What Is a Blockchain?

A blockchain is a ledger maintained across multiple computers rather than by a single database server. Participants follow a protocol that defines valid transactions, how records are grouped, how new history is accepted, and how each node verifies the resulting state. Cryptographic hashes link records so that altering earlier data changes later references and becomes detectable.

The defining idea is not “data stored on the internet.” It is replicated state plus a rule-governed process for deciding which updates become part of the accepted history.

Transactions are proposed changes. Blocks or similar ordered batches package accepted transactions. Nodes receive, validate, store, and relay protocol data. Cryptographic hashes create compact fingerprints of records. Consensus rules determine which history or state the network treats as canonical when participants receive competing information.

Some blockchains use an account model; others use unspent transaction outputs or other state models. The state model determines what a valid transaction is allowed to consume and create.

A user creates a transaction and authorizes it, commonly with a digital signature. Nodes check syntax, authorization, balances or inputs, and protocol-specific validity. The transaction is propagated to peers. A block producer or validator includes it in a candidate block or ordered batch. Other nodes independently verify the candidate. If it satisfies the protocol's acceptance rules, nodes update local state to reflect the transaction.

Consensus is needed because different nodes can see messages in different orders, and multiple candidate histories can temporarily exist.

A blockchain cannot verify that a real-world claim was true when entered. If an oracle reports a false price or a user signs a malicious transaction, the ledger can faithfully preserve the wrong input. Public blockchains are not automatically private; data can be visible even when user names are pseudonymous. Smart contracts can contain bugs, and bridges or wallets can fail even if the base chain continues operating correctly.

NIST IR 8202 describes blockchains as tamper-evident and tamper-resistant digital ledgers implemented in a distributed fashion and commonly without a central authority. The report separates the core mechanisms—hashing, digital signatures, consensus, and distributed data—from the marketing claims often attached to the term.

That distinction is essential for evaluating a proposed use case: ask which trust problem the blockchain actually solves and which risks remain outside the ledger.

1. Explain the difference between a blockchain as a replicated data structure and the consensus mechanism used to agree on its history.

2. Trace a transaction from creation to validation, inclusion in a block, propagation, and eventual acceptance by other nodes.

3. Why does linking blocks with cryptographic hashes make later tampering detectable but not physically impossible?

4. Compare permissionless and permissioned blockchain systems. Which trust assumptions change?

5. Identify one property that comes from cryptography, one that comes from consensus, and one that comes from network replication.

6. Correct this statement: “A blockchain is trustworthy simply because every record is encrypted.”