Blockchain

A blockchain is a shared digital ledger that many computers keep in identical copies. New records are grouped into blocks and added in time order. Each block carries a cryptographic fingerprint of the block before it, which links the blocks into a chain. Because every copy is checked against the others and each block depends on the one before it, an entry that has been accepted is extremely hard to change without the rest of the network noticing.

What Is a Blockchain?

A blockchain is a distributed ledger: a record of transactions or other data that is copied across a network of computers called nodes. Each node stores the ledger and runs software that follows the same rules for adding to it. When a new batch of records is accepted, every honest node adds the same block to its own copy, so the copies stay in step.

Three properties separate a blockchain from a spreadsheet saved on a shared drive:

1. Append-only structure. Data is added at the end of the chain. Earlier blocks are not edited or overwritten. A correction is made by adding a new entry that reverses or updates the old one, the way an accountant posts an adjusting entry instead of erasing a line in the books.
2. Cryptographic linking. Every block contains the hash of the previous block. That makes the chain’s history self-checking: anyone can recompute the hashes and see whether something was altered.
3. Shared control. On a public blockchain, the right to add the next block is decided by a consensus process rather than by an administrator. No one has a master password that lets them rewrite the past.

Because of these properties, the trust a blockchain asks for moves to a different place. You do not need to trust a single record keeper to keep honest books. You need to trust the rules of the protocol, the cryptography, and the assumption that no one controls a majority of the network. You also still need to trust whoever entered the data, because the chain records what it is given.

Blockchains became widely known through Bitcoin, which launched in January 2009. Since then, thousands of other networks and tokens have been built on the same idea, along with decentralized finance (DeFi) applications, non-fungible tokens and smart contracts.

How Does a Blockchain Work?

Hashing: A Fingerprint for Data

A cryptographic hash function takes any input, whether a single word or an entire file, and produces a fixed-length output called a hash. Bitcoin uses SHA-256, which always produces a 256-bit result, usually written as 64 hexadecimal characters (the digits 0 to 9 and letters a to f).

A good hash function has a few properties that make blockchains possible:

• Deterministic: the same input always produces the same hash.
• Fast to compute, practically impossible to reverse: you can hash data in a fraction of a second, but you cannot work backward from a hash to the original input.
• Sensitive to change: altering even one character of the input produces a completely different hash with no visible relationship to the old one.
• Collision resistant: it is computationally infeasible to find two different inputs that produce the same hash.

The sensitivity property is the one that protects the ledger. If someone changes a single digit in a recorded transaction, the hash of that data changes beyond recognition, and the mismatch is obvious to anyone who recomputes it.

Blocks and the Chain

A block has two parts: a header and a body. The body holds the data itself, which on Bitcoin is a list of transactions. The header is a short summary that includes, on Bitcoin:

• the hash of the previous block’s header
• a Merkle root, a single hash that summarizes every transaction in the block (transactions are hashed in pairs, then the results are hashed in pairs, and so on until one hash remains)
• a timestamp
• the current difficulty target
• a nonce, a number miners change while searching for a valid block

The header is then hashed to produce the block’s own identifier. The next block puts that identifier into its header, and so on. The chain begins with a genesis block, the first block that has no predecessor. Bitcoin’s genesis block is dated January 3, 2009.

The Merkle root matters for two reasons. It ties every transaction to the header, so changing any transaction changes the root and therefore the block hash. It also lets a lightweight wallet confirm that a transaction is included in a block by checking a short chain of hashes instead of downloading the whole block.

Worked Example: Why Tampering Gets Caught

The hashes below are shortened, made-up labels for illustration. Real hashes are 64 characters long.

Imagine a small chain of three blocks:

• Block 1 records “Ana pays Ben 5 units.” Its header hash is A1.
• Block 2 stores A1 as its “previous hash,” records “Ben pays Cara 2 units,” and has its own hash, B2.
• Block 3 stores B2 as its previous hash, records “Cara pays Dev 1 unit,” and has hash C3.

Now suppose Ben edits his copy of Block 1 so it reads “Ana pays Ben 50 units.” Recomputing Block 1’s hash no longer gives A1. It gives something unrelated, say X9. Block 2 still says the previous hash is A1, so the link between Block 1 and Block 2 is broken.

