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)

f647d1d1f50f8e4fdd173a329068d4fb62fdb9e790164fc3b024f3b011d6d919

Revealed seed (published after the draw)

9566ba6787e7c920416b9695f4080e5b44d71da3c5103a935de1f327d1b8f410

Beacon round (emitted after close) & its public value

Round 6,523,767 on the drand chain 8990e7a9aaed2ffed73dbd7092123d6f289930540d7651336225dc172e51b2ce

19439d64d4810e78c6f250bd8c2fa40c3a774aaa195079fc0526fdfcf7231835

Re-fetch it: curl https://api.drand.sh/8990e7a9aaed2ffed73dbd7092123d6f289930540d7651336225dc172e51b2ce/public/6523767

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' '9566ba6787e7c920416b9695f4080e5b44d71da3c5103a935de1f327d1b8f410' | sha256sum

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

The 906 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 #1,191:

$seed    = '9566ba6787e7c920416b9695f4080e5b44d71da3c5103a935de1f327d1b8f410';
$tickets = [ /* the 906 ticket numbers in the draw, sorted ascending */ ];
$total   = count($tickets);
$msg     = 'draw:01a0c096-b2f4-70f9-86ae-11d99be7be63:'.$total.':19439d64d4810e78c6f250bd8c2fa40c3a774aaa195079fc0526fdfcf7231835';   // 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 #1191

3. Ticket #1,191 belongs to the winner

That ticket number is held by exactly one entry - Natasha O. from Plymouth. 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

4a67126070bf709b1d2344c795446b118ec41d082b69a78ef7bf5161aee5ea00

How ticket numbers are assigned

This competition has a fixed set of 1,256 tickets, and on this draw you choose your own ticket number when you enter, from the numbers still available. They aren’t handed out one after another in entry order. Your number is fixed the instant you buy, timestamped, and independently verifiable there and then.

Whichever number you pick doesn’t decide anything on its own. The winner is picked at the draw (shown above), from a secret value committed before entries closed and a public randomness beacon that didn’t exist until after they closed, so the number you chose can’t affect who wins. Every number that’s taken 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.