Page last updated: January 8, 2022
A type of attack on a decentralized network where a group gains control of the majority of nodes. This would allow them to defraud the blockchain by reversing transactions and double spending ether and other tokens.
Most generally, this represents an EOA or contract that can receive (destination address) or send (source address) transactions on the blockchain. More specifically, it is the rightmost 160 bits of a Keccak hash of an ECDSA public key.
The standard way to interact with contracts in the Ethereum ecosystem, both from outside the blockchain and for contract-to-contract interactions.
An Application Programming Interface (API) is a set of definitions for how to use a piece of software. An API sits between an application and a web server, and facilitates the transfer of data between them.
assert(false) compiles to
0xfe, an invalid opcode, which uses up all remaining gas and reverts all changes. When an
assert() statement fails, something very wrong and unexpected is happening, and you will need to fix your code. You should use
assert() to avoid conditions that should never, ever occur.
Smart contract security
A positional number representation where the most significant digit is first in memory. The opposite of little-endian, where the least significant digit is first.
In Ethereum, a sequence of blocks validated by the proof-of-work system, each linking to its predecessor all the way to the genesis block. There is no block size limit; it instead uses varying gas limits.
What is a Blockchain?
An abstract instruction set designed for efficient execution by a software interpreter or a virtual machine. Unlike human-readable source code, bytecode is expressed in numeric format.
The first of two hard forks for the Metropolis development stage. It included EIP-649 Metropolis Difficulty Bomb Delay and Block Reward Reduction, where the Ice Age was delayed by 1 year and the block reward was reduced from 5 to 3 ether.
Compiling Smart Contracts
When numerous nodes (usually most nodes on the network) all have the same blocks in their locally validated best blockchain. Not to be confused with consensus rules.
The block validation rules that full nodes follow to stay in consensus with other nodes. Not to be confused with consensus.
A company or other organization that operates without hierarchical management. DAO may also refer to a contract named "The DAO" launched on April 30, 2016, which was then hacked in June 2016; this ultimately motivated a hard fork (codenamed DAO) at block 1,192,000, which reversed the hacked DAO contract and caused Ethereum and Ethereum Classic to split into two competing systems.
Decentralized Autonomous Organizations (DAOs)
Decentralized application. At a minimum, it is a smart contract and a web user interface. More broadly, a Dapp is a web application that is built on top of open, decentralized, peer-to-peer infrastructure services. In addition, many Dapps include decentralized storage and/or a message protocol and platform.
Introduction to Dapps
A type of dapp that lets you swap tokens with peers on the network. You need ether to use one (to pay transactions fees) but they are not subject to geographical restrictions like centralized exchanges – anyone can participate.
Short for "decentralized finance," a broad category of dapps aiming to provide financial services backed by the blockchain, without any intermediaries, so anyone with an internet connection can participate.
Decentralized Finance (DeFi)
A network-wide setting that controls how much computation is required to produce a proof-of-work.
A short string of data a user produces for a document using a private key such that anyone with the corresponding public key, the signature, and the document can verify that (1) the document was "signed" by the owner of that particular private key, and (2) the document was not changed after it was signed.
A cryptographic algorithm used by Ethereum to ensure that funds can only be spent by their owners. It's the preferred method for creating public and private keys. Relevant for account address generation and transaction verification.
A period of 32 slots (6.4 minutes) in the Beacon Chain-coordinated system. Validator committees are shuffled every epoch for security reasons. There's an opportunity at each epoch for the chain to be finalized.
A design document providing information to the Ethereum community, describing a proposed new feature or its processes or environment (see ERC).
Introduction to EIPs
In the context of cryptography, lack of predictability or level of randomness. When generating secret information, such as private keys, algorithms usually rely on a source of high entropy to ensure the output is unpredictable.
An account created by or for human users of the Ethereum network.
A label given to some EIPs that attempt to define a specific standard of Ethereum usage.
Introduction to EIPs
A proof-of-work algorithm for Ethereum 1.0.
The native cryptocurrency used by the Ethereum ecosystem, which covers gas costs when executing transactions. Also written as ETH or its symbol Ξ, the Greek uppercase Xi character.
Currency for our digital future
Events and Logs
A stack-based virtual machine that executes bytecode. In Ethereum, the execution model specifies how the system state is altered given a series of bytecode instructions and a small tuple of environmental data. This is specified through a formal model of a virtual state machine.
Ethereum Virtual Machine
A human-readable form of EVM bytecode.
A default function called in the absence of data or a declared function name.
A service carried out via smart contract that dispenses funds in the form of free test ether that can be used on a testnet.
Finality is the guarantee that a set of transactions before a given time will not change and can't be reverted.
A change in protocol causing the creation of an alternative chain, or a temporal divergence in two potential block paths during mining.
