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)

ff33b1fe2d89db68121323816ead50c43f3be4681a43b4efe3e41db2aa26621f

Revealed seed (published after the draw)

c666ef6b5991a71b55f27661a9d388d6730e0ffc332a6ecd0d2ad7f83a526a86

Beacon round (emitted after close) & its public value

Round 6,316,347 on the drand chain 8990e7a9aaed2ffed73dbd7092123d6f289930540d7651336225dc172e51b2ce

450cd9103563947e7d4c50c8b150cf45e40c84acc38d44f1a8b8b157c3090365

Re-fetch it: curl https://api.drand.sh/8990e7a9aaed2ffed73dbd7092123d6f289930540d7651336225dc172e51b2ce/public/6316347

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' 'c666ef6b5991a71b55f27661a9d388d6730e0ffc332a6ecd0d2ad7f83a526a86' | sha256sum

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

The 252 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 #46:

$seed    = 'c666ef6b5991a71b55f27661a9d388d6730e0ffc332a6ecd0d2ad7f83a526a86';
$tickets = [ /* the 252 ticket numbers in the draw, sorted ascending */ ];
$total   = count($tickets);
$msg     = 'draw:019f7704-94ca-72b1-9e99-9b21e50550af:'.$total.':450cd9103563947e7d4c50c8b150cf45e40c84acc38d44f1a8b8b157c3090365';   // 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 #46

3. Ticket #46 belongs to the winner

That ticket number is held by exactly one entry - Rowena E. from Worthing. 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

daaa4308db0a2a87fa7e188c1767b7d99102100902493e30eff460e8c205ea20

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' 'Ut3yuNoWyYYMUWROe0i6JcwiUvMrKfks11Y6MZ0y' | sha256sum        // => 8b786d9183ec21c7f4665cdb8716d0281e9a93a0fdebe7e7b8dcba902a67420c

// 2. re-run the shuffle: position -> ticket number
$seed   = 'Ut3yuNoWyYYMUWROe0i6JcwiUvMrKfks11Y6MZ0y';
$domain = 300;   // 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.