Ethereum Classic is a

blockchain The blockchain is a distributed ledger with growing lists of Record (computer science), records (''blocks'') that are securely linked together via Cryptographic hash function, cryptographic hashes. Each block contains a cryptographic hash of th ...

-based

distributed computing Distributed computing is a field of computer science that studies distributed systems, defined as computer systems whose inter-communicating components are located on different networked computers. The components of a distributed system commu ...

platform that offers

smart contract A smart contract is a computer program or a Transaction Protocol Data Unit, transaction protocol that is intended to automatically execute, control or document events and actions according to the terms of a contract or an agreement. The objective ...

(scripting) functionality. It is

open source Open source is source code that is made freely available for possible modification and redistribution. Products include permission to use and view the source code, design documents, or content of the product. The open source model is a decentrali ...

and supports a modified version of Nakamoto consensus via transaction-based state transitions executed on a public Ethereum Virtual Machine (EVM). Ethereum Classic maintains the original, unaltered history of the

Ethereum Ethereum is a decentralized blockchain with smart contract functionality. Ether (abbreviation: ETH) is the native cryptocurrency of the platform. Among cryptocurrencies, ether is second only to bitcoin in market capitalization. It is open-s ...

network. The Ethereum project's mainnet was initially released via Frontier on 30 July 2015. However, due to a hack of a third-party project, The DAO, the Ethereum Foundation created a new version of the Ethereum mainnet on 20 July 2016 with an ''irregular state change'' implemented that erased the DAO theft from the Ethereum blockchain history. The Ethereum Foundation applied their trademark to the new, altered version of the Ethereum blockchain. The older, unaltered version of Ethereum was renamed and continued on as Ethereum Classic. Ethereum Classic's native

Ether In organic chemistry, ethers are a class of compounds that contain an ether group, a single oxygen atom bonded to two separate carbon atoms, each part of an organyl group (e.g., alkyl or aryl). They have the general formula , where R and R� ...

token is a

cryptocurrency A cryptocurrency (colloquially crypto) is a digital currency designed to work through a computer network that is not reliant on any central authority, such as a government or bank, to uphold or maintain it. Individual coin ownership record ...

traded on digital currency exchanges under the currency code ETC. Ether is created as a reward to network nodes for a process known as "

mining Mining is the Resource extraction, extraction of valuable geological materials and minerals from the surface of the Earth. Mining is required to obtain most materials that cannot be grown through agriculture, agricultural processes, or feasib ...

", which validates computations performed on Ethereum Classic's EVM. Implemented on 11 December 2017, the current ETC monetary policy seeks the same goals as

bitcoin Bitcoin (abbreviation: BTC; Currency symbol, sign: ₿) is the first Decentralized application, decentralized cryptocurrency. Based on a free-market ideology, bitcoin was invented in 2008 when an unknown entity published a white paper under ...

: being mechanical, algorithmic, and capped. ETC can be exchanged for network transaction fees or other assets, commodities, currencies, products, and services. Ethereum Classic provides a decentralized

Turing-complete In computability theory, a system of data-manipulation rules (such as a model of computation, 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 ...

virtual machine In computing, a virtual machine (VM) is the virtualization or emulator, emulation of a computer system. Virtual machines are based on computer architectures and provide the functionality of a physical computer. Their implementations may involve ...

, the Ethereum Virtual Machine (EVM), which can execute scripts using an international network of public nodes. The virtual machine's

instruction set In computer science, an instruction set architecture (ISA) is an abstract model that generally defines how software controls the CPU in a computer or a family of computers. A device or program that executes instructions described by that ISA, s ...

is Turing-complete, in contrast to others like Bitcoin Script. Gas, an internal transaction pricing mechanism, is used to mitigate spam and allocate resources on the network.

Milestones

Frontier

Several codenamed prototypes of the Ethereum platform were developed by the Ethereum Foundation, as part of their proof-of-concept series, prior to the official launch of the Frontier network. Ethereum Classic followed this codebase after the DAO incident.

The DAO bailout

On 20 July 2016, as a result of the exploitation of a flaw in The DAO project's

software, and subsequent theft of $50 million worth of Ether, the Ethereum network split into two separate blockchains – the altered history was named Ethereum (ETH) and the unaltered history was named Ethereum Classic (ETC). * The new chain with the altered history was branded as

(code: ETH) with the BIP-44 Coin Index 60 and EVM Chain ID 1 attributed to it by the trademark-owning Ethereum Foundation. On this new chain, the history of the theft was erased from the Ethereum blockchain. * Some members of the Ethereum community ignored the change and continued to participate on the original Ethereum network. The non-fork chain with an unaltered history continued on as Ethereum Classic (code: ETC) with the BIP-44 Coin Index 61 and EVM Chain ID 61.

