What is Equihash?

Equihash is a memory-hard proof-of-work algorithm based on the generalized birthday problem, a well-studied computational problem in computer science and cryptography. It was designed by Alex Biryukov and Dmitry Khovratovich at the University of Luxembourg in 2016. The algorithm requires miners to find solutions to a specific instance of the birthday problem by sorting and matching hash values across large lists stored in memory. The standard Equihash configuration uses parameters (n=200, k=9), which requires approximately 700 MB of memory to solve efficiently, though different parameter sets can adjust the memory and computation requirements.

The mathematical foundation of Equihash in the birthday problem provides provable memory-hardness, meaning that any significant reduction in memory usage results in an exponential increase in computation time. This property makes it one of the few mining algorithms with a formal theoretical basis for its ASIC-resistance claims. The algorithm is asymmetric by design — solving is memory-intensive and relatively slow, while verification is fast and lightweight, which is ideal for blockchain networks where nodes must quickly validate blocks. Despite its strong theoretical foundation, specialized Equihash ASICs were developed by 2018, prompting some projects to adopt modified parameter sets (like Equihash 144,5) to restore GPU-mining viability.

Equihash was first adopted as the mining algorithm for Zcash, the pioneering privacy cryptocurrency launched in October 2016 that introduced zero-knowledge proofs (zk-SNARKs) to blockchain technology. The choice of Equihash was intended to ensure fair and decentralized mining in Zcash's early days. Following the development of Equihash ASICs, the Zcash community debated but ultimately chose not to fork to resist ASICs, while other projects like Horizen (formerly ZenCash), Komodo, and Beam adopted Equihash variants with different parameters. Bitcoin Gold specifically forked from Bitcoin and selected Equihash with the parameter set (144,5) as its mining algorithm, explicitly designed to resist the existing Equihash ASICs and maintain GPU mining accessibility.