Ethereum Foundation and Open Anonymity Project Launch zkAPI on Mainnet

The Ethereum Foundation and Open Anonymity Project have launched zkAPI on Ethereum mainnet. The system allows users to pay for metered API services without connecting their usage to a specific billing identity.

Users deposit credits into an Ethereum vault and generate zero-knowledge proofs on their own devices to authorize spending. These proofs confirm that a user has sufficient funds to cover a charge without revealing their identity or the specific deposit used.

In runtime-key mode, a payment server validates the proof and provides a short-lived API key with a set spending limit. The user then sends prompts directly to the AI provider. Once the key expires, a signed receipt is used to settle the charge against the user's private balance. In this mode, the payment server does not view the prompt content, and the AI provider does not learn the user's billing identity.

The system also offers a proxy mode that routes requests through the zkAPI server, though this allows the intermediary to see the traffic. The technical implementation utilizes Groth16 proofs on the BN254 curve, Poseidon hashes, and a 32-level Merkle tree. Nullifiers are used to prevent double spending, and the vault handles deposits, closing balances, and escape withdrawals.

The current mainnet vault supports USDC credits, with a Sepolia deployment available for testing. Users retain the ability to withdraw funds through the Ethereum contract even if the zkAPI servers go offline. The local client is compatible with standard OpenAI and Ollama APIs, enabling existing applications to connect via a local endpoint.

The Ethereum Foundation noted that the design could be applied to other services, including blockchain RPC queries, VPN bandwidth, and image or video generation. The project is based on an Ethereum Research design by Vitalik Buterin and Davide Crapis, with Open Anonymity contributing to the development of the client, server, and contracts.

The Foundation clarified that zkAPI does not provide full privacy for all data. It does not hide IP addresses, request timing, or prompt contents. Additionally, providers may still be able to link sessions through conversation histories, writing styles, or other personal details.