Verify this draw

How this draw is provable

Before entries closed we published a commitment: a one-way fingerprint (SHA-256) of a secret random seed. After the draw we revealed the seed. Because the commitment was public before entries closed, the seed could not have been changed afterwards to draw a particular ticket.

The draw also folds in a public randomness beacon (drand, the League of Entropy) whose value is only published after entries close. Anyone can re-fetch the exact beacon round below and confirm the same value went into the draw.

Commitment (published before entries closed)

ec19729669bbaf4e1a174dc3a8e9ddf263763646225ad21bd709c9ab6da7dae6

Revealed seed (published after the draw)

676b5e2729c401ec05e0ec472118a9413dfda6604874eaea5573c74acc0c6650

Beacon round (emitted after close) & its public value

Round 6,315,079 on the drand chain 8990e7a9aaed2ffed73dbd7092123d6f289930540d7651336225dc172e51b2ce

cf6fe9dc1543e84be4e64325743b4a31d7b5065496aa441cc31bc69a0dc33a8f

Re-fetch it: curl https://api.drand.sh/8990e7a9aaed2ffed73dbd7092123d6f289930540d7651336225dc172e51b2ce/public/6315079

Check it yourself

1. The seed matches the commitment

Run SHA-256 on the revealed seed - it equals the commitment above. For example, in a terminal:

printf '%s' '676b5e2729c401ec05e0ec472118a9413dfda6604874eaea5573c74acc0c6650' | sha256sum

2. The seed and the beacon draw the winning ticket number

The 54 ticket numbers that went into this draw are lined up in ascending order; the locked-in seed mixed with the post-close beacon value shown above (both are in the $msg below) picks one. Because the seed was fixed before entries closed and the beacon value didn't exist until after, the result was set in advance yet could not be steered. Sort the numbers in play (their fingerprint is below, and the full set is in the entrants list) and recompute - you'll get ticket #36:

$seed    = '676b5e2729c401ec05e0ec472118a9413dfda6604874eaea5573c74acc0c6650';
$tickets = [ /* the 54 ticket numbers in the draw, sorted ascending */ ];
$total   = count($tickets);
$msg     = 'draw:019f6f4a-7f93-7303-8de5-ad49001dd490:'.$total.':cf6fe9dc1543e84be4e64325743b4a31d7b5065496aa441cc31bc69a0dc33a8f';   // note: the trailing value is the drand beacon above
$span    = 1 << 60;                        // 60-bit space
$limit   = intdiv($span, $total) * $total; // reject above this to remove bias
$i = 0;
do {
    $n = (int) hexdec(substr(hash_hmac('sha256', $msg.':'.$i, $seed), 0, 15));
    $i++;
} while ($n >= $limit);
echo $tickets[$n % $total];                 // => winning ticket #36

3. Ticket #36 belongs to the winner

That ticket number is held by exactly one entry - Chloe F.. We publish a fingerprint of every ticket number in the draw, so the set of numbers can't be altered after the fact, and we hold the full list in our tamper-evident audit trail, available to the Advertising Standards Authority on request.

Ticket-numbers fingerprint

6bd80753ed259819e1f1c820d0512cf38a2ba73e0ab0c9bce722fe7dd2d66d54

How ticket numbers are assigned

You don’t choose your ticket number (unless a competition lets you choose your own), and it isn’t simply the next number in order. When you buy, the system gives you a number using a secret shuffle that was set when the competition opened, before anyone had bought a ticket, and can’t be changed after.

Because that shuffle is secret and gives everyone a different number:

• Two people can never end up with the same number.
• Nobody, not you and not us, can guess which number a purchase will get. On this draw the number itself doesn’t decide anything anyway: the winner is picked by the draw above, which nobody can predict.
• Your number is fixed the instant you buy, and recorded straight away.

We keep the shuffle secret so no one can work out in advance which number a purchase will get, which is what stops anyone targeting a winning ticket. Every number that’s given out still appears in the competition’s entrants list, so the numbers in play are public and can’t be quietly changed later.

// 1. the key matches the fingerprint published at open:
printf '%s' '4TIxWKBuJHzdVPT16VdmgARX3sksVtky2vJzEraS' | sha256sum        // => 3238cebaa141be578789f61152422927530006aa9464f7fcdb2aff9756d1a9b5

// 2. re-run the shuffle: position -> ticket number
$seed   = '4TIxWKBuJHzdVPT16VdmgARX3sksVtky2vJzEraS';
$domain = 1000;   // the pool the numbers are shuffled within
$prf = fn($v,$r) => (int) hexdec(substr(hash('sha256', "$seed:$r:$v"), 0, 8));
$permute = function(int $pos) use ($domain, $prf) {
    $half = (int) ceil(((int) ceil(log($domain, 2))) / 2);
    $mask = (1 << $half) - 1;
    $x = $pos - 1;
    do {
        $l = ($x >> $half) & $mask; $r = $x & $mask;
        for ($i = 0; $i < 4; $i++) { $n = $l ^ ($prf($r, $i) & $mask); $l = $r; $r = $n; }
        $x = ($l << $half) | $r;
    } while ($x >= $domain);
    return $x + 1;
};
// $permute(1), $permute(2), ... are the ticket numbers, in the order they were issued.