Preface#

My blog has always been hosted on Cloudflare — what the community affectionately calls the “cyber Bodhisattva” — where I freeload its edge nodes and various free services (I even wrote a post about deploying website analytics on Cloudflare). As a developer constantly harassed by bots yet annoyed by traditional CAPTCHAs, I had long wanted an “invisible” solution — one that neither makes users click traffic lights and pick fire hydrants, nor fails to stop scripts.

Lately I’ve been tinkering with and maintaining a project called Lap, which is exactly such a lightweight, open-source CAPTCHA alternative. Under the hood it relies on Proof-of-Work and browser instrumentation. No images, no puzzles, no tracking — the user is verified in the background while their browser does “a little maths.”

Below I’ll walk you through, from a first-person perspective, what Lap is and how to run it on Cloudflare at zero cost.

1. What Is Lap / Core Features#

At its heart, Lap is a CAPTCHA replacement: when someone submits your form, their browser quietly performs a series of hash computations to prove “I am not a low-cost script,” and only then is it allowed through. A few things I really like about it:

• Zero infrastructure: A single Cloudflare Worker plus a single D1 (SQLite) database is enough. No Docker, no Redis, no VPS, and no always-on process — fully Serverless.
• Free-tier friendly: The whole thing fits comfortably inside Cloudflare’s free Workers + D1 allowances, so personal projects are basically zero-cost.
• Self-hosted widget: The Worker serves the frontend widget itself from /widget.js, so it depends on no third-party CDN or npm package — no supply-chain risk.
• Privacy first: No telemetry, no cookies, no cross-site tracking.
• Runs at the edge: Challenges are issued from the Cloudflare PoP nearest the user.

Fun fact: Lap is actually a rebrand fork I made of Cap (by tiago, under Apache-2.0), porting its originally Node + Redis/Valkey architecture into a pure Cloudflare Serverless setup and renaming the components (<cap-widget> → <lap-widget>). All the cryptographic design and frontend interaction come from Cap’s work, so please also go give the original project a Star.

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 1. Browser requests a challenge  POST /:siteKey/challenge
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        │
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        ▼
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   Lap Worker issues a challenge { challenge, token } (with c/s/d params)
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        │
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        ▼
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 2. Browser brute-forces a nonce so that sha256(salt + nonce) starts with target (this is the PoW)
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        │
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        ▼
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 3. Browser submits the solution  POST /:siteKey/redeem → Worker verifies and issues a one-time redeem token
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        │
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        ▼
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 4. Your backend exchanges the token at POST /siteverify for { success: true }

2. Deployment Walkthrough (Workers + D1, recommended)#

Lap offers three deployment paths: Workers + D1 (recommended), Pages Functions, and GitHub Actions for CI/CD. Below I’ll take the most common, CI-used Workers + D1 route.

What you’ll need:

• A Cloudflare account x1 (free is fine)
• Node.js 18+ (Node 22 recommended)
• A working brain 🧠 x1
• A decent network environment x1

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git clone https://github.com/lenmei233/Lap.git lap
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cd lap/cloudflare
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npm install
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npx wrangler login      # opens a browser to authorize your Cloudflare account

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npx wrangler d1 create lap-serverless

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[[d1_databases]]
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binding = "DB"
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database_name = "lap-serverless"
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database_id = "0f9c1a2b-3c4d-5e6f-7a8b-9c0d1e2f3a4b"   # <- yours

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npx wrangler d1 migrations apply lap-serverless --remote

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npx wrangler secret put ADMIN_KEY
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# paste a long random string, e.g. openssl rand -hex 32

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npm run deploy      # runs sync-widget, then wrangler deploy

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curl https://lap-serverless.<your-subdomain>.workers.dev/health
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# {"ok":true,"service":"lap-serverless","version":"1.0.0"}

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cd cloudflare
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node scripts/sync-widget.mjs        # generates public/ + src/assets/
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npx wrangler pages project create lap-serverless --production-branch main
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npx wrangler pages deploy public --project-name lap-serverless

3. Create a site key and embed the widget#

After deployment you can’t use it directly yet — you first need to mint a site key for your website. Its secret is shown only once, so store it safely.

