong address cannot usually be reversed.

Bitcoin works because different participants perform different roles.

• Users receive, hold, and send bitcoin. A user may control their own keys held in a wallet or use a custodian. Users give the network economic meaning by choosing whether to accept bitcoin and which rules or services they trust.
• Nodes run Bitcoin software and independently verify the rules. A full node checks that transactions have valid signatures, that bitcoin have not already been spent, that blocks satisfy proof of work, and that new bitcoin follow the issuance schedule. Nodes share valid data with peers and reject invalid data.
• Miners gather pending transactions, build candidate blocks, and compete through proof of work. A successful miner broadcasts a block and may receive the allowed block subsidy and transaction fees. Miners help order transactions and make the ledger costly to rewrite.
• Developers review, write, and propose software changes. They do not own Bitcoin's rules. A software change matters only if participants choose to run software that follows those rules.

These roles limit one another:

• a user cannot spend bitcoin without the required valid signature
• a miner can propose a block, but nodes decide whether it follows the rules
• a node verifies using its own rules, but consensus requires compatible rules across the network
• developers can propose software, but participants choose whether to run it

Some people perform several roles. A miner can run a node and also be a user. A beginner does not need to perform every role to explain how the roles balance one another.

Spot question: Why does a node verify transactions? Name at least one rule that a node checks.

Proof of work

Proof of work is the process miners use to compete for the right to propose the next block.

A hash function turns data into a fixed-length result. A small change to the input produces an unpredictable new result. A miner changes a small piece of data in a proposed block and hashes it repeatedly. A result is valid only when its number is below the network's target, a threshold set by the rules; a lower target means fewer valid results and more attempts. There is no shortcut, so miners may need to make many attempts.

When a miner finds a valid result, it broadcasts the block. Nodes can check the proof quickly, even though finding it required significant computation and energy. If the block includes a transaction, that transaction receives its first confirmation: its record has been included in one valid block. Each valid block added after it adds another confirmation, making the transaction's place in the blockchain more costly to rewrite. This difference is important: work is costly to produce and easy to verify.

Proof of work serves several purposes:

• it gives independent miners a way to compete without a central scheduler
• it makes the ordering of transactions costly to rewrite
• it links network security to real-world equipment and energy
• it supports a public method for choosing between competing valid histories: the chain with the most accumulated work

Mining does not solve useful equations for an outside purpose. Its purpose is to secure the ordering of Bitcoin transactions. The network periodically adjusts the target: when blocks arrive too quickly it lowers the target, and when they arrive too slowly it raises it, aiming for about one block every ten minutes on average as computing power changes.

Proof of work involves a real energy cost. Supporters argue that this cost is what anchors Bitcoin's security and can create demand for flexible or otherwise unused energy. Critics question the amount and source of energy used. A responsible evaluation asks what energy is used, where it comes from, what security it provides, and what alternatives are being compared.

Bitcoin's rules define a predictable issuance schedule and a maximum supply of 21 million bitcoin (21,000,000 BTC). New bitcoin enter circulation through the block subsidy paid to miners who produce valid blocks.

Halving

Every 210,000 blocks, the subsidy is cut in half. This event is called a halving. Because blocks are produced approximately every ten minutes on average, a halving occurs roughly every four years. The exact calendar date is not fixed.

The subsidy began at 50 BTC per block. It falls by half in repeated steps until it becomes too small to continue. Miners also receive transaction fees. Over time, fees are expected to become a more important part of miner revenue.

The 21 million limit is enforced by nodes. If a miner creates more bitcoin than the rules allow, validating nodes reject the block. A software developer can write different code, but cannot force other people to run it.

Spot question: What are the advantages of Bitcoin having a limited supply? What would be the effect of increasing the supply of Bitcoin on purchasing power? Who would reject an increase in supply?

One bitcoin (1 BTC) can be divided into 100,000,000 satoshis. A satoshi, often shortened to sat, is the smallest unit recorded on Bitcoin's base layer.

You do not need to buy or use a whole bitcoin. Bitcoin software can display an amount in bitcoin or sats, just as a currency can use a main unit and smaller units. Many people use sats for everyday amounts because whole numbers are easier to read. Both on-chain and Lightning payment requests can be shown in sats.

