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Salted Unique Identifiers (OPRF)

Every verification result comes with a unique identifier tied to the user's ID. By default, it is derived purely from the ID data, your domain, and the request scope — which means anyone with complete knowledge of the ID chip data (for example the government that issued the ID) could recompute it and recognize the user.

Salted unique identifiers (NullifierType.SALTED) close this gap. The user's device derives the identifier with the help of an extra secret (the "salt") held by an independent network of servers, using a technique called an OPRF (Oblivious Pseudo-Random Function):

  • The servers never see the user's data — they only receive a blinded value.
  • No single server holds the whole secret, so nobody can recompute the identifier on their own — not even with full knowledge of the ID data.
  • The result is still deterministic: the same ID yields the same identifier for your app, so you can still recognize returning users.

Use this when the identifier itself is sensitive — for example anonymous voting, whistleblowing platforms, or any personhood check where users must not be linkable to their real-world identity by anyone.

To request one:

  • Set uniqueIdentifierType to NullifierType.SALTED — available both on the SDK's request() method and as a prop on the @zkpassport/ui QR code component (shown below).
  • Add .facematch("strict") to your query. Salted identifiers require the strict FaceMatch mode, which confirms the ID is being used by its actual holder; the request throws an error without it.
import { ZKPassportQRCode, NullifierType } from "@zkpassport/ui/react";

<ZKPassportQRCode
  name="ZKPassport"
  logo="https://zkpassport.id/logo.png"
  purpose="Prove your personhood"
  scope="personhood"
  uniqueIdentifierType={NullifierType.SALTED}
  query={(queryBuilder) => queryBuilder.facematch("strict").done()}
  onResult={({ verified, uniqueIdentifier }) => {
    if (verified) console.log("Unique identifier", uniqueIdentifier);
  }}
/>;
info

The onResult callback also reports the identifier's type as uniqueIdentifierType. In dev mode, mock proofs return NullifierType.SALTED_MOCK instead of NullifierType.SALTED, so you can tell real salted identifiers apart from test ones.