Rock Paper Scissors

Summary

The RockPaperScissors contract contains a flaw where, after the first round, the revealDeadline is not reset when players commit their moves in subsequent rounds. This allows a malicious player to commit their move, immediately reveal it (since revealDeadline is stale from the first round), and then use timeoutReveal to unfairly win the round before the opponent has a chance to commit or reveal their move.

Vulnerability Details

In the commitMove function, the revealDeadline is only set if both players' commitments are submitted and it's the first round:

However, in rounds beyond the first, if a player commits first and then calls revealMove, the stale revealDeadline (from round 1) might already have passed. This allows them to:

1. Commit their move.

2. Immediately call revealMove.

3. Wait a moment.

4. Call timeoutReveal, claiming the opponent didn't reveal on time — even though the opponent never got a chance to commit in that round.

This leads to a forced win that breaks the fairness of the game.

Impact

• Unfair advantage: A malicious player can abuse the outdated revealDeadline to force wins.

• Race condition: It creates a timing race where the first committer has the power to lock out the opponent.

• Game logic flaw: The expected order of commit → commit → reveal → reveal is violated.

• Undermines fairness: A core value of Rock-Paper-Scissors is simultaneous reveal, which this bug breaks.

Tools Used

• Manual code review

• Logic tracing of commitMove, revealMove, and timeoutReveal

• Inference of state transitions across multiple rounds

Recommendations

Update the commitMove function to reset revealDeadline on every round, once both players have committed:

Ensure this block always runs, even after the first round. You might also refactor the condition logic to explicitly handle revealDeadline reset in every round.

Additionally, consider adding a state check in revealMove to prevent revealing before both commits are received:

This ensures the reveal phase begins only when both commitments are present, maintaining fair play.