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Packs

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Published odds

Packs::odds(pack) returns the Odds a pack publishes: the chance of each rarity and of each item, before anything is drawn. A site that makes its members draw at random says at which odds.

Opening a pack

A pack opens in two steps, so that neither the site nor the member can choose the draw:

  1. Commit. commit(member, pack, key) draws a secret seed of 16 to 64 bytes from the injected SeedSource — SecureSeeds in production, a fixed seed in tests — and returns a PendingOpening: the key, the member, the pack and the seed in hexadecimal. The site keeps it and shows the member its commitment(), the SHA-256 of the seed. Nothing is given yet. A PendingOpening is rebuilt from what was kept with its constructor.
  2. Open. The member, having seen the commitment, gives a client seed of 1 to 256 bytes of UTF-8 text: words they type, or bytes their own device draws. open(pending, clientSeed, moment) draws the pack's items — each on its own: a rarity by weight, then an item of that rarity — and gives a copy of each, under the key followed by # and the place of the item.

The same pending opening and client seed opened again return the opening they made; a key that opened another pack, or for another member, or from another seed or client seed, is refused.

The draw is the one both seeds and the key give: Xoshiro256StarStar seeded by the HMAC-SHA256, under the seed, of the client seed, a line feed and the key. The site's seed is fixed before the member's client seed is known, and the member chooses theirs without knowing the site's: the site cannot keep only the seeds whose draw suits it, and the member cannot try seeds until one gives a legendary. A client seed the site chooses for the member gives no such guarantee.

Checking a draw

The PackOpening keeps the seed in hexadecimal, its commitment and the client seed. The member checks afterwards that the seed matches the commitment shown and that verify(opening) replays the same items. The verification draws with the pack as the catalog holds it now: a pack whose sets or weights changed since no longer verifies its old openings.