To hide the edit, Ben would have to rebuild Block 2 so it points to X9, which changes Block 2’s hash, which forces him to rebuild Block 3, and so on to the newest block. On a proof-of-work chain, every rebuilt block requires redoing the computational work that made it valid in the first place. Meanwhile, every other node still holds the original chain ending in C3. When Ben broadcasts his version, the other nodes compare it with theirs, see that it does not match the chain the network built, and reject it.

Transaction Process

The steps differ from network to network. Here is how a payment works on Bitcoin.

1. You create a transaction in a wallet. A crypto wallet is software that holds your private keys and gives you an interface to the blockchain. It builds a message that says which coins you are spending and which address receives them.
2. You sign it. The wallet uses your private key to produce a digital signature. Anyone can verify the signature with your public key, but no one can produce it without the private key, so the network knows the owner authorized the payment.
3. The transaction is broadcast. Nodes check that the signature is valid and the coins have not already been spent, then pass valid transactions along to other nodes.
4. It waits in the mempool. Each node keeps a memory pool of valid transactions that are not yet in a block. Miners usually pick the transactions that pay the highest fee per unit of block space, so a low fee can mean a longer wait.
5. A miner includes it in a candidate block. Each miner assembles its own candidate block from the mempool, so candidate blocks differ.
6. Mining begins. Miners hash their candidate block header over and over, changing the nonce each time, looking for a hash equal to or below the network’s target. The 32-bit nonce runs out after about 4.3 billion values, which modern mining hardware exhausts in well under a second, so miners also change an “extra nonce” inside the block and keep searching.
7. A valid block is found and shared. The first miner to find a qualifying hash broadcasts the block. Other nodes check it, which takes a single hash calculation, and add it to their copy of the chain. The winning miner collects the block reward plus the fees of the transactions it included.
8. Confirmations accumulate. Your transaction now has one confirmation, and each later block adds another. A common rule of thumb for larger Bitcoin payments is to wait for six confirmations, about one hour with blocks arriving roughly every 10 minutes.

Bitcoin’s 10-minute average is maintained by a difficulty adjustment. Every 2,016 blocks, roughly every two weeks, the network resets the target so blocks keep arriving at about the same pace whether more or less computing power joins.

Consensus: Proof of Work and Proof of Stake

A distributed ledger needs a way for nodes that do not know or trust one another to agree on which block comes next. That is the job of the consensus mechanism.

Proof of work (PoW) is the system described above. The hashing itself is the “work,” and a valid block proves that its creator spent real computing effort and electricity. Rewriting history would require redoing that work faster than the honest network is extending the chain, which is why PoW chains get harder to attack as more computing power secures them. The cost is energy: the same work that secures Bitcoin is why the network consumes so much electricity. Bitcoin mining today is done almost entirely on specialized hardware (ASIC miners).

Proof of stake (PoS) replaces computing power with locked-up capital. Validators deposit, or “stake,” the network’s native token. The protocol selects validators to propose blocks, and other validators attest that the blocks are valid. On Ethereum, which switched from proof of work to proof of stake in September 2022 in an upgrade known as the Merge, a validator stakes 32 ETH, and a new slot for a block opens every 12 seconds. Validators that break the rules, for example by signing two conflicting blocks, can have part of their stake destroyed, a penalty called slashing. PoS uses far less energy than PoW because no one has to win a hashing race.

Other designs exist, such as delegated proof of stake and various voting-based protocols used on private networks, but PoW and PoS cover the largest public blockchains.

What Does a Blockchain Look Like?

A blockchain has no visual form of its own; it is data stored on disk. Most people “see” one through a block explorer, a website or app that reads the chain and displays it in a readable form. A typical explorer view shows:

• a list of recent blocks with their height (position in the chain), timestamp and number of transactions
• for each block, its hash and the hash of the previous block
• for each transaction, the sending and receiving addresses, the amount, the fee and the number of confirmations

An address looks like a long string of letters and numbers. A transaction ID is the hash of the transaction data. If you have ever been given a “TXID” after sending crypto, that is the identifier you can paste into an explorer to follow the payment.