Security vulnerabilities disclosed

On 28 May 2016, a paper was released detailing security vulnerabilities with the DAO that could allow Ether to be stolen. On 9 June 2016, Peter Vessenes publicly disclosed the existence of a critical security vulnerability overlooked in many Solidity contracts, a recursive call bug. On 12 June 2016, Stephan Tual publicly claimed that the DAO funds were safe despite the newly-discovered critical security flaw.

Carbon vote

On 15 July 2016, a short notice on-chain vote was held on the DAO hard

fork In cutlery or kitchenware, a fork (from 'pitchfork') is a utensil, now usually made of metal, whose long handle terminates in a head that branches into several narrow and often slightly curved tines with which one can spear foods either to h ...

.Andreas M. Antonopoulos, Gavin Wood (2018):
Ethereum timeline
. In ''Mastering Ethereum: Building Smart Contracts and DApps'', page 329. O'Reilly Media; 424 pages. Of the 82,054,716 ETH in existence, only 4,542,416 voted, for a total voter turn out of 5.5% of the total supply on 16 July 2016; 3,964,516 ETH (87%) voted in favor, 1/4 of which came from a single address, and 577,899 ETH (13%) opposed the DAO fork. The expedited process of the carbon vote drew criticism from opponents of the DAO fork. Proponents of the fork were quick to market the vote as an effective consensus mechanism, pushing forward with the DAO fork four days later.

Block 1,920,000

The first Ethereum Classic block that was not included in the forked Ethereum chain was block number 1,920,000, which was generated by Ethereum Classic miners on 20 July 2016.

Defuse Difficulty Bomb

A mechanism called the "Difficulty Bomb" was designed to push the Ethereum chain from proof-of-work consensus mechanism to proof-of-stake in the future by exponentially increasing the difficulty of mining. This Difficulty Bomb was added to the network on block 200,000 in an upgrade named "Ice Age". While Ethereum Classic participants debated the merits of the Difficulty Bomb, a network upgrade called "Die Hard" at block 3,000,000 delayed the effects of the mechanism. Once the network participants came to consensus on the issue, Ethereum Classic upgraded its network on block 5,900,000 to permanently defuse the Difficulty Bomb. This abandoned a future with proof-of-stake and committed the network to the proof-of-work consensus mechanism.

EVM standard protocol parity

After the Ethereum Classic and

Foundation hardfork in 2016, multiple versions of the EVM existed. This led to the realization of the need for an interoperable EVM standard to connect multiple EVMs. In an attempt to modernize the Ethereum Classic EVM, several protocol upgrades were scheduled to activate features that the

network already enabled over the past years. Atlantis, activated in September 2019, enabled the Agharta upgrade, which included the outstanding Byzantium changes. Agharta was followed by the incorporation of the Constantinople patches through the January 2020 upgrade. Finally, with the Phoenix upgrade, Ethereum Classic achieved protocol parity with Ethereum, allowing for fully cross-compatible applications between the two networks. The Ethereum Classic development community continues to maintain protocol parity with the greater EVM standard. Development moves slowly, only updating stable versions of EVM standard.

ETChash mining algorithm

After a series of 51% attacks on the Ethereum Classic network in 2020, a change to the underlying Ethash mining algorithm was considered by the community to prevent being a minority proof-of-work chain in the Ethash mining algorithm where Ethereum is dominating the hashrate. After evaluating various options such as Monero's RandomX or the standardized SHA-3-256, it was eventually decided to double the Ethash epoch duration from 30,000 to 60,000 in order to reduce the DAG size and prevent Ethash miners to easily switch to Ethereum Classic. This modified Ethash is also referred to as ''ETChash'' or ''Thanos'' upgrade.

Leading Proof-of-Work Smart Contract Platform

Following

’s transition to a proof-of-stake consensus mechanism in 2022, Ethereum Classic achieved a notable milestone by becoming the largest and most secure

proof-of-work Proof of work (also written as proof-of-work, an abbreviated PoW) is a form of Cryptography, cryptographic proof (truth), proof in which one party (the ''prover'') proves to others (the ''verifiers'') that a certain amount of a specific computatio ...

supporting

functionality. In 2025, the network’s ETChash hashrate surpassed 300 terahashes per second (TH/s), a level not observed since

's DeFi summer boom cycle. Ethereum Classic subsequently established itself as the leading blockchain utilizing the Ethash algorithm family. This increase in computational power indicates greater participation by miners, broader network adoption, and enhanced network security.

