Ethereum Classic
Ethereum Classic is a blockchain-based distributed computing platform that offers smart contract functionality. Ethereum Classic was created in a hard fork with the mainline Ethereum blockchain, and maintains the original, unaltered ledger prior to the attempt to reverse a hacking attack on the Ethereum-based DAO in July 2016. It is now the largest smart contract platform secured by a proof-of-work consensus mechanism, following Ethereum’s transition to proof-of-stake in 2022. It is open source and supports a modified version of Nakamoto consensus via transaction-based state transitions executed on a public Ethereum Virtual Machine.
Ethereum Classic maintains the original, unaltered history of the Ethereum 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] token is a cryptocurrency 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", which validates computations performed on Ethereum Classic's EVM.
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.| Date | Block | Milestone name |
| 2015-07-30 | 0 | Frontier |
| 2015-07-30 | 1 | 5M20 Era 1 |
| 2015-09-08 | 200,000 | Ice Age |
| 2016-03-15 | 1,150,000 | Homestead |
| 2016-07-20 | 1,920,000 | "Ethereum Classic" Rebrand |
| 2016-10-24 | 2,500,000 | Gas Reprice |
| 2017-01-13 | 3,000,000 | Die Hard |
| 2017-12-11 | 5,000,000 | Gotham |
| 2017-12-11 | 5,000,001 | 5M20 Era 2 |
| 2018-05-29 | 5,900,000 | Defuse Difficulty Bomb |
| 2019-09-12 | 8,772,000 | Atlantis |
| 2020-01-11 | 9,573,000 | Agharta |
| 2020-03-17 | 10,000,001 | 5M20 Era 3 |
| 2020-06-01 | 10,500,839 | Phoenix |
| 2020-11-28 | 11,700,000 | Thanos |
| 2021-07-23 | 13,189,133 | Magneto |
| 2022-02-12 | 14,525,000 | Mystique |
| 2022-04-25 | 15,000,001 | 5M20 Era 4 |
| 2022-09-15 | 15,950,000 | Largest PoW EVM |
| 2024-02-04 | 19,250,000 | Spiral |
| 2024-05-30 | 20,000,001 | 5M20 Era 5 |
| TBD | 25,000,001 | 5M20 Era 6 |
The DAO bailout
On 20 July 2016, as a result of the exploitation of a flaw in The DAO project's smart contract software, and subsequent theft of $50 million worth of Ether, the Ethereum network split into two separate blockchains – the altered history was named Ethereum and the unaltered history was named Ethereum Classic.- The new chain with the altered history was branded as Ethereum 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 with the BIP-44 Coin Index 61 and EVM Chain ID 61.
Security vulnerabilities disclosed
Carbon vote
On 15 July 2016, a short notice on-chain vote was held on the DAO hard fork. 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 voted in favor, 1/4 of which came from a single address, and 577,899 ETH 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
| Date | ETC EVM Upgrade | ETH EVM Version |
| 2016-10-24 | Gas Reprice | Tangerine Whistle |
| 2017-01-13 | Die Hard | Spurious Dragon |
| 2019-09-12 | Atlantis | Byzantium |
| 2020-01-11 | Agharta | Constantinople + Petersburg |
| 2020-06-01 | Phoenix | Istanbul |
| 2021-07-23 | Magneto | Berlin |
| 2022-02-12 | Mystique | London |
| 2024-02-04 | Spiral | Shanghai |
After the Ethereum Classic and Ethereum 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 Ethereum 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 Ethereum’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 blockchain supporting smart contract functionality. In 2025, the network’s ETChash hashrate surpassed 300 terahashes per second, a level not observed since Ethereum'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 blockchain, which is a continuously growing list of records, called "blocks", which are linked and secured using cryptography. By design, the blockchain is inherently resistant to modification of the data. It is an open, distributed ledger 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. 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 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.