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Ribaunt protects your forms and sensitive actions by making the browser perform a small but measurable amount of computational work before your server accepts a submission. Unlike traditional CAPTCHAs, this happens silently in the background: your server issues a cryptographic puzzle, the browser grinds through SHA-256 hashes until it finds a valid answer, and your server checks the proof before proceeding. No image grids, no checkbox rituals — just math.

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The challenge-response flow

Every Ribaunt verification follows a three-step cycle.
1

Server issues challenge tokens

Your server calls createChallenge(), which produces an array of signed JWT tokens. Each token encodes a random challenge string, the difficulty level (how many leading zeros the winning hash must start with), an expiry timestamp, and a unique token ID (jti) for replay prevention. The tokens are signed with your RIBAUNT_SECRET, so they cannot be forged or tampered with by the client.
import { createChallenge } from 'ribaunt';

// Issue 4 challenges at difficulty 5, valid for 60 seconds
const challenges = createChallenge(5, 4, 60);
2

Browser solves the challenges

The browser receives the tokens and runs the solver. For each token it decodes the challenge string and difficulty, then iterates through candidate nonces — 0, 1, 2, … — computing SHA-256(challenge + nonce) for each one using the Web Crypto API. The moment it finds a nonce whose hash starts with the required number of leading zeros, it records that nonce and hash as the solution and moves on to the next token.
3

Server verifies the solutions

The browser submits the original tokens alongside the solutions it found. Your server calls verifySolution(), which:
  • Verifies the JWT signature with RIBAUNT_SECRET
  • Checks that the token has not expired
  • Recomputes SHA-256(challenge + nonce) and confirms it starts with the required leading zeros
  • Checks the jti against the replay store to ensure the token has never been used before
Only if every check passes does verifySolution() return true.
import { verifySolution } from 'ribaunt';

const valid = await verifySolution(tokens, solutions);

if (!valid) {
  return res.status(400).json({ error: 'Invalid CAPTCHA solution' });
}

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Challenge tokens

Each challenge token is a standard signed JWT. When decoded, its payload contains:
FieldDescription
challengeA random base64 string generated fresh for each token
difficultyThe number of leading zeros the SHA-256 hash must start with
expiresA Unix timestamp (seconds) after which the token is rejected
jtiA UUID that uniquely identifies this token, used for replay prevention
The tokens are signed with RIBAUNT_SECRET, an environment variable you set on your server. Your server is the only party that can produce or validate tokens — the browser only ever sees the signed, opaque JWT.
Never expose RIBAUNT_SECRET to the browser. In Next.js, do not prefix it with NEXT_PUBLIC_.

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Difficulty

The difficulty parameter controls how many leading zeros the winning hash must have. Because each additional zero cuts the probability of a random hash qualifying by a factor of 16, every increment roughly doubles the expected number of hash attempts — and therefore the solve time.
SettingApproximate solve timeRecommended use
createChallenge(4, 4, 30)MillisecondsFast / background checks
createChallenge(5, 4, 60)~1 secondModerate / form submission
createChallenge(5, 8, 120)~2 secondsHigh / sensitive actions
Values above 6 may cause browsers to hang. Do not let users or external input control difficulty, amount, or ttlSeconds without validation.

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Stateless design

Ribaunt does not require a database to issue or verify challenges. All the information the server needs to verify a solution — the challenge string, the required difficulty, and the expiry — is encoded directly in the signed JWT token. Verification reduces to checking the JWT signature and recomputing the hash; no round-trips to a data store are necessary for the core proof check. Replay protection is the one area that requires state. Without it, a single valid solution could be resubmitted repeatedly within the token’s TTL window. Ribaunt handles this with a lightweight replay store:
  • In the default local mode, an in-process Map tracks used token IDs within the current Node.js process.
  • In remote mode, you supply a distributed store (such as Redis or Valkey) so that multiple processes or serverless function instances share a consistent view of which tokens have already been consumed.
This keeps your server infrastructure simple — no session tables, no challenge databases — while still providing the replay protection that makes the scheme secure.
For more detail on configuring replay protection, including a full Redis example, see Replay Protection.

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Secure context requirement

The browser solver uses the Web Crypto API (crypto.subtle), which browsers only expose in secure contexts. This means client-side solving works on:
  • https:// origins (production)
  • http://localhost (local development)
Plain LAN addresses such as http://192.168.x.x may not expose crypto.subtle, especially on mobile browsers. Always serve your application over HTTPS in production.