Offchain play

A Surround game is a chain of signed messages. The chain sees it only when it must, and then checks every step it is shown against the same rules the players used.

Terms and session keys

Before a game starts, each player picks a fresh Stark-curve session key, and both wallets sign the game's terms. The terms fix:

• the chain, the channel contract and the game id;
• both wallets and their session keys (seat 0 plays black, seat 1 white);
• the proof adapter allowed to settle the game;
• the board size, komi and rules version;
• the dispute response window (5 minutes to 7 days);
• the clock, if any: the referee's key and the time-control settings;
• the digest of the rated ticket the game was paired by, or 0.

The wallets sign the terms as SNIP-12 typed data under the domain arbiter. Onchain, each signature is checked by its account's is_valid_signature, so any Starknet account works, including a smart wallet. A signed-in player's browser key may sign in its wallet's place, under a delegation the wallet signed once (rated games only, for at most a week and an hour).

Game ids

A game's id is a hash of its seats: both wallets and both session keys (gameIdOf in the SDK). It is the only id the channel will open the game under. If players could choose any id, a losing player could open another game under the real one's id, with other wallets, and keep the real game from ever settling. Fresh session keys for every game make every id new.

Steps

Every action is a step, signed by the acting player's session key. Each signature covers:

• the terms;
• the step's sequence number;
• the running transcript hash, which chains every earlier step;
• the step itself.

A Go step is one of:

The sequence number and the transcript chain make replays and forks detectable: a step signed for one history does not verify in another.

Clients verify every signature they receive and never sign from a state they did not compute themselves. That invariant is what lets the chain check far fewer signatures than there are steps (below).

Rules

The steps follow Surround's rules of Go: captures before the suicide check, positional superko, area scoring with komi, and dead stones agreed by the players after two passes. They're described for players under Gameplay. A proof never decides life and death: the players agree, or they play it out.

Final-signature authentication

When a batch of steps reaches the chain, Cairo verifies one signature per player: the one on that player's last step in the batch. A player with no step in the batch needs none.

This suffices because each signed message commits, through the transcript hash, to every step before it. If player P's last step is step j and P's signature on it verifies, then P signed a message committing to steps 1 through j, including all of P's own earlier steps, and P has no step after j. The opponent's later steps are covered by the opponent's final signature in the same way. Every step is therefore endorsed by the player who took it, provided honest clients never sign from a state they didn't verify.

Signatures are not part of any hash, so an unverified signature field cannot change the state a replay produces. The result is exactly the state full verification would produce on any history both players really signed, and it removes about 83% of a proof's trace.

Players keep their transcripts

There is no data availability service. Each player keeps the complete transcript, and must respond if the opponent opens a dispute. An uncooperative opponent can force a game onchain and make it cost transactions; Disputes explains how that ends.