In Day 6 of my blockchain series, I take our Rock, Paper, Scissors dApp and make it way more secure by adding testing and automatic verifications in Reach. Because we’ve been changing a lot, I start from a fresh version and rewrite the program as a full review. Along the way, I clean up the logic by using enums for hands (Rock/Paper/Scissors) and outcomes, which makes the code easier to read than juggling raw numbers. I also pull the “who won?” logic into a dedicated `winner` function so it’s testable and reusable. Then I get into the fun part: proving things about the program. I add assertions as test cases (like specific Rock vs Scissors scenarios), and I also use `forall` to express stronger guarantees—like “no matter what the inputs are, `winner` always returns a valid outcome,” and “if both hands match, it’s always a draw.” Reach can verify these insanely fast using symbolic execution, so you’re not brute-forcing billions of values. Finally, I protect Alice’s hand using `makeCommitment` (commitment + salt), add `hasRandom` to the participant interface/front end, and use `unknowable` to ensure Bob can’t learn Alice’s hand or salt early. The result compiles cleanly, runs with basically the same front-end output, and removes exploitable gameplay bugs—though we still need to handle non-participation next.