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)

5b51f1ac199b21acaac243f7ab4559f54391040708216bbee79b4cdcc5ee01a9

Revealed seed (published after the draw)

afe77f27d6eb00c3a94ca6ed5cb2e2b16cbdd30c799c8e836c39b5bd85580f55

Beacon round (emitted after close) & its public value

Round 6,284,667 on the drand chain 8990e7a9aaed2ffed73dbd7092123d6f289930540d7651336225dc172e51b2ce

878b7c14d6b3ac7347123a5050e08a2a80aed8b2307f0227e5a0fec87ba1a258

Re-fetch it: curl https://api.drand.sh/8990e7a9aaed2ffed73dbd7092123d6f289930540d7651336225dc172e51b2ce/public/6284667

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' 'afe77f27d6eb00c3a94ca6ed5cb2e2b16cbdd30c799c8e836c39b5bd85580f55' | sha256sum

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

Every one of the 120 tickets (numbered 1 to 120) went into this draw. The winner is drawn from the locked-in seed mixed with the post-close beacon value shown above - both are in the $msg below. 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. Recompute it and you'll get ticket #47:

$seed  = 'afe77f27d6eb00c3a94ca6ed5cb2e2b16cbdd30c799c8e836c39b5bd85580f55';
$total = 120;                       // tickets in the draw (numbers 1..120)
$msg   = 'draw:019f38ec-037e-7201-bd0a-0c58543b08ac:'.$total.':878b7c14d6b3ac7347123a5050e08a2a80aed8b2307f0227e5a0fec87ba1a258';   // 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 >= $limit);
echo ($n % $total) + 1;                     // => winning ticket #47

3. Ticket #47 belongs to the winner

That ticket number is held by exactly one entry - Amy R. from Washington. 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

1c4732eaf0b754ef54c34467457cbbe4d233b277b73e72505aaef7dbb4c5bd08

How ticket numbers are assigned

This competition has a fixed set of 120 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.