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)

ca79c00f9c6d113edf62e2346f249e080565c64ab7e014efa33ae6bc26b5f6d7

Revealed seed (published after the draw)

a1702a39e13b438b4dae4f462a52a3181b6f43c157db529837015a39d4276aa3

Beacon round (emitted after close) & its public value

Round 6,624,687 on the drand chain 8990e7a9aaed2ffed73dbd7092123d6f289930540d7651336225dc172e51b2ce

ce4599064962e7bfc17c4cdf07da4a4e25e851f10e97e8636b696ba4bbfcbd51

Re-fetch it: curl https://api.drand.sh/8990e7a9aaed2ffed73dbd7092123d6f289930540d7651336225dc172e51b2ce/public/6624687

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' 'a1702a39e13b438b4dae4f462a52a3181b6f43c157db529837015a39d4276aa3' | sha256sum

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

Every one of the 1,256 tickets (numbered 1 to 1,256) 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. 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. Recompute it and you'll get ticket #1,081:

$seed  = 'a1702a39e13b438b4dae4f462a52a3181b6f43c157db529837015a39d4276aa3';
$total = 1256;                       // tickets in the draw (numbers 1..1256)
$msg   = 'draw:01a17490-aeea-73c3-91cd-b46629c4e1a4:'.$total.':ce4599064962e7bfc17c4cdf07da4a4e25e851f10e97e8636b696ba4bbfcbd51';   // 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 #1081

3. Ticket #1,081 belongs to the winner

That ticket number is held by exactly one entry - Sarah B. from Alcester. 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

5ce0b332048c8b439b13c437dd094fad611cc2f47cf572924f6f18cdfb09900a

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.