What is Ethash?

Ethash is a memory-hard proof-of-work hashing algorithm that was specifically designed for the Ethereum blockchain. It works by generating a large dataset called the DAG (Directed Acyclic Graph), which starts at approximately 1 GB and grows by about 8 MB every 30,000 blocks (roughly every 5 days). During mining, the algorithm performs a series of pseudo-random lookups into this DAG, requiring miners to store and rapidly access the entire dataset. The hash computation involves mixing data from 64 different DAG pages per nonce attempt, making memory bandwidth the primary bottleneck rather than computational speed.

The growing DAG size is a deliberate mechanism to maintain ASIC resistance over time, as it continually increases the memory requirements for mining hardware. This design heavily favored GPU mining, since modern graphics cards possess both the large memory capacity and high memory bandwidth needed for efficient Ethash computation. The algorithm successfully maintained GPU-dominated mining for several years, though Ethereum-specific ASICs were eventually developed by manufacturers like Bitmain and Innosilicon starting in 2018. Ethash also provides strong security properties, with the DAG dependency making it resistant to various optimization shortcuts that could undermine the fairness of the proof-of-work process.

Ethash was developed by the Ethereum team, building on earlier work called Dagger-Hashimoto, and launched with the Ethereum mainnet on July 30, 2015. It served as Ethereum's proof-of-work algorithm throughout the network's most formative years, supporting one of the largest GPU mining ecosystems in cryptocurrency history. On September 15, 2022, Ethereum transitioned from proof-of-work to proof-of-stake in an event known as "The Merge," rendering Ethash mining on Ethereum obsolete. However, the algorithm lives on through Ethereum Classic, which continues to use a variant of Ethash, as well as other forks and projects that adopted the algorithm. Many former Ethereum GPU miners redirected their hardware to mine Ethereum Classic and other Ethash-based coins after The Merge.