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.

Commitment (published before entries closed)

414daa20b3ea317ea3cb4a42d3c12f522eea504013fe4e5fb3ca8cce1aa8d3e8

Revealed seed (published after the draw)

96c2428fcda1e8bd2720fdd90b4e07c2680d9237d370be9c19f692d8adc10a5b

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' '96c2428fcda1e8bd2720fdd90b4e07c2680d9237d370be9c19f692d8adc10a5b' | sha256sum

2. The seed draws the winning ticket number

Every one of the 120 tickets (numbered 1 to 120) went into this draw. The seed picks one - and because it comes from the locked-in seed, it couldn't be steered once entries were seen. Recompute it and you'll get ticket #47:

$seed  = '96c2428fcda1e8bd2720fdd90b4e07c2680d9237d370be9c19f692d8adc10a5b';
$total = 120;                       // tickets in the draw (numbers 1..120)
$msg   = 'draw:019f0b30-7fd5-7169-8a55-4fe1489b4c23:'.$total;
$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 - Joanne H. from Street. 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.