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)

93ba50b9c421f86016769c80e5e2c1e0e7058bc90607a520b02b02a02e92f1f3

Revealed seed (published after the draw)

ab11be8298e2f46b6299d19588137dc2968d1f6118f464e719e6cd6c7b4005ad

Beacon round (emitted after close) & its public value

Round 6,270,267 on the drand chain 8990e7a9aaed2ffed73dbd7092123d6f289930540d7651336225dc172e51b2ce

1fabde7641ff94a30d251606d6997bafcb4ea55e32d014da9f2b285258b2884d

Re-fetch it: curl https://api.drand.sh/8990e7a9aaed2ffed73dbd7092123d6f289930540d7651336225dc172e51b2ce/public/6270267

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' 'ab11be8298e2f46b6299d19588137dc2968d1f6118f464e719e6cd6c7b4005ad' | sha256sum

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

The 236 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 #84:

$seed    = 'ab11be8298e2f46b6299d19588137dc2968d1f6118f464e719e6cd6c7b4005ad';
$tickets = [ /* the 236 ticket numbers in the draw, sorted ascending */ ];
$total   = count($tickets);
$msg     = 'draw:019f0fa6-1c8d-71cd-b4bf-15943d149c2d:'.$total.':1fabde7641ff94a30d251606d6997bafcb4ea55e32d014da9f2b285258b2884d';   // 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 #84

3. Ticket #84 belongs to the winner

That ticket number is held by exactly one entry - Natalie C. from South Molton. 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

e78f6b27f788e67abbb80236fb53771fe8b76a4beab2148dd61605517161de41

How ticket numbers are assigned

This competition has a fixed set of 280 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.