How to Play Chain Bluff: Mexican Train Dominoes With a Cryptographic Bluff Layer

Chain Bluff is the classic Mexican Train domino game with one twist: every face-down placement is a SHA-256 commitment, which means you can lie. Rules, the bluff mechanic, when to call, and the cryptography that makes online dominoes actually fair.

Chain Bluff is the fifth game on 6proclub. It is a four-player domino game built on top of Mexican Train — a domino variant most readers have either played at a kitchen table or never heard of — with a single addition that changes the entire feel of the game: every face-down placement is a cryptographic commitment, which means players can bluff, and other players can call.

The base game is a clean tile-laying race. The bluff layer turns it into something closer to poker. This post covers both.

The basic rules

Chain Bluff is played by four players with a double-9 domino set (55 tiles, ranging from 0|0 to 9|9). Each player starts with 8 tiles in hand; the remaining tiles form the boneyard that players draw from when they cannot play.

A match consists of 3 rounds. The player with the lowest total pip count across the three rounds wins. (Pip count = the sum of dots on tiles still in hand when the round ends — lower is better.)

The board layout

The board has five tracks:

• Four personal trains, one per player
• One Mexican Train, shared by everyone

All five trains start at a central hub tile. Round 1's hub is the 9-9 double (the highest tile in the set). Subsequent rounds use 8-8, then 7-7.

The first tile placed on any train must match the hub value. After that, each subsequent tile must match the open end of the train it is being added to.

A turn, in order

On your turn, you may:

1. Play a tile from your hand onto a legal train. Then your turn ends.
2. If you cannot play, draw one tile from the boneyard.
   • If the drawn tile is legal, you may play it immediately.
   • If it is not, your turn ends and your personal train becomes open (see below).

Open trains

A train is closed by default — only its owner may add tiles to it. A train becomes open when its owner draws a tile and cannot play it on that turn. While the train is open, any player may add tiles to it. The train closes again when its owner makes any play (on their own train or the Mexican Train).

The Mexican Train is always open to everyone.

Doubles

When a player places a double (any tile with matching pips, like 5-5 or 7-7), the doubled value must be satisfied before normal play continues. The next player must play a tile matching that double on the same train. If they cannot, they draw a tile and try to play it. If they still cannot, the double remains pending for the player after them — and so on, around the table, until somebody satisfies it.

Multiple doubles can stack. A clean string of doubles is the most dramatic moment in the game.

Round end

The round ends when:

• A player plays their last tile (they "go out"), or
• Nobody can play and the boneyard is empty

The player who goes out scores 0 pips for that round. Everyone else scores the sum of pips remaining in their hand. After three rounds, the player with the lowest cumulative pip total wins the match.

The bluff layer

This is where Chain Bluff diverges from standard Mexican Train.

When you place a tile, you have an additional choice: place it face-up (visible to everyone, as in normal dominoes) or place it face-down as a commitment. A face-down tile is just a SHA-256 hash on the server — your client has committed to a specific tile, but no other player can see what it is.

There are two reasons to place face-down:

Reason 1 — You have a legal play and you want to keep the game moving. A face-down tile is treated as legal provisionally. The game continues. If nobody calls you within two turns, the tile is auto-revealed at expiry and the game proceeds. No penalty.

Reason 2 — You do not have a legal play and you want to bluff one anyway. You commit to a tile that does not actually match the open end. The game treats your move as legal in the moment. If nobody calls you within two turns, you have effectively skipped the "draw and pass" sequence — a meaningful tempo gain in late-round endgames.

Calling a bluff

Within two turns of any face-down placement, any player may call. The bluffer must then reveal the committed tile (the original tile, not a substitute — the SHA-256 commitment binds them).

• If the placement was legal (truth): the caller takes the penalty.
• If the placement was a lie: the bluffer takes the penalty.

A penalty is a fixed pip surcharge added to the offender's hand for round-end scoring. The exact value is configured per match (typically equivalent to drawing 2-3 tiles).

Bluffs that are not called within two turns expire as provisionally-true. They are auto-revealed and the original placement stands. No penalty in either direction.

When to bluff

The bluff is most attractive in two specific situations:

• You're stuck and the boneyard is light. If the boneyard has only 2-3 tiles left, drawing into a non-playable tile costs you a turn and exposes your situation. Bluffing the play keeps your tempo and your information advantage. Risk: getting called.
• You're close to going out. If you are down to your last 2-3 tiles, every turn matters. A bluff that goes uncalled saves you a draw and pushes you closer to the round win. Risk: a called bluff adds penalty pips that wipe out your low-hand lead.

