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)

53405287340ed532fb655c010141eadd21ff9f26e28482376891da245ec15a00

Revealed seed (published after the draw)

dcc727b3d43252855eddf5429d25b6dc7405f1b8ec2b8f64429136771c52fa9f

Beacon round (emitted after close) & its public value

Round 6,324,987 on the drand chain 8990e7a9aaed2ffed73dbd7092123d6f289930540d7651336225dc172e51b2ce

ba5314e49470641865661fea0e2b692efbce233d6e6849085efe1735bc29ff1b

Re-fetch it: curl https://api.drand.sh/8990e7a9aaed2ffed73dbd7092123d6f289930540d7651336225dc172e51b2ce/public/6324987

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' 'dcc727b3d43252855eddf5429d25b6dc7405f1b8ec2b8f64429136771c52fa9f' | sha256sum

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

The 240 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 #82:

$seed    = 'dcc727b3d43252855eddf5429d25b6dc7405f1b8ec2b8f64429136771c52fa9f';
$tickets = [ /* the 240 ticket numbers in the draw, sorted ascending */ ];
$total   = count($tickets);
$msg     = 'draw:019f7f9e-9be7-73ce-ab4d-51fb92ae0652:'.$total.':ba5314e49470641865661fea0e2b692efbce233d6e6849085efe1735bc29ff1b';   // 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 #82

3. Ticket #82 belongs to the winner

That ticket number is held by exactly one entry - Sarah N. from Telford. 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

07fd719e96e10bcd6284a731b5c05d4149eb26d038a72cf8a1b048e146dc498c

How ticket numbers are assigned

This competition has a fixed set of 270 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.