Data & cryptography
What is a hash function?
A hash function is a calculation that turns data of any size into a short string of fixed length — a digital fingerprint that changes completely if even one bit of the input changes.
Blockchain concepts, one card each
Thirty cards in five collections. Each one explains a single concept and links to three related cards.
Data & cryptography
Fingerprints, keys and signatures: the maths that lets strangers check each other. About this collection
Data & cryptography
What is SHA-256?
SHA-256 is a specific, standardised hash function from the SHA-2 family that turns any input into a 256-bit fingerprint, and it is the hash function at the heart of Bitcoin.
Data & cryptography
What is a Merkle tree?
A Merkle tree is a way of hashing many items in pairs, level by level, until a single hash — the Merkle root — stands for all of them at once.
Data & cryptography
What is a public key?
A public key is the shareable half of a key pair: a value calculated from a private key that lets anyone check signatures made with that private key, without being able to make them.
Data & cryptography
What is a private key?
A private key is a secret number that lets its holder create digital signatures — and on a blockchain, whoever can sign for an address controls whatever is recorded to it.
Data & cryptography
What is a digital signature?
A digital signature is a short piece of data, made with a private key, that proves the key holder approved one specific message and that the message has not been altered since.
Data & cryptography
What is a blockchain address?
A blockchain address is a short public identifier, usually derived from a public key by hashing, that tells the network where units or tokens are recorded.
Blocks & network
Nodes, transactions and blocks: how information moves and gets recorded. About this collection
Blocks & network
What is a node?
A node is any computer that runs a blockchain's software, keeps a copy of its data and exchanges new transactions and blocks with other nodes.
Blocks & network
What is a peer-to-peer network?
A peer-to-peer network is a network in which computers connect directly to each other as equals, sharing data without routing it through a central server.
Blocks & network
What is a blockchain transaction?
A blockchain transaction is a signed instruction, broadcast to the network, that asks for the shared record to be changed in one specific way — for example, moving units between addresses or calling a smart contract.
Blocks & network
What is the mempool?
The mempool, short for memory pool, is the waiting area in which a node keeps valid transactions that have been broadcast but not yet included in a block.
Blocks & network
What is a block?
A block is a batch of transactions bundled together with a header and added to a blockchain as one unit, once the network accepts it as valid.
Blocks & network
What is a block header?
A block header is the small summary at the top of every block — exactly 80 bytes in Bitcoin — that links the block to its parent and commits to all of its transactions.
Blocks & network
What is a genesis block?
The genesis block is the first block of a blockchain — block number zero — which has no parent and is written directly into the network's software instead of being produced by consensus.
Agreement (consensus)
Nonces, proof of work, proof of stake, finality and forks: how strangers agree on one history. About this collection
Agreement (consensus)
What is a nonce?
A nonce is a "number used once": in proof-of-work mining it is the header field that miners change again and again to get a new block hash, and on Ethereum it is also the counter that numbers each account's transactions.
Agreement (consensus)
What is proof of work?
Proof of work is a consensus method in which the right to add the next block goes to whoever first finds a block hash below a target — something that takes enormous computation to find but a single calculation to check.
Agreement (consensus)
What is proof of stake?
Proof of stake is a consensus method in which participants lock up the network's own units as a deposit, take turns proposing and voting on blocks, and can lose part of that deposit if they break the rules.
Agreement (consensus)
What is a validator?
A validator is a participant in a proof-of-stake network who has locked a deposit and, in exchange, may propose blocks and must vote on which blocks are correct.
Agreement (consensus)
What is finality?
Finality is the point after which a transaction recorded on a blockchain can no longer be reversed or replaced under the network's rules.
Agreement (consensus)
What is a fork in blockchain?
A fork is a split in a blockchain: either a short-lived branch when two valid blocks appear at once, or a change to the rules that some or all nodes adopt.
Contracts & tokens
Gas, smart contracts, tokens, stablecoins and layer 2: what runs on top of a chain. About this collection
Contracts & tokens
What is gas?
Gas is the unit Ethereum uses to measure how much computational work a transaction needs; the fee a sender pays is the gas used multiplied by a fee per unit of gas.
Contracts & tokens
What is a smart contract?
A smart contract is a program stored on a blockchain that runs exactly as written whenever a transaction calls it, with every node executing it and checking the same result.
Contracts & tokens
What is a token?
A token is a unit recorded and managed by a smart contract on an existing blockchain, which can stand for almost anything — a vote, a ticket, a game item or a claim on something held elsewhere.
Contracts & tokens
What is a stablecoin?
A stablecoin is a token designed to keep a steady value relative to something else — usually one national currency — by means of reserves, collateral or an algorithm.
Contracts & tokens
What is a layer 2?
A layer 2 is a system built on top of a base blockchain that processes transactions away from the base chain but relies on that chain for security or final settlement.
Safety & rules
Recovery phrases, phishing, 51% attacks, MiCA and the travel rule. About this collection
Safety & rules
What is a recovery phrase?
A recovery phrase, also called a seed phrase, is a list of 12 to 24 ordinary words that encodes the master secret from which a wallet derives all of its private keys.
Safety & rules
What is phishing?
Phishing is a type of fraud in which someone imitates a trusted person, company or website to trick you into revealing a secret — such as a password or recovery phrase — or into approving an action yourself.
Safety & rules
What is a 51% attack?
A 51% attack is an attempt by someone who controls the majority of a blockchain's block-producing power — computing power in proof of work, or staked deposits in proof of stake — to rewrite recent history or block certain transactions.
Safety & rules
What is MiCA?
MiCA — the Markets in Crypto-Assets Regulation, Regulation (EU) 2023/1114 — is the European Union's common rulebook for issuing crypto-assets and for companies that provide crypto-asset services.
Safety & rules
What is the travel rule?
The travel rule is an anti-money-laundering requirement that makes service providers send identifying information about the sender and the recipient along with a transfer of crypto-assets, as banks already do for wire transfers.