Unit bias is the mistaken feeling that a lower price per whole unit makes an asset cheaper or more accessible. The number of units is a design choice. Owning 100,000 sats and owning 0.001 BTC describe the same amount.

When comparing prices, focus on the share of the total supply, the amount of value involved, and the risks. Do not assume that an asset with many cheap-looking units offers better value than an asset divided into smaller units.

Show your understanding: How many sats are in 0.01 BTC? Why does the price of one whole bitcoin not prevent a person from using a smaller amount?

As Module 1 introduced, sound money describes money whose supply is difficult to expand and whose properties support reliable use across time. The term expresses a judgment based on criteria, so it should be examined rather than accepted as a label.

Bitcoin combines properties that did not previously exist together in one asset:

Bitcoin can perform the three functions of money, but not equally in every place:

• As a store of value, its supply is predictable, but its market price is volatile.
• As a medium of exchange, it can move without a bank, but acceptance, fees, and payment tools vary.
• As a unit of account, amounts can be stated in bitcoin or sats, but most goods are still priced in fiat currency.

Bitcoin therefore presents both benefits and trade-offs.

Potential benefits include direct ownership, predictable supply, global transfer, open participation, transparent rules, and resistance to unilateral censorship.

Trade-offs include price volatility, personal security responsibility, the fact that a valid payment is usually difficult or impossible to reverse, changing transaction fees, dependence on electricity and communications for digital transfer, public transaction records, and limited acceptance.

Whether you call Bitcoin sound money should follow from your criteria. Compare it with fiat money, precious metals, and other assets using the same standards.

Local-value check: Does bitcoin have value in your community? Separate three questions in your answer: What market price can be observed? Where, if anywhere, is it accepted? What personal preference or bias affects your judgment? Price, local usefulness, and personal approval are related but not identical.

Digital assets can look similar on a screen while relying on very different systems. Bitcoin has public supply rules and no issuer that can create new bitcoin outside those rules. A stablecoin normally depends on an issuer, reserves, legal claims, or a mechanism intended to track another currency. Other crypto-assets may have different people or organizations controlling issuance, changing rules, or operating important parts of the system.

When comparing any digital asset, ask: Who controls issuance or changes? What can I verify independently? Who holds the keys? Which company, issuer, or other counterparty must I trust? This Bitcoin-only Diploma uses the comparison only to clarify Bitcoin. It does not recommend acquiring, trading, or promoting any asset.

The comparison below distinguishes categories and design choices. It is not an endorsement, recommendation, or a claim that any asset is safe, reliable, or suitable for you.

Bitcoin (BTC) is a digital asset on the Bitcoin network. Its maximum supply and issuance schedule are public rules. People can hold bitcoin directly through keys and can independently verify Bitcoin rules by running suitable software. It can be sent globally without needing a central Bitcoin issuer to approve every transaction. Its market price can change sharply, and losing keys or recovery information can mean permanent loss; a mistaken payment is usually hard or impossible to reverse. Fees, confirmation time, electricity, communications, public transaction records, and local acceptance can also affect use. A wallet provider or exchange can add its own counterparty and custody risks.

Some projects use Bitcoin's code as a starting point but operate under different rules or networks. These are often called forks. A shared code history does not make a fork part of Bitcoin or give it Bitcoin's rules, participants, or security.

Stablecoins, such as USD Coin (USDC) and Tether (USDT), are digital tokens designed to track a reference value, often one unit of a national currency such as the U.S. dollar. Some may be useful for moving a token value between compatible services or networks. Their design, reserves, redemption rights, and controls vary substantially. A stablecoin can lose its intended value (“depeg”), and users may depend on an issuer, reserve custodian, bank, legal redemption process, blockchain operator, or bridge. Issuers or platforms may freeze tokens or restrict access where their rules or laws allow. Reserve quality, transparency, insolvency, regulation, smart-contract errors, scams, and network fees are additional risks.