Blockchain Decentralization

On a public blockchain, the ledger is spread across nodes in many locations, run by different people and organizations. Each full node downloads every block, checks every transaction against the protocol’s rules, and keeps its own copy. That redundancy protects the data, because an altered copy’s hashes will not match the others. It also removes the need for a central gatekeeper: no single node can change a balance, permanently censor a valid transaction or rewrite a past block. Changing the rules themselves requires most of the network to adopt new software.

It helps to separate the roles:

• Full nodes verify and store the chain. Anyone with enough storage and bandwidth can run one.
• Miners (on PoW) or validators (on PoS) produce new blocks. They are usually also full nodes, but most full nodes are not miners.
• Light clients, which include many mobile wallets, keep only block headers and rely on full nodes for the rest.

Decentralization is a spectrum, not a yes-or-no label. A network with a handful of large mining pools or a few dominant staking providers is more concentrated than one where block production is widely spread, even though both are technically decentralized.

Blockchains can also hold data other than payments. A private blockchain might track legal agreements, identity records or a company’s inventory. Large files are usually not stored on the chain directly. Instead, the document is hashed and the hash (or a token representing it) is recorded on-chain, which proves the document existed in that exact form at that time without filling every node’s disk with copies of it.

Blockchain Transparency

On a public blockchain, every transaction is visible to anyone, either by downloading the full chain or through a block explorer.

This makes it possible to follow funds from address to address. When crypto exchanges have been hacked, investigators and the public have been able to trace the stolen coins on-chain because every movement is recorded, even though the thieves were not named on the ledger.

Transparency does not mean users are identified by name. The chain shows addresses, not identities, which is why Bitcoin is better described as pseudonymous than anonymous. Your address works like a pen name. If it is ever tied to you, for example through an exchange account that verified your identity, a reused address or a public post, your whole transaction history at that address becomes readable. Some networks add privacy features that hide amounts or addresses, and private blockchains may not expose their data publicly at all.

Is Blockchain Secure?

A blockchain’s security comes from several layers working together:

• Order and linking. Each block contains the previous block’s hash, so altering any block invalidates everything after it.
• Distribution. Thousands of nodes hold copies, and a node presenting an altered history is outvoted by the copies that match.
• Economic cost. On PoW chains, rewriting history means redoing the work. On PoS chains, it means controlling a large share of the staked tokens and risking having that stake slashed.
• Signatures. Only the holder of a private key can authorize spending from its address.

None of this makes blockchains impenetrable. The main weaknesses are worth understanding separately because they fail in different ways.

The 51% Attack

If one person or group controls more than half of a proof-of-work network’s computing power, they can produce blocks faster than everyone else combined. That would let them reverse their own recent transactions (spending coins, then replacing the block that recorded the payment, a “double spend”) or block other transactions from being confirmed. They still could not forge signatures or spend coins from addresses they do not control.

On a large network such as Bitcoin, gathering that much hashing power would require an enormous amount of specialized hardware and electricity, and the honest network keeps extending the chain while an attacker tries to catch up. Smaller chains with little computing power behind them are far more exposed, and several have suffered such attacks.

Proof of stake has its own version of this threshold: an attacker would need to control a large share of all staked tokens. On Ethereum, that means acquiring a very large amount of ETH, and misbehavior can be punished by slashing the attacker’s own stake.

Code Vulnerabilities

Blockchains are software. Bugs in the core protocol, in a smart contract, or in a bridge that moves tokens between chains can be exploited. Many of the largest losses in crypto have come from flawed smart contracts or bridges rather than from breaking the underlying chain’s cryptography.

Keys and Custodians

The chain can be secure while the user is not. If someone steals your private key, the network treats their transactions as yours, and there is no help desk to reverse them. If you lose your key, the coins at that address are effectively gone.

Many people also keep crypto on exchanges, which hold the keys on their behalf. When an exchange collapses or is hacked, customers can lose funds even though the blockchain itself worked as designed. Mt. Gox, which collapsed in 2014, and FTX, which failed in November 2022, were failures of companies holding customer assets, not of the Bitcoin or Ethereum ledgers.

Finality

On proof-of-work chains, a recent block can occasionally be replaced if two miners find blocks at nearly the same time and the network later settles on the other branch. This is why confirmations matter: each additional block makes reversal less likely. Proof-of-stake networks such as Ethereum have an explicit notion of finality, a checkpoint after which reversing blocks would require destroying a large amount of staked ETH.

