Ethereum Foundation Launches zkAPI for Private AI Payments

The Ethereum Foundation has launched zkAPI, a payment system for AI models and other metered APIs that keeps payments private from both the user and the provider. Built with the Open Anonymity Project, the system is now live on Ethereum mainnet.

Vittorio Rivabella of the dAI team described the problem: every AI API call carries an identity, linking payment to account and payment method. This setup means each prompt a user sends joins a record tied to both.

With zkAPI, users deposit credits into a vault contract on Ethereum, creating a private note that only they can spend. To authorize spending, software generates a small zero-knowledge proof showing the funded note covers the request and hasn't been spent before. The zkAPI server checks the proof and issues a short-lived API key with a capped dollar limit without learning which note or person the request came from.

The system relies on two cryptographic components: commitments inside a Merkle tree, allowing a proof to confirm a note's validity without revealing its identity; and nullifiers, one-way serial numbers tied to the note's secret that expose attempts to spend the same balance twice without revealing anything else about the user. Prompts travel directly from the user's device to the AI provider using the temporary key, while the zkAPI server separately settles metered usage against the user's private balance once the key expires.