Many crypto casinos promise fair play, but without Provably Fair it's just words. We have implemented this Provably Fair system in mobile apps for iOS and Android, where each round can be verified locally in 2-3 milliseconds. Verification uses the cryptographic commit-reveal protocol with hardware-generated client seed. Order such an implementation — contact us.
We pay special attention to client_seed generation — the foundation of trust between player and casino. The server cannot influence the outcome because the seed is created directly on the device using secure mechanisms: on iOS — SecRandomCopyBytes from the Security framework, on Android — SecureRandom CryptoProvider. The generated 32 bytes are converted to a hex string and displayed to the user. Additionally, the interface must allow the user to manually change client_seed — this is a standard requirement for transparent systems. This approach eliminates any possibility of server-side manipulation, as the seed is generated in the app's isolated environment and is not accessible for substitution.
The Commitment scheme on Wikipedia describes the theoretical foundation.
How is client_seed generated on a mobile device?
Only hardware CSPRNG is used:
// iOS var clientSeedBytes = Data(count: 32) clientSeedBytes.withUnsafeMutableBytes { SecRandomCopyBytes(kSecRandomDefault, 32, $0.baseAddress!) } let clientSeed = clientSeedBytes.map { String(format: "%02x", $0) }.joined() // Android val clientSeedBytes = ByteArray(32) SecureRandom().nextBytes(clientSeedBytes) val clientSeed = clientSeedBytes.joinToString("") { "%02x".format(it) } The user must be able to manually change client_seed — a standard practice for transparent systems. An input field with a "Refresh" button generates a new random seed and displays it.
Client-side verification
After a round, the app should provide a verification screen. The user sees:
| Parameter | Value |
|---|---|
| Server Seed Hash | a1b2c3... (shown before round) |
| Server Seed | deadbeef... (revealed after) |
| Client Seed | f00f... |
| Nonce | 42 |
| HMAC Result | 0x3f2a... |
| Outcome | 6 (from HMAC mod 6 + 1) |
The verification calculation is performed locally in the app — the user sees exactly what is computed. SHA-256 and HMAC-SHA-256 are available in CryptoKit (iOS) and javax.crypto (Android) with no dependencies.
Additionally: a link to a third-party verifier (e.g., provablyfair.org) — this increases trust even if the client never uses it.
Why is local client_seed generation important?
If client_seed comes from the server, the casino could theoretically pick a seed that yields a desired result. Local generation on the device eliminates this attack: the seed is known only to the client until disclosure. Our experience shows that users trust apps where they control the seed.
Commit-reveal scheme
The standard scheme works as follows:
- Before a round, the server publishes
server_seed_hash = SHA256(server_seed). - The client generates
client_seed(random 32 bytes via CSPRNG on the device). - Round result:
HMAC-SHA256(server_seed, client_seed + nonce), where nonce is a round counter. - After the round, the server reveals
server_seed. The client checks:SHA256(server_seed) == server_seed_hash.
The mobile app handles steps 2 and 4. Notably, local verification is 10 times faster than server-side — it requires no data exchange with the backend and ensures 100% transparency.
What's included in the work
- Development of client_seed generation module with UI for manual updates.
- Integration of HMAC calculation with result (e.g., dice roll 0-5).
- Verification screen displaying all parameters and automatic check.
- Server_seed rotation support: reveal old and publish new hash.
- API documentation and testing on 10,000 rounds.
- Deployment to App Store and Google Play with guideline compliance.
Process of work
- Analytics: study your server scheme, determine HMAC format and nonce.
- Design: draw UX for verification, write API specification.
- Implementation: code in Swift/Kotlin, using CryptoKit/javax.crypto. Run 1000 rounds on a test server.
- Test: check compatibility with server, automatically verify 10,000 rounds.
- Deploy: publish to stores, provide source code access.
Time estimates
From 3 to 5 working days for basic implementation. If custom outcome generation is needed (e.g., for poker) — up to 10 days. The cost is calculated individually after analyzing your backend.
Comparison: local vs server-side verification
| Parameter | Local (ours) | Server-side |
|---|---|---|
| Network dependency | No | Yes |
| Transparency for user | Full | Limited |
| Fraud possibility | Eliminated | Theoretically possible |
| Verification speed | Instant | Depends on ping |
Trust and guarantees
Our guarantee: source code is open for audit, all hashes are public. We have 5 years of experience in mobile development and have implemented Provably Fair for 15+ projects. Get a consultation on integrating Provably Fair into your mobile app — just write to us.