A security model for certain layer 2 solutions where, to increase speed, transactions are rolled up into batches and submitted to Ethereum in a single transaction. They are assumed valid but can be challenged if fraud is suspected. A fraud proof will then run the transaction to see if fraud took place. This method increases the amount of transactions possible while maintaining security. Some rollups use validity proofs.
The initial test development stage of Ethereum, which lasted from July 2015 to March 2016.
A virtual fuel used in Ethereum to execute smart contracts. The EVM uses an accounting mechanism to measure the consumption of gas and limit the consumption of computing resources (see Turing complete).
Gas and Fees
The first block in a blockchain, used to initialize a particular network and its cryptocurrency.
Go Ethereum. One of the most prominent implementations of the Ethereum protocol, written in Go.
A permanent divergence in the blockchain; also known as a hard-forking change. One commonly occurs when nonupgraded nodes can't validate blocks created by upgraded nodes that follow newer consensus rules. Not to be confused with a fork, soft fork, software fork, or Git fork.
A fixed-length fingerprint of variable-size input, produced by a hash function. (See keccak-256)
A wallet using the hierarchical deterministic (HD) key creation and transfer protocol.
A value used to generate the master private key and master chain code for an HD wallet. The wallet seed can be represented by mnemonic words, making it easier for humans to copy, back up, and restore private keys.
The second development stage of Ethereum, launched in March 2016 at block 1,150,000.
A network structure meant to optimize the querying of information from across the blockchain by providing an efficient path to its storage source.
An Ethereum address encoding that is partly compatible with the International Bank Account Number (IBAN) encoding, offering a versatile, checksummed, and interoperable encoding for Ethereum addresses. ICAP addresses use a new IBAN pseudo-country code- XE, standing for "eXtended Ethereum," as used in nonjurisdictional currencies (e.g., XBT, XRP, XCP).
A user interface that typically combines a code editor, compiler, runtime, and debugger.
Integrated Development Environments
Once a contract's (or library's) code is deployed, it becomes immutable. Standard software development practices rely on being able to fix possible bugs and add new features, so this represents a challenge for smart contract development.
Deploying Smart Contracts
Also known as a "password stretching algorithm," it is used by keystore formats to protect against brute-force, dictionary, and rainbow table attacks on passphrase encryption, by repeatedly hashing the passphrase.
Smart contract security
A JSON-encoded file that contains a single (randomly generated) private key, encrypted by a passphrase for extra security.
An open source on-disk key-value store, implemented as a lightweight, single-purpose library, with bindings to many platforms.
A special type of contract that has no payable functions, no fallback function, and no data storage. Therefore, it cannot receive or hold ether, or store data. A library serves as previously deployed code that other contracts can call for read-only computation.
Smart Contract Libraries
Short for "main network," this is the main public Ethereum blockchain. Real ETH, real value, and real consequences. Also known as layer 1 when discussing layer 2 scaling solutions. (Also, see testnet)
A data structure used in Ethereum to efficiently store key-value pairs.
The third development stage of Ethereum, launched in October 2017.
Referring to the Ethereum network, a peer-to-peer network that propagates transactions and blocks to every Ethereum node (network participant).
Also known as a "deed," this is a token standard introduced by the ERC-721 proposal. NFTs can be tracked and traded, but each token is unique and distinct; they are not interchangeable like ETH and ERC-20 tokens. NFTs can represent ownership of digital or physical assets.
Non-Fungible Tokens (NFTs)
ERC-721 Non-Fungible Token Standard
A software client that participates in the network.
Nodes and Clients
In cryptography, a value that can only be used once. There are two types of nonce used in Ethereum- an account nonce is a transaction counter in each account, which is used to prevent replay attacks; a proof-of-work nonce is the random value in a block that was used to satisfy the proof-of-work.
When a miner finds a valid block, another miner may have published a competing block which is added to the tip of the blockchain first. This valid, but stale, block can be included by newer blocks as ommers and receive a partial block reward. The term "ommer" is the preferred gender-neutral term for the sibling of a parent block, but this is also sometimes referred to as an "uncle".
A rollup of transactions that use fraud proofs to offer increased layer 2 transaction throughput while using the security provided by Mainnet (layer 1). Unlike Plasma, a similar layer 2 solution, Optimistic rollups can handle more complex transaction types – anything possible in the EVM. They do have latency issues compared to Zero-knowledge rollups because a transaction can be challenged via the fraud proof.
One of the most prominent interoperable implementations of the Ethereum client software.
A method by which a cryptocurrency blockchain protocol aims to achieve distributed consensus. PoS asks users to prove ownership of a certain amount of cryptocurrency (their "stake" in the network) in order to be able to participate in the validation of transactions.
A number, derived via a one-way function from a private key, which can be shared publicly and used by anyone to verify a digital signature made with the corresponding private key.
Data returned by an Ethereum client to represent the result of a particular transaction, including a hash of the transaction, its block number, the amount of gas used, and, in case of deployment of a smart contract, the address of the contract.
