Provably fair Coin Flip and Stack

How Coin Flip commit and reveal works, how Stack crash points come from a published SHA-256 hash chain, and how to check a round yourself.

Two Skill.io games run on randomness: Coin Flip and Stack. Both are built so you can check the result yourself instead of taking our word for it.

How do you prove a Coin Flip is fair?

Before you pick a side, the flip screen shows the hash of the server seed, which is already fixed. Your browser adds a seed of its own, which you can change, and when someone accepts your flip their seed goes in too. So the server cannot choose a seed after seeing your pick.

After the flip the server seed is revealed. The match page checks that it matches the hash you saw before you picked and works out the result in your browser. This is called a commit and reveal: the hash is a fingerprint of the seed, so the seed cannot be swapped without the fingerprint no longer matching.

How is a Stack crash point decided?

Stack is a crash game against the house. One multiplier climbs for everyone in the round, and at a point nobody can know in advance, the round crashes.

Every crash point comes from a chain of SHA-256 hashes that Skill.io publishes before its first round. Run any round's hash through SHA-256 and you get the hash of the round before it, all the way back to the first one published. No round can be changed once the chain is out.

Can Skill.io know a result before the round?

No. The results also depend on a salt that cannot be known when the chain is published: the hash of a Bitcoin block mined after it. So nobody, including Skill.io, can know a result before its round.

How do I check a round myself?

After every round you get its hash. The fairness page shows the chain and lets you work out any crash point yourself.

What is the house edge in Stack?

Perfect play returns 98% of what you stake over time, and nothing returns more: no cash-out plan, auto-bank target, script or bot. The other 2% is the house edge. It also means about 2 rounds in 100 crash the moment they launch.

Any lock that is not perfect returns less than 98%, and how much less depends on your timing.

Where does skill come in?

The skill is the tower. A chevron slides across about once a second and you lock it with a tap or the Space bar. A clean lock keeps your tower steady. An early, late or missed lock makes it sway, and sway takes your multiplier below the round's. Your multiplier never goes above the round's and never below 1.00x.

A lock counts from when you tapped, not when it reached the server, as long as it arrives within 0.8 seconds. Your payout is worked out on the server from your own locks.

Can I try Stack without risking money?

Yes. Stack has practice credits. They are play money and never touch your balance.