Quickstart

Add an age check to a Node.js app. You'll create a session on your server, open it in the browser and read the verified result back.

1. Get an API key#

Create a business account and click New app.
Copy the secret key that's shown once. It starts with lkp_sk_.
Store it as a server-side environment variable. Never ship it to browsers or commit it to git.

.env

LKP_API_URL=https://lkp-verifier.onrender.com
LKP_SECRET_KEY=lkp_sk_...

2. Create a verification session#

A session describes what you want to know. Create one per verification attempt, on your server, when the user clicks your “Verify” button.

app.post("/api/verify/start", async (req, res) => {
  const r = await fetch(`${process.env.LKP_API_URL}/v1/verification-sessions`, {
    method: "POST",
    headers: {
      Authorization: `Bearer ${process.env.LKP_SECRET_KEY}`,
      "Content-Type": "application/json",
    },
    body: JSON.stringify({
      requirements: [{ kind: "AGE_OVER", minAge: 18 }],
      // "age_estimation" (fastest selfie check), "full_id" (passport/ID card), or "either" (user picks)
      kycWorkflow: "age_estimation",
      reference: req.session.cartId, // optional: your own ID, echoed back
    }),
  });
  const session = await r.json();
  req.session.lkpSessionId = session.id; // remember which session belongs to this user
  res.json({ url: session.url });
});

The response contains the session ID and the hosted URL where the user completes the check:

{
  "id": "vs_3f9c2a…",
  "object": "verification_session",
  "status": "pending",
  "claims": ["Age >= 18"],
  "url": "https://lkp-three.vercel.app/verify/vs_3f9c2a…",
  "expiresAt": "2026-11-06T15:30:00.000Z",
  "result": null
}

3. Present the verification flow#

You can open verification as an in-page modal dialog directly on your website, embed it into a custom page or checkout container, open a popup window, or redirect to LKP. See Modal, embed and hosted flow for all options.

checkout.html

<button id="verify">Verify my age (18+)</button>
<script src="https://lkp-three.vercel.app/sdk/lkp.js"></script>
<script>
  document.getElementById("verify").onclick = async () => {
    const { url } = await fetch("/api/verify/start", { method: "POST" }).then((r) => r.json());
    // Opens a sleek in-page modal dialog right on your site (never blocked by popup blockers)
    const { sessionId, status } = await LKP.verify(url, { mode: "modal" });
    if (status === "verified") await fetch("/api/verify/complete", { method: "POST" });
  };
</script>

The user unlocks their LKP wallet with a passkey (or creates one in seconds), and their browser generates a zero-knowledge proof in ~40ms. First-time users take a quick 1-2s selfie or scan an ID document right there.

Privacy notice for your users

You can display this short notice next to your verify button:
"The site never sees your identity document; LKP deletes Didit session data after the check; Didit may keep limited fraud and audit records as required by law."

4. Confirm the result on your server#

The browser's status is a hint for your UI. Before granting anything, fetch the session with your secret key:

app.post("/api/verify/complete", async (req, res) => {
  const session = await fetch(
    `${process.env.LKP_API_URL}/v1/verification-sessions/${req.session.lkpSessionId}`,
    { headers: { Authorization: `Bearer ${process.env.LKP_SECRET_KEY}` } },
  ).then((r) => r.json());

  if (session.status !== "verified") return res.status(403).json({ error: "Not verified" });

  // Stable, anonymous, app-specific user ID
  req.session.verifiedUser = session.result.nullifier;
  res.json({ ok: true });
});

Bind the session to your user

Look up the session ID you stored for this user in step 2. Don't accept a session ID from the browser, or someone could replay a session that another person completed.

Next steps#

Add webhooks so your backend hears about verifications without polling.
Use mode: "ONE_TIME_ACTION" for votes, giveaways and anything a person should only do once. See the API reference.
Watch conversion and unique users on your app's overview in the console.