An attack that consists of an attacker contract calling a victim contract function in such a way that during execution the victim calls the attacker contract again, recursively. This can result, for example, in the theft of funds by skipping parts of the victim contract that update balances or count withdrawal amounts.
An encoding standard designed by the Ethereum developers to encode and serialize objects (data structures) of arbitrary complexity and length.
A type of layer 2 scaling solution that batches multiple transactions and submits them to the Ethereum main chain in a single transaction. This allows for reductions in gas costs and increases in transaction throughput. There are Optimistic and Zero-knowledge rollups which use different security methods to offer these scalability gains.
The fourth and final development stage of Ethereum, otherwise known as Ethereum 2.0.
Ethereum 2.0 (Eth2)
A family of cryptographic hash functions published by the National Institute of Standards and Technology (NIST).
A proof-of-stake chain that is coordinated by the Beacon Chain and secured by validators. There will be 64 added to the network as part of the Eth2 shard chain upgrade. Shard chains will offer increased transaction throughput for Ethereum by providing additional data to layer 2 solutions like optimistic rollups and ZK-rollups.
A scaling solution that uses a separate chain with different, often faster, consensus rules. A bridge is needed to connect these sidechains to Mainnet. Rollups also use sidechains, but they operate in collaboration with Mainnet instead.
A computer programming term that describes an object of which only a single instance can exist.
A program that executes on the Ethereum computing infrastructure.
Introduction to Smart Contracts
A hard fork of the Ethereum blockchain, which occurred at block 2,675,000 to address more denial-of-service attack vectors and clear state (see Tangerine Whistle). Also, a replay attack protection mechanism (see nonce).
An ERC-20 token with a value pegged to another asset's value. There are stablecoins backed by fiat currency like dollars, precious metals like gold, and other cryptocurrencies like Bitcoin.
ETH isn't the only crypto on Ethereum
Depositing a quantity of ether (your stake) to become a validator and secure the network. A validator checks transactions and proposes blocks under a proof-of-stake consensus model. Staking gives you an economic incentive to act in the best interests of the network. You'll get rewards for carrying out your validator duties, but lose varying amounts of ETH if you don't.
Stake your ETH to become an Ethereum validator
A layer 2 solution where a channel is set up between participants, where they can transact freely and cheaply. Only a transaction to set up the channel and close the channel is sent to Mainnet. This allows for very high transaction throughput, but does rely on knowing number of participants up front and locking up of funds.
A hard fork of the Ethereum blockchain, which occurred at block 2,463,000 to change the gas calculation for certain I/O-intensive operations and to clear the accumulated state from a denial-of-service attack, which exploited the low gas cost of those operations.
Short for "test network," a network used to simulate the behavior of the main Ethereum network (see Mainnet).
ERC-20 Token Standard
A fee you need to pay whenever you use the Ethereum network. Examples include sending funds from your wallet or a dapp interaction, like swapping tokens or buying a collectible. You can think of this like a service charge. This fee will change based on how busy the network is. This is because miners, the people responsible for processing your transaction, are likely to prioritise transactions with higher fees – so congestion forces the price up.
At a technical level, your transaction fee relates to how much gas your transaction requires.
Reducing transaction fees is a subject of intense interest right now. See Layer 2
A concept named after English mathematician and computer scientist Alan Turing- a system of data-manipulation rules (such as a computer's instruction set, a programming language, or a cellular automaton) is said to be "Turing complete" or "computationally universal" if it can be used to simulate any Turing machine.
Staking in Ethereum
A security model for certain layer 2 solutions where, to increase speed, transactions are rolled up into batches and submitted to Ethereum in a single transaction. The transaction computation is done off-chain and then supplied to the main chain with a proof of their validity. This method increases the amount of transactions possible while maintaining security. Some rollups use fraud proofs.
A high-level programming language with Python-like syntax. Intended to get closer to a pure functional language. Created by Vitalik Buterin.
Software that holds private keys. Used to access and control Ethereum accounts and interact with smart contracts. Keys need not be stored in a wallet, and can instead be retrieved from offline storage (i.e. a memory card or paper) for improved security. Despite the name, wallets never store the actual coins or tokens.
The third version of the web. First proposed by Dr. Gavin Wood, Web3 represents a new vision and focus for web applications- from centrally owned and managed applications, to applications built on decentralized protocols (see Dapp).
Web2 vs Web3
The smallest denomination of ether. 1018 wei = 1 ether.
A special Ethereum address, composed entirely of zeros, that is specified as the destination address of a contract creation transaction.
A rollup of transactions that use validity proofs to offer increased layer 2 transaction throughput while using the security provided by Mainnet (layer 1). Although they can't handle complex transaction types, like Optimistic rollups, they don't have latency issues because transactions are provably valid when submitted.
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