The bluff is least attractive when the open ends are obvious and tightly tracked — in early rounds with full hands, every player can usually deduce what is plausible from what tiles they hold and what has been played.

When to call

Calling math is approximate but tractable:

• Track which numbers are accounted for. With 55 tiles total and four hands of 8 (32 tiles in hands at start), the boneyard holds 23. Each face-up tile played reduces the unknown set. Late in the round, you can often calculate exactly which tiles can still match a given open end.
• Watch the order of placement. A player who could obviously have played face-up but chose face-down is signaling something — either they are bluffing, or they are setting up a future bluff and want you to grow suspicious. The latter is a deeper play.
• Calling is asymmetric. A correctly called bluff penalizes the bluffer; an incorrect call penalizes you. But the information you gain from a correct call extends past the round — once a player has been caught bluffing, their face-down placements get called more aggressively for the rest of the match, which they know, which constrains their late-game options.

The rough rule: call when you can mentally reduce the set of legal tiles for the bluffer to fewer than 4 candidates and your read of the player suggests bluff > truth in that subset. Calling on pure suspicion is usually a losing trade.

Why this works online

Chain Bluff requires provably-fair commitment at two layers, and this is where the cryptography earns its keep.

Layer 1 — the shuffle commit

Before any tiles are dealt, the server commits to its random seed by publishing the SHA-256 hash of (server-seed, client-seed). The client seed is contributed by the players themselves. The shuffle of all 55 tiles, who deals them, who gets which 8, the order of the boneyard — all of it is deterministically derived from those seeds.

At match end, the server reveals its seed. Anyone can hash it together with the client seed and verify that the original commit hash matches. They can then re-run the shuffle algorithm and confirm that every tile dealt came from a fair shuffle that the server could not have rigged after the fact.

Layer 2 — the per-bluff commit

Every face-down placement also publishes a SHA-256 hash: hash(tile, nonce). This is what makes the bluff layer work. The bluffer cannot wait to see what their opponents do and then change which tile they "had" placed. The hash binds them to a specific tile from the moment of placement. Reveals — at call time, or at the two-turn auto-reveal — are verified by re-hashing and matching against the original commit.

If a reveal does not match its commit, the bluffer is treated as having forfeited the bluff and takes a heavy penalty — significantly worse than getting called fairly.

The combined effect is that Chain Bluff online is cryptographically as fair as Mexican Train at a kitchen table with no cheating uncle, plus the bluff mechanic that the kitchen-table version cannot really support (because face-down tiles in person can be peeked at, swapped, or denied).

Strategy notes for new players

A handful of things that are not obvious until you have played a few matches:

• The opening matters more than you think. With a fresh hand of 8 and a hub of 9-9, the tiles you can play immediately determine your pace. If you have a tile matching 9, play it on your own train to keep your train closed. If you do not, you are about to draw — and your train will open.
• Doubles are a double-edged tool. Playing a double constrains your opponents but it also reveals information about your hand. If you play 7-7, you are signaling that you have a 7-related tail in your hand to chain off. Track who plays which doubles and you learn a lot about who has what.
• The Mexican Train is everyone's escape valve. Save tiles that are useful on the Mexican Train for moments when your opponents have constrained your personal options. Burning Mexican-Train-eligible tiles early is a mistake.
• Bluffing before round 3 is usually wrong. Until the third round, your pip lead or deficit is small enough that the cost of a called bluff outweighs the benefit. Save the bluff for round 3 closing positions where every saved turn matters.
• Watch the boneyard count. When the boneyard hits 3 or fewer, the round is in its final phase. This is when bluffs spike, calls tighten, and round-end calculation matters most.

The verification

After every Chain Bluff match, you can pull the verification record from the match details page. It includes:

• The shuffle commit hash, server seed reveal, and client seed
• Every face-down placement's commit hash and reveal
• The full move history with timestamps

Re-running the shuffle and re-hashing every commit reproduces the full match deterministically. Any deviation would be detectable by anyone with the transcript. We have not had a verification fail in production, and the architecture is built so that we cannot — the server has no path to producing a winning shuffle without leaving evidence.

Where to play

Head to /chainbluff/setup to host or join a 4-player table. The first round is gentle. By the third round somebody is bluffing.