# Blockchain Technology
Confidence: high
Last verified: 2026-05-22
Generation: human_only


## TL;DR

Blockchain is a distributed, immutable ledger where transactions are grouped into blocks, cryptographically linked in a chain. Satoshi Nakamoto's Bitcoin (2009) was the first. Consensus mechanisms: Proof-of-Work (Bitcoin, mining), Proof-of-Stake (Ethereum since 2022, validators stake). Blockchain = decentralized trust without intermediaries.

## Core Explanation

Block: transactions + timestamp + hash of previous block. Changing any past block invalidates all subsequent hashes — computationally infeasible. Merkle tree: efficiently verify transaction inclusion. Smart contracts (Ethereum): code stored on blockchain, executed by validators. Beyond currency: supply chain, digital identity, voting, DeFi. Public vs. private (permissioned) blockchains. 51% attack: controlling majority of hash power/stake enables double-spending.

## Further Reading

- [Bitcoin: A Peer-to-Peer Electronic Cash System (Nakamoto, 2008)](undefined)