# What is a blockchain? > A blockchain is a growing list of records, called blocks, that are linked together using cryptography ---  From: [Wikipedia Blockchain](https://en.wikipedia.org/wiki/Blockchain) --- # Example: Bitcoin > Bitcoin (₿) is a decentralized digital currency, without a central bank or single administrator, that can be sent from user to user on the peer-to-peer bitcoin network without the need for intermediaries --- # Structure Logically, a blockchain can be seen as consisting of several layers: * infrastructure (hardware) * networking (node discovery, information propagation and verification) * consensus (proof of work, proof of stake) * data (blocks, transactions) * application (smart contracts/decentralized applications, if applicable) --- We will focus on data and consensus (proof of work) --- ## Our toolbox 🧰 * Hash function A cryptographic hash function (CHF) is a mathematical algorithm that maps data of arbitrary size to a bit array of a fixed size. It is a one-way function, that is, a function which is practically infeasible to invert or reverse the computation. https://andersbrownworth.com/blockchain/hash --- * Nonce https://en.wikipedia.org/wiki/Cryptographic_nonce#Hashing The PoW requires miners to find a number called a nonce, such that when the block content is hashed along with the nonce, the result is numerically smaller than the network's difficulty target --- * Difficulty Every 2,016 blocks (approximately 14 days at roughly 10 min per block), the difficulty target is adjusted based on the network's recent performance, with the aim of keeping the average time between new blocks at ten minutes https://andersbrownworth.com/blockchain/block --- Demo part 2 Blockchain Demo: Public / Private Keys & Signing https://andersbrownworth.com/blockchain/public-private-keys/ --- ### Questions?