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)

5d4d13d00f270d9731ae4bdb6efa8707dccb42a2b8756e16f6904ffcfc2ae1cf

Revealed seed (published after the draw)

3965cffe902b17cb6f80839e9df5420df150599f8d3580a5ddaadf48a73efb9d

Beacon round (emitted after close) & its public value

Round 6,290,427 on the drand chain 8990e7a9aaed2ffed73dbd7092123d6f289930540d7651336225dc172e51b2ce

b4fee0e58c20301c038279111efbb775585ca616513f78459bea99e42a9bbf3b

Re-fetch it: curl https://api.drand.sh/8990e7a9aaed2ffed73dbd7092123d6f289930540d7651336225dc172e51b2ce/public/6290427

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' '3965cffe902b17cb6f80839e9df5420df150599f8d3580a5ddaadf48a73efb9d' | sha256sum

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

The 123 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 #149:

$seed    = '3965cffe902b17cb6f80839e9df5420df150599f8d3580a5ddaadf48a73efb9d';
$tickets = [ /* the 123 ticket numbers in the draw, sorted ascending */ ];
$total   = count($tickets);
$msg     = 'draw:019f5cfa-0049-7096-9a37-ed064c37db4d:'.$total.':b4fee0e58c20301c038279111efbb775585ca616513f78459bea99e42a9bbf3b';   // 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 #149

3. Ticket #149 belongs to the winner

That ticket number is held by exactly one entry - Jasmine P.. 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

7aae91185b79a81c2a241a34f91c4bac8da8ed6dbd7031f1902793d95b1dc25e

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.