Every Concordium account comes from an identity a regulated identity provider verified. With your Concordium wallet you can prove facts about that identity, such as "an adult" or "lives in the EU", as a zero-knowledge proof: the other side learns that the fact holds, and nothing else about you. x402 Doctor never sees your identity or a key.
Pick facts your identity has. Nothing is revealed: the wallet proves each fact without showing your birth date, country or name.
Uses the Concordium Wallet for Web (Chrome or Brave). Make the payTo account the active account in the wallet first: the wallet proves with the active account. The proof is then checked with Concordium's verifier.
Serve this file at . Concordium x402 facilitators that check sellers (this one) read it before they sponsor a payment to your payTo. It is public: it holds the proof and the facts proved, nothing else.
Give this file to your agent. It sends it with a payment, as extensions["concordium-buyer-proof"] in the x402 payment payload, to sellers that ask for it (their 402 names the facts in extra.buyerProof). This facilitator checks it before it sponsors the payment: it must come from the account that pays. It is not a secret: it holds the proof and the facts proved, nothing else, and is worthless from any other account.