Ethash

Ethash is a proof-of-work hashing algorithm that was used by the Ethereum network from its launch in 2015 until the transition to proof-of-stake in 2022. It is designed to be memory-hard, requiring large amounts of RAM and memory bandwidth to compute, with the goal of reducing the advantage of specialized hardware and keeping mining accessible to a broad range of devices. A central concept in Ethash is the Directed Acyclic Graph (DAG), a large data set generated anew for each epoch and used as a source of data during mining. The DAG is regenerated periodically and grows in size over time, which increases the memory requirements for miners.

Mining with Ethash involves producing a valid nonce by running a Hashimoto-based computation. For a given block header, miners derive a seed hash from the block’s epoch, use it to generate the DAG, and repeatedly perform the Hashimoto function, which makes random accesses into the DAG to produce a mix hash. The final result is a 256-bit digest that must be below a target determined by the network’s difficulty. If the digest meets the target, the miner broadcasts a valid block header with the corresponding nonce.

Epochs of the protocol last 30,000 blocks (roughly five days under typical conditions), and the DAG is updated at the start of each epoch. This design favors devices with substantial memory bandwidth and makes device-specific optimizations more challenging to exploit without large RAM capacities.

With Ethereum’s transition to proof of stake in 2022, Ethash ceased to be used for consensus validation on the main network. It remains of historical and technical interest, and may continue to be relevant to certain test networks and other projects that still employ Ethash.