Rules of Cofferwit

The vault

A vault hides a code of 4, 5 or 6 digits, drawn by the server. The positions are called A, B, C… from left to right; a leading 0 and repeats are allowed. The vault carries clues: true statements about its code, enough for a single code to satisfy them all, and none too many (removing any one of them reopens the lock).

Pick

You have 3 attempts. A wrong attempt tells you the first clue, in the order of the list, that your code breaks, and costs 60 seconds: if less time was left, the vault held. The vault is cracked on the first right attempt; it holds after 3 wrong attempts, when time runs out or if you give up.

Forging

Choose a tier: the server draws your code (you can draw two others). Tick clues in the catalogue of true statements about your code, then check: the forge counts the codes still possible and, when only one is left, says which clues are superfluous. A vault can be published when it allows only one code and no clue is superfluous. You keep 3 active vaults at most and publish 1 vault a day; a vault stays 14 days or 50 picks (during the calibration, its 14 days run from Nov 1, 00:00). A draft not published is discarded after 24 hours. Once ranked play has started, the vaults published during the calibration no longer count toward the limit of active vaults.

Daily offers

On your first visit of the day, the game offers you 3 vaults from other members (or house vaults): “within reach”, rated about 100 below you, “your level”, and “tough”, rated about 100 above. The offers change at midnight; a taken offer keeps its pick until it ends. The settings choose the tier of the offers. You pick the same ranked vault only once.

Calibration until Nov 1, 00:00: picks are not ranked yet (no rating, no loot, no forge points, no prizes). Your times are used to set the times of the tiers.

Ratings, loot and forge points

Every thief has a rating (1,000 at the start), and so does every vault: they move as in chess (Elo), according to the chance of cracking the two ratings expected. Only the picks of the daily offers move them.

Expected chance of cracking: 1 / (1 + 10^{(vault rating − thief rating) / 400}). Thief’s result: 1 if they crack it on the 1st attempt, 0.8 on the 2nd, 0.6 on the 3rd, 0 if the vault holds. The thief’s rating moves by 32 × (result − expected chance) over their first 20 ranked picks, by 16 × after that; the vault’s, the other way, by 40 × over its first 10 picks, by 20 × after that.

• Loot of a cracked vault: 2 × the base of the tier (10 in bronze, 15 in silver, 20 in gold) × the outcome (1 on the 1st attempt, 0.8 on the 2nd, 0.6 on the 3rd) × (1 − the expected chance of cracking), at least 1. Cracking a tough vault pays more than a vault you were expected to crack.
• Forge points of a vault that holds: 20 × the thief’s chance of cracking it. Holding against a strong thief pays more. These points stay provisional until the vault is settled.
• For the same thief and the same forger, 3 loots and 3 forge points count at most per month.
• Settlement of a vault when it leaves (14 days, 50 picks, or withdrawal): points counted × a factor. Factor 0, walled-up vault, if nobody cracked it in 5 picks or more; 0.5 if nobody cracked it in fewer than 5; 1.25, well-tuned vault, with at least 10 picks and 20 to 60% cracked; 1 otherwise. So a vault nobody cracks earns nothing.

Daily vault

Every day at 00:05, a hard house vault, the same for everyone: bronze on Monday and Tuesday, silver on Wednesday and Thursday, gold from Friday to Sunday. One pick each, ranked on the daily leaderboard (cracked first, then fewer attempts, then faster); no rating, no loot, a title for the first. The code and the deduction path show the next day.

Duels

A duel is offered from the Duels page or a member’s profile. Flash duel: the same house vault for both (hard, or standard when the tier’s stock has no hard vault left), to pick within 24 hours of the acceptance; the best pick wins (cracked first, then fewer attempts, then more time left), and the code shows when the duel is settled. Forge duel: each player has 24 hours to forge a vault for the other, then 24 hours to pick the one received; not forging loses the duel, not picking scores 0. Cofferwit ratings do not move in duels.

Season and prizes

A season lasts a calendar month. A pick counts for the month it ends in, a vault for the month of its settlement. Two leaderboards with prizes, each with its own prizes: a member can win both in the same month.

• Thieves: the loot of the month. You need at least 8 ranked picks in the month. On equal loot: the best rating of the month, then the fewest ranked picks, then the first to reach that loot.
• Forgers: the forge points of the vaults settled in the month. You need at least 8 ranked picks by other members on your vaults settled in the month, cracked or not (those of accounts linked to yours do not count). On equal points: the most vaults settled at a factor of 1 or more, then the first to reach those points.

Prizes in credits: 1st 150, 2nd 100, 3rd 60, 4th to 10th 25. Places are counted among the members who meet the condition only; a deleted, archived or blocked account takes no place. A prize needs loot, or forge points, above 0: a place at 0 is not paid. No place is shared: if everything stays equal, the oldest account goes first. The prizes are paid on the 1st of the next month around 00:15, and the leaderboards of the month then keep the places paid. On the 1st of the month, thief ratings move halfway back to 1,000. A player without an account is ranked but takes no prize place: the next member moves up to it.

Definitions

• “prime digit”: 2, 3, 5 or 7; 0 and 1 are not prime.
• 0 is even, and a multiple of 3 and of 4.
• “the difference between X and Y”: the absolute value of the subtraction.
• “X divides Y”: X is 2 or more, Y is not 0, and Y is a multiple of X.
• “the number AB”: the two-digit number read from left to right, a leading 0 allowed (07 is 7).
• “the code, read as a number”: 0451 is 451; it is divisible by k if it is not 0 and a multiple of k.
• “exactly”: no more, no less.

Solving a vault with a program skews your rating and takes the fun out of the game.