Types of Blockchains

Not every blockchain is open to the public. The main categories differ in who can read the data, who can submit transactions and who can produce blocks.

• Public (permissionless) blockchains such as Bitcoin and Ethereum let anyone read the ledger, send transactions and, with the right equipment or stake, help produce blocks. They offer the most resistance to censorship and tampering, at the cost of speed and efficiency.
• Private blockchains are run by a single organization that decides who may participate. They can be faster and keep data confidential, but they rely on that organization in much the same way a conventional database does.
• Consortium blockchains are governed by a group of organizations, such as several banks or members of a supply chain. Block production is shared among the members.
• Permissioned blockchains is a broad term for any network where participation requires approval. Private and consortium chains are both permissioned.

The trade-off is consistent: the more participants are vetted and the fewer validators there are, the faster and cheaper the system can be, and the more it depends on trusting those validators.

Blockchain vs. Bitcoin vs. Cryptocurrency

These terms are often used interchangeably, but they name different things.

Blockchain

Blockchain is the record-keeping technology: linked blocks, hashing, distribution and consensus. The core idea predates Bitcoin. In 1991, researchers Stuart Haber and W. Scott Stornetta described a method for timestamping digital documents so the timestamps could not be backdated or altered, chaining each record to the ones before it. Their work, published in the Journal of Cryptology, is widely cited as an ancestor of the blockchain.

A blockchain can record many kinds of data: payments, votes, product histories, identity records, property titles and more. It does not need a currency at all, although public blockchains usually have one to pay validators and fees.

Bitcoin

Bitcoin is a specific digital currency and network that uses a blockchain to keep a public ledger of payments. Its pseudonymous creator, Satoshi Nakamoto, published the white paper “Bitcoin: A Peer-to-Peer Electronic Cash System” on October 31, 2008, and the network went live in January 2009. The paper described a system for sending payments directly between people without a trusted third party.

Bitcoin was the first working application of a blockchain at scale. The supply of bitcoin is capped at 21 million coins, and the block reward miners receive is cut in half every 210,000 blocks, roughly every four years.

Cryptocurrency

Cryptocurrency is the broader category of digital assets recorded on blockchains. Bitcoin is one cryptocurrency. Ether is the native asset of Ethereum. Many other tokens are created on top of existing blockchains through smart contracts rather than having their own network.

A simple way to keep them apart: blockchain is the ledger, cryptocurrency is a kind of asset recorded on it, and Bitcoin is one particular asset and network.

Blockchain vs. a Traditional Database

A conventional database, the kind behind a bank’s account system or an online store, is run by one organization. Administrators can create, read, update and delete records. Access is controlled by the owner, and users trust the owner to keep accurate data.

A public blockchain differs in several ways:

• Control: many independent parties maintain it, rather than one administrator.
• Edits: records are append-only. The usual database operations of updating and deleting are deliberately unavailable for confirmed data.
• Verification: every full node checks every change against shared rules before accepting it.
• Performance: a central database can handle far more operations per second and change data instantly. A blockchain trades speed for resistance to tampering and for not needing a trusted operator.
• Privacy: a central database can keep records confidential. A public blockchain exposes them to everyone, pseudonymously.

The practical question for any project is whether those trade-offs are worth it. If a single trusted organization already keeps the records and everyone is satisfied with that, a regular database is simpler and faster. A blockchain earns its overhead when several parties who do not fully trust one another need to share one record, or when no central record keeper is available or wanted.