Characteristics

As with other cryptocurrencies, the validity of each ether is provided by a

, which is a continuously growing list of records, called "blocks", which are linked and secured using

cryptography Cryptography, or cryptology (from "hidden, secret"; and ''graphein'', "to write", or ''-logy, -logia'', "study", respectively), is the practice and study of techniques for secure communication in the presence of Adversary (cryptography), ...

. By design, the blockchain is inherently resistant to modification of the data. It is an open,

distributed ledger A distributed ledger (also called a shared ledger or distributed ledger technology or DLT) is a system whereby replicated, shared, and synchronized digital data is geographically spread (distributed) across many sites, countries, or institutions. I ...

that records transactions between two parties efficiently and in a verifiable and permanent way. Unlike bitcoin, Ethereum Classic operates using accounts and balances in a manner called state transitions. This does not rely upon unspent transaction outputs (UTXOs). The state denotes the current balances of all accounts and extra data. The state is not stored on the blockchain, it is stored in a separate Merkle Patricia tree. A

cryptocurrency wallet A cryptocurrency wallet is a device, physical medium, program or an online service which stores the Public-key cryptography, public and/or private keys for cryptocurrency transactions. In addition to this basic function of storing the keys, a cr ...

stores the public and private "keys" or "addresses" which can be used to receive or spend Ether. These can be generated through BIP 39 style mnemonics for a BIP 32 "HD wallet". In the Ethereum tech stack, this is unnecessary as it does not operate in a UTXO scheme. With the private key, it is possible to write in the blockchain, effectively making an ether transaction. To send Ether to an account, the Keccak-256 hash of the public key of that account is needed. Ether accounts are pseudonymous in that they are not linked to individual persons, but rather to one or more specific addresses.

Ether

''ETC'' is a fundamental token for operation of Ethereum Classic, which thereby provides a public

for transactions. It is used to pay for Gas, a unit of computation used in transactions and other state transitions. Within the context of Ethereum Classic it might be called ether, but it should not be confused with ETH, which is also called ether. It is listed under the currency code ETC and traded on

exchanges, and the Greek uppercase Xi character (Ξ) is generally used for its

currency symbol A currency symbol or currency sign is a graphic symbol used to denote a currency unit. Usually it is defined by a monetary authority, such as the national central bank for the currency concerned. A symbol may be positioned in various ways, acc ...

. It is also used to pay for transaction fees and computational services on the Ethereum Classic network.

Addresses

Ethereum Classic addresses are composed of the prefix "0x", a common identifier for

hexadecimal Hexadecimal (also known as base-16 or simply hex) is a Numeral system#Positional systems in detail, positional numeral system that represents numbers using a radix (base) of sixteen. Unlike the decimal system representing numbers using ten symbo ...

, concatenated with the rightmost 20 bytes of the Keccak-256 hash (

big endian '' Jonathan_Swift.html" ;"title="Gulliver's Travels'' by Jonathan Swift">Gulliver's Travels'' by Jonathan Swift, the novel from which the term was coined In computing, endianness is the order in which bytes within a word (data type), word of d ...

) of the ECDSA

public key Public-key cryptography, or asymmetric cryptography, is the field of cryptographic systems that use pairs of related keys. Each key pair consists of a public key and a corresponding private key. Key pairs are generated with cryptographic alg ...

(the curve used is the so-called ''secp256k1'', the same as bitcoin). In hexadecimal, two digits represent a byte, meaning addresses contain 40 hexadecimal digits. An example of an Ethereum Classic address is 0xb794f5ea0ba39494ce839613fffba74279579268. Contract addresses are in the same format, however, they are determined by sender and creation transaction nonce. User accounts are indistinguishable from contract accounts given only an address for each and no blockchain data. Any valid Keccak-256 hash put into the described format is valid, even if it does not correspond to an account with a private key or a contract. This is unlike bitcoin, which uses base58check to ensure that addresses are properly typed.

Monetary policy

On 11 December 2017, the total supply of Ether on Ethereum Classic was hard capped at ETC 210,700,000 via the Gotham hard fork upgrade. This added a bitcoin-inspired deflationary emission schedule that is documented in Ethereum Classic Improvement Proposal (ECIP) 1017. The emission schedule, also known as "5M20", reduces the block reward by 20% every 5,000,000 blocks. Socially, this block reward reduction event has taken the moniker of "the fifthening."

