The Tool Desk
Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →A provably fair giveaway lets entrants check that the organizer committed to a secret before the draw inputs were fixed, then reproduce the published result after the secret is revealed. The code below is a compact Node.js example of that idea—not a complete giveaway system. It assumes the entrant list and rules have already been frozen and that everyone agrees on how the draw maps a cryptographic value to a winner.
What a verifiable giveaway can—and cannot—prove
A commit-reveal draw starts with a secret server seed. Before the remaining inputs are fixed, the organizer publishes the seed’s SHA-256 hash. After entries close and the entrant list is frozen, the organizer calculates the result using the secret and the agreed inputs, then reveals the seed. Entrants can hash the revealed seed to check the commitment and rerun the documented calculation to check the result. Changing the seed after publishing its hash would produce a different hash.
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This establishes a narrow, useful guarantee: the disclosed seed matches the earlier commitment, and the published result follows from the disclosed inputs and algorithm. It does not prove that the organizer included every eligible entrant, applied the rules fairly, or will award the prize. Those parts require transparent procedures and evidence outside the cryptographic calculation.
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What to publish before entries close
Post the commitment and draw rules somewhere participants can access before the draw inputs are finalized. To make independent replay possible, publish:
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- The SHA-256 commitment to the server seed.
- The entry cutoff, eligibility rules, and handling of duplicate or invalid entries.
- The ordered entrant list, or a digest together with an accessible method for checking the exact list.
- The number of winners and the precise algorithm, including how it maps a digest to an entrant index.
Freeze the entrant list and rules before calculating the outcome. A seed cannot make a draw auditable if participants cannot establish which list and rules were used.
A compact Node.js commit-and-reveal example
This 10-line example uses only Node.js’s built-in crypto module. It derives a deterministic pseudorandom stream by hashing the seed and a counter, then uses rejection sampling to select an unbiased index. It assumes entries is the already-frozen, ordered array of eligible entrants and that the organizer has securely retained serverSeed. Publish the hash of that seed before fixing the rest of the draw inputs; after the draw, reveal the seed and publish the list and result so entrants can rerun the calculation.
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const { createHash, randomBytes } = require('node:crypto');
const serverSeed = randomBytes(32).toString('hex');
const commit = createHash('sha256').update(serverSeed).digest('hex');
// Publish commit before the entrant list and other draw inputs are fixed.
const entries = ['entrant-1', 'entrant-2', 'entrant-3']; // frozen ordered list
const n = entries.length, limit = Math.floor(2 ** 32 / n) * n;
let i = 0, x;
do { const h = createHash('sha256').update(`${serverSeed}:${i++}`).digest(); x = h.readUInt32BE(0); } while (x >= limit);
const winnerIndex = x % n;
console.log({ commit, winner: entries[winnerIndex], winnerIndex });The rejection step discards values at or above limit. That keeps the accepted 32-bit values evenly divisible across the number of entries; simply taking a digest value modulo the entrant count without this check can favor some indices when the range does not divide evenly. This example uses a single seed and counter-based SHA-256 derivation rather than a standardized giveaway protocol. For a real draw, document the exact code or algorithm, input encoding, list order, and output format so independent implementations agree.
The sample’s illustrative entrant names are not a real participant list. Replace them with the exact frozen list and make that list available to entrants. A public commitment alone does not reveal or authenticate the list.
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How entrants verify the draw
- Check that the commitment was published before the entrant list and other draw inputs were finalized.
- After the organizer reveals the server seed, calculate its SHA-256 hash and compare it with the published commitment.
- Use the published ordered entrant list and the exact algorithm to recompute the selected index and winner.
- Check that the published winner corresponds to that index and that the revealed inputs match the announced cutoff and eligibility rules.
For the example, the verification implementation must use the same UTF-8 string format, counter starting at zero, first four digest bytes interpreted as a big-endian unsigned integer, and rejection condition. Otherwise, two implementations could disagree even when both use SHA-256.
Why not use Node.js randomInt for this draw?
Node.js documents crypto.randomInt(min, max) as returning an integer with an inclusive lower bound and exclusive upper bound while avoiding modulo bias. Its bounds must be safe integers, and the range must be less than 248. But an ordinary call to randomInt is not seeded for replay: entrants cannot reproduce its output from a later-revealed secret. A replayable draw needs a specified deterministic derivation and unbiased mapping, as in the example’s counter hash and rejection step. See the Node.js Crypto documentation v26.8.1.
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- The back of the "ticket" part has a place for Name/Address/City/Phone
- With a deposit ticket and keep ticket
- Perfect for a wide range of events including carnivals, school fundraisers, class rewards, and prize drawings.
When a hosted commit-reveal API may fit
A hosted service can provide commitment and reveal endpoints, but its workflow is a particular implementation, not a universal requirement. Provable.IO’s API reference documents a commit endpoint that returns a commitment ID and server hash, followed by a single-use reveal request with the client seed and generation parameters. The reference lists a default commitment TTL of 10 minutes; confirm the service’s current documentation and constraints before relying on that value.
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Whether you self-host or use an API, ensure the public record gives entrants the inputs needed to verify the commitment and reproduce the result. A provider’s endpoint workflow does not, by itself, settle whether the entrant list was complete or the rules were followed.
Quick Recap
Operational safeguards beyond the code
- Keep the unrevealed seed private until the draw; disclose it afterward so participants can check the commitment.
- Publish the commitment early enough that entrants can see it before the remaining draw inputs are fixed.
- Preserve the exact ordered entrant list, rules, commitment, revealed seed, and result record.
- Explain how to report a mismatch and who will resolve disputes; cryptography cannot compel the organizer to honor the result.
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