Blockchain vs. Banks

Blockchains are often described as a challenge to banking, especially for payments. Comparing a bank with Bitcoin’s network shows how different they are:

• Hours: banks process many transfers during business hours, typically five days a week. A blockchain runs 24 hours a day, every day of the year.
• Settlement time: a bank transfer can take one to three business days to clear. A Bitcoin payment is usually included in a block within minutes to an hour depending on fees, and is commonly treated as settled after six confirmations.
• Identity: banks must verify customers’ identities when they open accounts. Using a public blockchain directly requires only a wallet; identity checks happen at the edges, for example when you buy crypto through a regulated exchange.
• Reversibility: banks can freeze, reverse or charge back transactions in some circumstances. Confirmed blockchain transactions cannot be reversed by any authority; a refund requires the recipient to send a new payment.
• Fees: bank and card fees are set by institutions. Blockchain fees are set by demand for block space and can rise sharply when the network is congested.
• Protection: in the US, eligible bank deposits are covered by FDIC insurance up to the legal limit. Crypto held in a personal wallet has no such protection.
• Privacy: bank records are private between you, the bank and authorities. Public blockchain records are visible to everyone, tied to addresses rather than names.

How Are Blockchains Used?

Cryptocurrency transactions remain the most established use. Blockchains have also been applied, or are being tested, in a range of other fields. Large companies including Walmart, Pfizer, AIG, Siemens and Unilever have experimented with the technology.

Currency and Payments

Blockchains are the foundation of cryptocurrencies. Because the network is reachable by anyone with an internet connection, a payment can cross borders without passing through correspondent banks, currency controls or local banking infrastructure. That has particular appeal in countries with unstable currencies or limited banking.

Stablecoins, tokens designed to track a reference asset such as the US dollar, have become a widely used way to move dollar value on blockchains without exposure to the price swings of assets like bitcoin.

Banking and Finance

Financial institutions are interested in blockchains for settlement, the step where money and assets actually change hands after a trade. In US stock markets, the standard settlement cycle is one business day after the trade (T+1), and international trades can take longer. During that period, cash and securities are in transit, which ties up capital and creates the risk that one side fails to deliver. A shared ledger that records the transfer of both the asset and the payment at the same moment could, in principle, shorten that window sharply.

Smart Contracts

A smart contract is a program stored on a blockchain that runs automatically when its conditions are met. For example, a contract could hold funds from a buyer and release them to the seller once a delivery confirmation is recorded, without an escrow agent in between. Ethereum was built specifically to run such programs.

Smart contracts underpin decentralized exchanges, lending platforms and many other DeFi applications. Their weakness is that the code does exactly what it says, including any mistakes in it. Once deployed, a flawed contract may be difficult or impossible to fix.

Tokenization and NFTs

A blockchain can record ownership of a token that represents something else: a share of a fund, a bond, a piece of art or a ticket. Non-fungible tokens (NFTs) are unique tokens, each with its own identifier, used for digital collectibles, art and access passes. Tokenized versions of traditional assets aim to make ownership easier to transfer and divide.

The token proves who holds the token. Whether that also gives a legal claim to the underlying asset depends on the agreements and laws outside the chain.

Supply Chains

Suppliers can record where materials came from and each step they pass through on the way to the customer. This lets companies verify a product’s origin and support labels such as “organic,” “local” or “fair trade.”

Food safety is one of the clearest examples. IBM’s Food Trust platform was built to trace food products from their origin through each supplier. When an outbreak of E. coli, salmonella or listeria occurs, investigators have historically needed weeks to trace the source. A shared record of each handoff lets a company identify affected products and batches far more quickly.

Healthcare

Hospitals and clinics can record medical records, or hashes of them, on a blockchain once they are created and signed. The record then serves as proof that the document has not been changed since. Sensitive health data can be encrypted so only people holding the right keys can read it, which helps preserve patient privacy while keeping the record verifiable.

Property Records

Recording property ownership is often slow and paper-heavy. In many US counties, a deed must be delivered to the local recording office and entered into a central database and public index. When ownership is disputed, claims have to be reconciled with that index, which costs time and money and invites human error.

A blockchain-based registry could keep a permanent, time-ordered history of who owned a property and when, without scanning and tracking down physical files. It could matter most where formal records barely exist, such as in conflict zones or regions without a functioning recorder’s office, where proving ownership can be nearly impossible.

Voting

Researchers and some election officials have explored blockchain voting. One proposed design issues each eligible voter a single token and gives each candidate a wallet address; voters send their token to their chosen candidate, and the chain itself becomes the tally. West Virginia ran a blockchain-based mobile voting pilot for certain overseas and military voters in the November 2018 midterm elections.

