Vitalik Buterin Exposes Modexp: The Hidden ZK Scaling Roadblock

Ethereum's co-founder just spotlighted a critical flaw in zero-knowledge proofs—and it's choking scalability.

Modexp, the modular exponentiation function, is sucking up computational resources like a crypto hedge fund burns through investor capital. Buterin's warning throws cold water on ZK-rollup hype, revealing a bottleneck that could stall layer-2 adoption.

Here's the kicker: Until this gets fixed, 'instant' ZK proofs will keep costing more than a Wall Street broker's lunch. The irony? The very math meant to streamline blockchain is now throttling it.

Legacy code holding back efficiency

Modexp is one of Ethereum’s early precompiled contracts. It was built to handle modular exponentiation, a key operation behind RSA encryption and digital signatures. Buterin argued that these cryptographic functions can now be handled more efficiently using SNARK-based systems, which are better suited for zero-knowledge proofs and modern scaling solutions.

Buterin, as the original creator of modexp, admitted that it has now become a burden on Ethereum’s progress. He said its inefficiency makes zero-knowledge proof generation far more expensive and complex, slowing the development of ZK-based scaling solutions such as ZK-rollups and ZK-EVMs. He also warned that the precompile’s intricate code path has exposed the protocol to unnecessary consensus risks over time.

Buterin said improving modexp would take “heroic efforts” for a feature that very few apps use. He thinks Ethereum should stop trying to optimize it and instead focus on removing or replacing it. 

The feature is used by only a small number of applications. Most of these, such as RSA verification or cross-chain bridges, could migrate to newer proof methods based on modern cryptography. 

Although no EIP has been drafted so far, the topic is likely to be discussed by the developers in future Core developer calls and Ethereum Foundation updates. Should the change go through, it may prove a watershed moment in Ethereum’s development-the first meaningful step away from legacy cryptographic designs toward a future wholly optimized for zero-knowledge technology.

Rethinking Ethereum’s cryptography

Buterin’s criticism of modexp reflects his broader effort to make Ethereum’s cryptographic systems faster and easier to verify. He recently introduced the GKR protocol, a method to speed up complex mathematical checks in blockchain systems. He also called on developers to use clearer performance metrics for zero-knowledge and fully homomorphic encryption systems.

He also urged developers to use clearer performance metrics for zero-knowledge (ZK) and fully homomorphic encryption (FHE) systems. Instead of focusing on raw speed, he suggested measuring how much efficiency is lost when switching to cryptographic systems. 

Together, these ideas illustrate Buterin’s bigger ambition: to simplify the CORE of Ethereum and to make sure its security tools scale seamlessly with the growth of the network. His attention has increasingly turned to methods like SNARKs and ZK proofs, which promise stronger privacy and faster verification without a heavy computational load from older cryptography such as RSA.

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