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ORIGIN=https://lap-serverless.<your-subdomain>.workers.dev
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ADMIN_KEY=<the key you set earlier>
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curl -X POST "$ORIGIN/admin/keys" \
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  -H "x-admin-key: $ADMIN_KEY" \
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  -H "content-type: application/json" \
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  -d '{}'

Sample response:

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{
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  "id": "a1b2c3d4e5f60718293a4b5c",
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  "secret": "9f8e7d6c5b4a39281706f5e4d3c2b1a0",
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  "config": { "difficulty": 4, "challengeCount": 80, "saltSize": 32 }
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}

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<script src="https://lap-serverless.<your-subdomain>.workers.dev/widget.js"></script>
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<form method="POST" action="/signup">
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  <input name="email" type="email" required />
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  <lap-widget
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    data-lap-api-endpoint="https://lap-serverless.<your-subdomain>.workers.dev/<SITE_KEY>/">
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  </lap-widget>
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  <button type="submit">Sign up</button>
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</form>

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<script src=".../widget.js"></script>
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<script>
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  const lap = new Lap({ apiEndpoint: "https://.../<SITE_KEY>/" });
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  const { token } = await lap.solve();
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  // send the token to your backend yourself
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</script>

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lap-widget {
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  --lap-background: #11111b;
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  --lap-border-color: #313244;
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  --lap-border-radius: 12px;
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  --lap-color: #cdd6f4;
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  --lap-spinner-color: #89b4fa;
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}

4. Verify on your server (you must)#

A token from the browser means nothing until verified server-side at /siteverify, and each token is consumed on first use.

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curl -X POST "$ORIGIN/siteverify" \
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  -H "content-type: application/json" \
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  -d '{"secret":"<SITE_SECRET>","response":"<SITEKEY>:<ID>:<TOKEN>"}'
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# {"success":true}

Node.js / Express example:

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app.post("/signup", async (req, res) => {
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  const token = req.body["lap-token"];
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  const r = await fetch(`${process.env.LAP_ORIGIN}/siteverify`, {
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    method: "POST",
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    headers: { "content-type": "application/json" },
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    body: JSON.stringify({ secret: process.env.LAP_SECRET, response: token }),
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  });
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  const { success } = await r.json();
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  if (!success) return res.status(403).send("CAPTCHA failed");
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  // ...continue your business logic
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});

Python / Flask version:

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import os, requests
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from flask import request, abort
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@app.post("/signup")
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def signup():
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    r = requests.post(
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        f"{os.environ['LAP_ORIGIN']}/siteverify",
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        json={"secret": os.environ["LAP_SECRET"], "response": request.form.get("lap-token")},
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        timeout=10,
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    )
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    if not r.json().get("success"):
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        abort(403, "CAPTCHA failed")
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    # ...continue

5. Local dev and troubleshooting#

Local debugging needs no Cloudflare account:

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cd cloudflare
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npm install
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echo "ADMIN_KEY=dev-secret" > .dev.vars
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npx wrangler d1 migrations apply lap-serverless --local
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npx wrangler dev --local

The Worker runs at http://127.0.0.1:8787 against a local SQLite file, ready to use out of the box.

Common pitfalls:

• {"error":"Admin key not configured"}: ADMIN_KEY isn’t set — run wrangler secret put ADMIN_KEY.
• no such table: site_keys: migrations weren’t applied — run the d1 migrations apply step (add --remote for production).
• D1_ERROR: your wrangler.toml database_id is probably still the placeholder.
• Widget doesn’t show: check the browser console for a 404 on /widget.js, then confirm <lap-widget> is spelled correctly.
• Widget shows but never verifies: data-lap-api-endpoint must end in / and include the site key.

Closing#

What impresses me most about Lap is its “quietness” — no puzzle popups, no face scans, no cross-site tracking; it just lets the browser silently finish one computation in the background, and everything goes on as usual. Running it on Cloudflare means zero servers, zero ops, within the free tier, which is almost the perfect CAPTCHA replacement for personal blogs, forms, and small tools.

If you’re also tired of traditional CAPTCHAs and want to own your data, fork Lap and follow the steps above — ten minutes is all it takes to have your own invisible verification service. And if you could spare a Star, that would be even better (.

中文版：用 Cloudflare 免费部署属于自己的无感验证码：Lap 介绍与部署教程