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)

fc30ec0adb5c0aeed8a5aa0aa057a7176c6b5720a1ea0bef2df56eb95714465c

Revealed seed (published after the draw)

aa4854580906cc9705a3b2ee63c8b08276d9368cfd5359046ab5c29649b43d03

Beacon round (emitted after close) & its public value

Round 6,379,707 on the drand chain 8990e7a9aaed2ffed73dbd7092123d6f289930540d7651336225dc172e51b2ce

e15d9d75091b6850ec17ebca34a6208eab3302fb5d1350803a81ea5c8c4cc47f

Re-fetch it: curl https://api.drand.sh/8990e7a9aaed2ffed73dbd7092123d6f289930540d7651336225dc172e51b2ce/public/6379707

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' 'aa4854580906cc9705a3b2ee63c8b08276d9368cfd5359046ab5c29649b43d03' | sha256sum

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

The 75 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. The seed was committed before entries closed, and the public beacon that decides the winner didn't exist until after - so the result couldn't be set in advance or predicted. Sort the numbers in play (their fingerprint is below, and the full set is in the entrants list) and recompute - you'll get ticket #585:

$seed    = 'aa4854580906cc9705a3b2ee63c8b08276d9368cfd5359046ab5c29649b43d03';
$tickets = [ /* the 75 ticket numbers in the draw, sorted ascending */ ];
$total   = count($tickets);
$msg     = 'draw:019fe600-8b29-72ed-af0e-43fad0bafa07:'.$total.':e15d9d75091b6850ec17ebca34a6208eab3302fb5d1350803a81ea5c8c4cc47f';   // 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 #585

3. Ticket #585 belongs to the winner

That ticket number is held by exactly one entry - Sally S. from Northampton. 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

28f5ad2d926dd99109cc3bb7de8e68f74500aa4752f15f07de169d44ee90e5a1

How ticket numbers are assigned

On this competition you don’t choose your ticket number, 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 isn’t what picks the winner - that’s 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.

You can see them: every entry and its ticket number is listed on the competition’s entrants page.

// 1. the key matches the fingerprint published at open:
printf '%s' 'gUJp7g1fXqlwIyKFaRi3jHCMcEQnaIdR5qwdkjX4' | sha256sum        // => 379c854bc7a63cbbbe9afadd619460b0a6c556222fe6fe690a9a06f795099f87

// 2. re-run the shuffle: position -> ticket number
$seed   = 'gUJp7g1fXqlwIyKFaRi3jHCMcEQnaIdR5qwdkjX4';
$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.