Supporters point to tamper-resistant records and fast results. The harder problems lie outside the ledger: verifying that each voter is who they claim to be, keeping ballots secret, and securing the phones and apps voters use. A blockchain cannot fix a compromised device that casts the wrong vote before the data ever reaches the chain.

Pros and Cons of Blockchain

Pros

• Lower intermediary costs: a blockchain can replace some third-party verification, such as a bank confirming a payment or a notary witnessing a document, along with its fees. Merchants who accept cards pay processing fees to banks and card networks; a direct blockchain payment has only a network fee.
• Tamper resistance and accuracy: many independent computers verify each transaction under the same rules, and there is no single database to break into and rewrite.
• Availability: the network runs around the clock, and on some blockchains transactions confirm in seconds or minutes, which helps most with cross-border payments that otherwise wait on banking hours and time zones.
• Open code: many blockchains are open source, so anyone can audit the code, and upgrades take effect only if enough of the network adopts them.
• Access for the unbanked: anyone with an internet connection can hold and send value. The World Bank estimated, based on 2024 data, that about 1.3 billion adults worldwide have no bank account, most of them in developing economies that depend heavily on cash.

Cons

• Technology and energy cost: proof-of-work chains consume large amounts of electricity. The University of Cambridge’s Bitcoin Electricity Consumption Index compares the network’s estimated use with that of entire countries, and has put it above Finland’s. Some miners use solar, wind or excess natural gas that would otherwise be flared.
• Low throughput: Bitcoin handles roughly seven transactions per second. Visa, for comparison, has been cited at about 1,700 per second. Ethereum and newer networks handle more, but the need for many nodes to agree still limits base-layer capacity.
• Illicit use: pseudonymity has been exploited for crime. The best-known example is Silk Road, a dark web marketplace for illegal drugs that operated from January 2011 until the FBI shut it down in October 2013, with payments made in bitcoin.
• Regulatory uncertainty: rules differ widely by country and keep changing. So far, regulation has largely targeted crypto assets, exchanges and other products built on blockchains rather than the underlying technology.
• Storage growth: a chain only grows. Nodes that keep the full history need more storage every year.

Limitations Worth Understanding

The Scalability Trade-Off

A blockchain wants to be secure, decentralized and fast, and improving one usually costs another. Larger or more frequent blocks raise throughput, but they also make it harder for ordinary people to run full nodes, which concentrates the network. This is why many networks are scaling through additional layers rather than by enlarging the base chain. Bitcoin’s Lightning Network moves small payments off-chain and settles the totals on-chain later. Ethereum relies on rollups, which bundle many transactions and post compressed data back to the main chain, and its roadmap includes data-sharding upgrades designed to make that cheaper.

Garbage In, Garbage Out

A blockchain guarantees that data has not changed since it was recorded. It does not guarantee the data was correct to begin with. If a supplier records a false origin for a shipment, the chain will faithfully preserve the false claim. When a smart contract depends on outside information, such as a price or a delivery confirmation, it needs a data feed called an oracle, and the contract is only as trustworthy as that feed. This is sometimes called the oracle problem.

Immutability Cuts Both Ways

The same property that stops fraud stops corrections. Personal data written to a public chain cannot be deleted later, which clashes with privacy laws that give people a right to have their data erased. Projects handle this by keeping personal data off-chain and recording only hashes or references.

Governance

With no central owner, changing the rules of a public blockchain requires broad agreement. When a community disagrees, the chain can split into two versions in a hard fork, each with its own history after the split. Bitcoin Cash, which split from Bitcoin in 2017, is one example.

How Are Blockchains Stored?

Every full node stores the blockchain on its own disk, along with an index of the current state, such as which coins are unspent on Bitcoin or the balance and code of each account on Ethereum. These two networks track ownership differently. Bitcoin uses an unspent transaction output (UTXO) model, where your balance is the sum of unspent coins sent to your addresses. Ethereum uses an account model, more like a bank account, where each address has a balance that goes up and down.

Not every participant needs the whole history:

• Archive nodes keep every block and every past state. Block explorers and analytics services often run these.
• Full nodes verify every block from the beginning. Some run in “pruned” mode, deleting older block data once it has been checked while keeping what is needed to validate new blocks.
• Light clients keep only block headers and request proofs from full nodes when they need to confirm a transaction.

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