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)

c939376a4e622d9aa77cd22f76336c5ce242d496fd219f143d6d057e1cce868c

Revealed seed (published after the draw)

17ce9e27b88752caf99dcc38f4467c62a6e1ee124f0dc101dde09c0ebbe3e4f1

Beacon round (emitted after close) & its public value

Round 6,273,147 on the drand chain 8990e7a9aaed2ffed73dbd7092123d6f289930540d7651336225dc172e51b2ce

8667049c630080177449b63bc85859fecc4412c21f490f82d586cd338c4e91b0

Re-fetch it: curl https://api.drand.sh/8990e7a9aaed2ffed73dbd7092123d6f289930540d7651336225dc172e51b2ce/public/6273147

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' '17ce9e27b88752caf99dcc38f4467c62a6e1ee124f0dc101dde09c0ebbe3e4f1' | sha256sum

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

Every one of the 160 tickets (numbered 1 to 160) 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 #30:

$seed  = '17ce9e27b88752caf99dcc38f4467c62a6e1ee124f0dc101dde09c0ebbe3e4f1';
$total = 160;                       // tickets in the draw (numbers 1..160)
$msg   = 'draw:019f340e-bc64-718f-81eb-dbd0e65acd43:'.$total.':8667049c630080177449b63bc85859fecc4412c21f490f82d586cd338c4e91b0';   // 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 #30

3. Ticket #30 belongs to the winner

That ticket number is held by exactly one entry - Jessica I. from Langport. 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

dff2b11d40eb925a2793da0d0facf6e8f3959145da7f52ee01d396291e1d7acf

How ticket numbers are assigned

This competition has a fixed set of 160 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.