Ethereum Foundation and Open Anonymity launch zkAPI on mainnet
The system separates API payments from billing identities using zero-knowledge proofs, while providers still see prompts and network metadata.
The Ethereum Foundation and Open Anonymity Project have launched zkAPI on Ethereum mainnet, offering a way to pay for metered APIs without linking usage to a billing identity, according to an October 1 Foundation blog post.
Users deposit credits into an Ethereum vault, then authorize spending with zero-knowledge proofs generated on their devices. A proof verifies that a funded balance covers the charge without identifying the deposit or user.
In its runtime-key mode, a payment server checks the proof and issues a short-lived API key with a spending limit. Prompts then travel directly from the user’s device to the AI provider. When the key expires, a signed usage receipt determines the charge against the private balance.
The payment server does not see prompt content in that mode, while the AI provider sees requests without learning the billing identity behind the key, the Foundation said. A simpler proxy mode relays requests through the zkAPI server, allowing that intermediary to see traffic.
The system uses Groth16 proofs on the BN254 curve, Poseidon hashes and a 32-level Merkle tree. Nullifiers identify attempted double spending. Spend proofs are verified off-chain, while the vault verifies proofs for deposits, closing balances and escape withdrawals.
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Users can withdraw through the Ethereum contract even if the zkAPI servers become unavailable. The local client supports standard OpenAI and Ollama APIs, allowing existing applications to connect through a local endpoint.
The Foundation said the same design could support blockchain RPC queries, image and video generation, VPN bandwidth and machine-to-machine services. The live mainnet vault holds USDC credits, and a Sepolia deployment is available for testing.
zkAPI does not hide prompt contents, IP addresses or request timing. Providers may still link sessions through reused personal details, writing styles or conversation histories. The Foundation identified network anonymity and content privacy as separate limitations.
The implementation builds on an Ethereum Research design by Davide Crapis and Vitalik Buterin. Open Anonymity helped develop the client, server and contracts.