Other crypto-assets are a broad category of assets used by other networks or applications. Their rules may support functions such as paying network fees, staking, governance, or application use. Issuance, control, technical design, and what can be independently verified differ from one asset to another. Prices and liquidity can change rapidly, and developers, foundations, validators, administrators, or large holders may have significant influence, depending on the system. Rule changes, token issuance, smart-contract bugs, hacks, bridge failures, scams, platform failure, and unclear legal treatment can create loss or access risks. Holding a token does not automatically give a user ownership of a company, guaranteed income, or a right to redeem it for another asset.

Before using any digital asset, distinguish the asset itself from the service holding it for you. A familiar ticker, a stable-looking price, or a well-designed app does not remove the need to identify and compare custody, counterparty, technical, legal, and loss risks.

Learning how Bitcoin works is different from trying to profit from short-term price changes, persuading others to buy a token, or treating speculation as education. Trading and speculation can involve rapid decisions, loss, platform risk, and pressure to act without clear reasoning. Token promotion can create conflicts of interest and reward persuasion rather than clear explanation.

The purpose of the Bitcoin Diploma is to help you explain Bitcoin, ask better questions, and make safer decisions. It does not provide investment advice or tell you what to buy, sell, or hold.

Bitcoin appeared through a short series of public events.

Explore the source records: read the Bitcoin whitepaper (source PDF), inspect the Genesis Block (block record) and the first recorded person-to-person transaction (transaction record), and read the original pizza request. A block explorer is a convenient interface to public data, not a primary authority over Bitcoin's rules.

Satoshi Nakamoto is the pseudonym used by Bitcoin's unknown creator or creators. Satoshi communicated online, released the software as open source, and later stopped participating.

No central company owns Bitcoin. The software and rules continued because other people chose to run, review, and develop them. This origin matters: the creator did not remain as a permanent leader who could issue more coins or approve transactions.

The Genesis Block and the 2008 financial crisis give Bitcoin important historical context. However, evaluate Bitcoin by its design and operation, not only by a story about its creator.

In October 2008, Satoshi Nakamoto announced the Bitcoin whitepaper. Its central question was simple: how can people make online payments without letting the same digital unit be spent twice or putting one intermediary in control?

Bitcoin's public rules, peer-to-peer network, and digital signatures give participants meaningful roles. Users can choose to receive, hold, and send bitcoin; nodes can independently verify the rules; miners can propose blocks; and developers can propose software changes. No single role controls the network for everyone.

These properties can empower participants, but they also place more responsibility on them. Without a central authority to approve transactions, recover access, or reverse many errors, each person must be able to identify the role and risks they accept.

For a person who chooses self-custody, control of the keys brings direct control of bitcoin. No company can freeze a correctly controlled on-chain balance. No company can restore a lost seed phrase either.

Self-custody is therefore not just a technical feature. It requires protecting backups, verifying addresses, checking wallet behavior, and creating a recovery plan. The next module explains how to hold and use bitcoin carefully, including how to manage these risks and responsibilities.

Pause and consider how public rules, finite issuance, peer-to-peer transfer, and independent verification respond to the flaws identified in Module 1: changing supply, institutional control, restricted access, and reliance on trust. Why do nodes and miners have different roles, and why must double spending be prevented? Bitcoin replaces some institutional trust with verification and personal responsibility; it does not remove risk or trade-offs.

Key thought: Based on what you have learned about Bitcoin in this module, does it have value to you? Does the current price of bitcoin represent its value?

Key takeaways

• Bitcoin was created to make digital transfer possible without a central ledger keeper while preventing double spending.
• Its public ledger, digital signatures, peer-to-peer network, proof of work, and independent verification work together to maintain a shared transaction history.
• Bitcoin is decentralized in the sense that no single participant can unilaterally change balances or the rules for everyone.
• Users choose whether to hold or accept bitcoin; nodes verify the rules; miners propose blocks; and developers can propose, but not impose, software changes.
• Bitcoin has a public supply schedule with a maximum of 21 million BTC, and nodes reject blocks that break this rule.
• One bitcoin (1 BTC) divides into 100,000,000 sats, so people can use smaller units without needing a whole bitcoin.
• Bitcoin can be evaluated as sound money using monetary properties, while still recognizing the trade-offs: price volatility, limited acceptance, payments that are usually difficult or impossible to reverse, and the need for increased personal responsibility.

A moment to reflect

Which part of Bitcoin's design would you explain differently now, and why?