What Is Blockchain? Core Concepts and Fundamentals

Although definitions of "blockchain" vary, they can be divided into narrow and broad levels. The narrow definition refers to a data structure that links data blocks in chronological order with cryptographic protection, forming a decentralized shared ledger that securely stores simple, sequentially verifiable data. The broad definition treats blockchain as a decentralized infrastructure and computing paradigm that uses the cryptographic chain, distributed consensus, and programmable smart contracts; this broader sense is more commonly used.

In essence, blockchain is a distributed storage system. Its basic unit is the data block, which consists of a block header (containing metadata) and a block body (containing transactions). The block header stores version, previous‑block hash, difficulty target, timestamp, nonce, and the Merkle‑tree root used to verify the block’s transactions.

The block body is the list of transactions, including coinbase and regular transactions.

A hash function maps data of any length to a fixed‑size (typically 256‑bit) value; each block’s header is double‑hashed to produce the block’s hash, which uniquely identifies the block.

The chain structure links blocks together: each block contains pointers (hashes) to its predecessor and successor, forming a linear chain where only the first and last blocks have a single neighbor. Modifying any block would require recomputing all subsequent hashes, making tampering extremely difficult.

A Merkle tree is a binary hash tree whose leaves are the hashes of individual transactions; internal nodes are hashes of their child nodes. Any change in a transaction alters the root hash, enabling efficient verification of data integrity and supporting features such as zero‑knowledge proofs.

A timestamp is a digital signature that proves a file existed before a certain time; in blockchain, each new block receives a timestamp that is hashed and included in subsequent blocks, creating a decentralized, tamper‑resistant time‑stamping service.

Asymmetric encryption uses a pair of keys (public and private) so that data encrypted with one key can only be decrypted with the other; this underpins blockchain’s encryption, digital signatures, and authentication mechanisms.

The P2P (peer‑to‑peer) network connects nodes without a central authority, enabling decentralized communication and data exchange.

When a block or transaction is created, the originating node broadcasts it to the network. The Bitcoin propagation protocol, for example, involves broadcasting new transactions, collecting them into a block, performing proof‑of‑work, broadcasting the valid block, and having other nodes verify and accept it before extending the chain.

Each node continuously listens for incoming data, validates its correctness, stores valid data in a pool, forwards it to peers, and discards invalid data, ensuring that only legitimate information propagates through the blockchain network.