Ethereum Classic Improvement Proposal

The Ethereum Classic Improvement Proposal (ECIP) process enables engineers and computer scientists to propose modifications, upgrades, or fixes. Any software developer who is a GitHub user is allowed to make contributions to the ECIP process. There is a number of ECIP types, each listed in the table below.

Code is law

The people who continued with Ethereum Classic advocate for blockchain immutability, and the concept that "code is law" against the pro-fork side (Ethereum) which largely argued for extra-protocol intentionality, decentralized decision-making, and conflict resolution. Code is law refers to the idea that the code is above all else including law from outside forces such as a

government A government is the system or group of people governing an organized community, generally a State (polity), state. In the case of its broad associative definition, government normally consists of legislature, executive (government), execu ...

. The law is written into the code, therefore, anything the code allows is legal.

Attacks

The DAO fork replay attacks

On 20 July 2016, due to reliance on the same clients, the DAO fork created a replay attack where a transaction was broadcast on both the ETC and ETH networks. On 13 January 2017, the Ethereum Classic network was updated to resolve transaction replay attacks. The networks are now officially operating separately.

RHG sells stolen ETC

On 10 August 2016, the ETH proponent Robin Hood Group transferred 2.9 million stolen ETC to Poloniex in an attempt to sell ETC for ETH on the advice of Bitly SA; 14% was successfully converted to ETH and other currencies, 86% was frozen by Poloniex. On 30 August 2016, Poloniex returned the ETC funds to the RHG. They set up a refund contract on the ETC network.

Classic Ether Wallet website attack

On 29 June 2017, the Ethereum Classic Twitter account made a public statement indicating reason to believe that the website for Classic Ether Wallet had been compromised. The Ethereum Classic Twitter account confirmed the details released via Threatpost. The Ethereum Classic team worked with

Cloudflare Cloudflare, Inc., is an American company that provides content delivery network services, cybersecurity, DDoS mitigation, wide area network services, reverse proxies, Domain Name Service, ICANN-accredited domain registration, and other se ...

to place a warning on the compromised domain warning users of the

phishing Phishing is a form of social engineering and a scam where attackers deceive people into revealing sensitive information or installing malware such as viruses, worms, adware, or ransomware. Phishing attacks have become increasingly sophisticate ...

attack.

51% double spend attacks

Ethereum Classic (ETC) has experienced two major 51% double-spending attacks in its history. These attacks exploit the decentralized nature of the network by gaining control of more than 50% of its mining power, allowing malicious actors to reorganize the blockchain, manipulate transactions, and double-spend digital assets. The first significant incident occurred in January 2019, when Ethereum Classic was targeted by a double-spending attack resulting in approximately US$1.1 million worth of ETC being fraudulently spent. The second major 51% attack took place in August 2020, on the fourth anniversary of the 2016 hard fork. This attack involved a series of deep chain reorganizations, leading to estimated losses of US$5.6 million and US$1.68 million from centralized exchanges, respectively.Howarth, Dan. "Hackers Launch Third 51% Attack on Ethereum Classic This Month." Decrypt. Decrypt, 18 Nov. 2020. Web. 4 Apr. 2023. In response, the Ethereum Classic development community considered several network upgrades and ultimately adopted a modified version of the Ethash

(PoW) consensus algorithm, known as ETChash. This change was implemented through the "ECIP-1099 Thanos Upgrade" in November 2020. The upgrade recalibrated the

Directed Acyclic Graph In mathematics, particularly graph theory, and computer science, a directed acyclic graph (DAG) is a directed graph with no directed cycles. That is, it consists of vertices and edges (also called ''arcs''), with each edge directed from one ...

(DAG) by reducing its size and slowing its growth rate by half, enabling 3GB Ethash mining hardware to continue securing the network. This adjustment also positioned Ethereum Classic to benefit from Ethash-compatible hardware rendered obsolete by blockchains adhering to the original DAG growth schedule. Following the Thanos Upgrade, Ethereum Classic's hashrate began a steady increase, eventually becoming the largest blockchain secured by the Ethash mining algorithm. In 2025, the ETChash network hashrate surpassed 300 terahashes per second (TH/s), a level not seen since Ethereum's "DeFi summer." As a result, Ethereum Classic solidified its position as the leading proof-of-work

platform.

References

External links

* {{Authority control 2015 software Blockchains Cross-platform software Cryptocurrency projects Currencies introduced in 2015 Ethereum