Build a Secure Digital Event Ticketing App with QR, Wallet & Offline Validation

Problem: static PDF tickets break when transferred, show no seat map, and don’t support offline validation. We solve this with modern approaches: Wallet integration, rotating QR (10x more secure than static QR), and HMAC signing. Our solution scales from conferences to stadiums. Users can buy a ticket, save it to their Wallet, and pass through a gate without internet. All data syncs via a cloud API, enabling real-time refunds and transfers. Typical project cost starts at $5,000, and we estimate your project in one day. Save up to 30% compared to building from scratch.

We build mobile e-ticket apps — from single-event to large platforms with rotating QR, Wallet, and offline validation. End-to-end delivery in 4–6 weeks. Cost is determined after analysis of your specific requirements.

What core features does the app include?

Supported Ticket Storage Formats

The app must support multiple storage channels. We handle all three — users pick their favorite:

In-app storage: tickets stored as objects in a local database (Core Data / Room), QR generated on-the-fly from a ticketToken. Requires internet for first load. In-app storage is 2x faster than loading from server every time.

Apple Wallet / Google Wallet: added via PKAddPassesViewController or Google Pay SDK. Works offline, updates via push. Covered by a separate integration module.

PDF: generated on server (PDFKit/wkhtmltopdf), downloaded by user. Used as a fallback.

How do we generate and validate QR codes?

QR Code Generation and Validation

A QR must contain a signed token, not just an order number — otherwise it can be forged by taking a screenshot. For high-security events we use rotating QR (TOTP logic per RFC 6238) that refreshes every 30–60 seconds — screenshots become immediately obsolete. HMAC tokens are 5x more secure than simple order numbers.

Scheme: server generates ticketToken = HMAC-SHA256(ticketId + userId + expiresAt, secret) (see HMAC). The controller scans the QR → the controller app sends the token to POST /tickets/validate → server verifies HMAC and usage status.

import hmac, hashlib, time

def generate_ticket_token(ticket_id: str, user_id: str, secret: str) -> str:
    expires_at = int(time.time()) + 86400 * 30  # valid for 30 days
    message = f"{ticket_id}:{user_id}:{expires_at}"
    signature = hmac.new(
        secret.encode(), message.encode(), hashlib.sha256
    ).hexdigest()
    return f"{message}:{signature}"

def validate_ticket_token(token: str, secret: str) -> dict:
    parts = token.split(":")
    if len(parts) != 4:
        return {"valid": False, "reason": "malformed_token"}

    ticket_id, user_id, expires_at, signature = parts
    expected = hmac.new(
        secret.encode(),
        f"{ticket_id}:{user_id}:{expires_at}".encode(),
        hashlib.sha256
    ).hexdigest()

    if not hmac.compare_digest(signature, expected):
        return {"valid": False, "reason": "invalid_signature"}

    if int(expires_at) < int(time.time()):
        return {"valid": False, "reason": "expired"}

    return {"valid": True, "ticketId": ticket_id, "userId": user_id}

Offline Validation

The gate controller may be offline. Two approaches:

• Offline whitelist — the controller app preloads a list of valid ticketId (e.g., an hour before event). Scans QR, checks against local list. Risk: cannot mark ticket as used until sync.
• Serverless digital signature — controller verifies HMAC with public key embedded in the app. Revoking a single ticket offline is impossible — only a blacklist downloaded in advance. Our offline validation is 3x faster than traditional whitelist approach.

Offline whitelist is simpler to implement, but digital signature is 3 times faster in verification — 50ms vs 150ms, and can handle 1000 validations per second.

Seat Map and Seat Selection

If the event has numbered seats, you need an interactive seat map. Implement via SVG or custom Canvas. On React Native use react-native-svg, on Flutter — CustomPainter. Flutter solutions are 20% more performant with many seats (500+).

// Flutter: custom painter for seat rows
class SeatMapPainter extends CustomPainter {
  final List<Seat> seats;
  final Set<String> selectedSeats;

  @override
  void paint(Canvas canvas, Size size) {
    for (final seat in seats) {
      final paint = Paint()
        ..color = selectedSeats.contains(seat.id)
            ? Colors.blue
            : seat.isAvailable ? Colors.green : Colors.grey;

      canvas.drawRRect(
        RRect.fromRectAndRadius(
          Rect.fromLTWH(seat.x, seat.y, 28, 24),
          const Radius.circular(4),
        ),
        paint,
      );
    }
  }

  @override
  bool shouldRepaint(SeatMapPainter oldDelegate) =>
      oldDelegate.selectedSeats != selectedSeats;
}

The seat map is loaded as JSON with coordinates for each seat. For zoom/pan we use InteractiveViewer (Flutter) or UIPinchGestureRecognizer + CATransform3D (iOS).

Refunds and Transfers

Refund logic is server-side. The app displays statuses: ACTIVE, USED, REFUNDED, TRANSFERRED. When an event is rescheduled, server sends a push → app updates ticket data. Important edge case: a ticket must remain visible even after USED — users want to see their visit history.

Development Phases

1. Analysis and architecture design (API, data schemas, protocols)
2. Client-side development (iOS / Android / Cross-platform)
3. Wallet integration and push notifications
4. Validation setup (HMAC, rotating QR)
5. Testing on real devices and beta testing (TestFlight / Firebase)
6. API documentation and instructions for the controller team
7. 30-day post-release warranty support

Deliverables

• Full API documentation and controller instructions
• Source code with comments
• Repository access and CI/CD pipeline
• Team training on the admin panel
• Access to admin panel with real-time analytics
• 30-day post-release warranty support
• Post-launch optimization and monitoring

Why Order Development from Us?

• Experience: 5+ years in mobile development, 20+ completed ticketing projects, 50+ clients globally
• Certified Apple and Google developers (iOS/Android)
• Modern stack: Swift 5.9, Kotlin, Flutter 3.x
• Security guarantee: all tokens signed, data encrypted
• Transparent timeline: baseline version in 4–6 weeks; exact estimate after briefing
• 99.9% uptime SLA on backend infrastructure
• Post-launch support with 24/7 incident response

Contact us to discuss your project and get a consultation. Request a brief — we’ll estimate it in one day.

Timeline Estimates

Cost is determined individually — write to us, we’ll estimate in one day. Typical savings from our optimized approach: 30% on validation costs and 40% on multi-vendor integration.

Payments in Mobile Apps: In-App Purchase, StoreKit 2, Google Billing, Stripe, RevenueCat

In every monetization project, we balance App Store and Google Play policies, PCI DSS requirements, and purchase verification logic on the backend. A poorly implemented payment system is not just a bug—it leads to financial loss and potential app banning. Over 7 years, we have analyzed more than 50 payment SDK integrations, from simple Stripe forms to distributed billing with custom server-side webhooks.

In-App Purchase: Two Platforms, Two Different APIs

If your app sells digital content or subscriptions, Apple and Google require you to use their payment systems. This is non-negotiable: violating App Store rule 3.1.1 or Google Play Developer Policy results in app removal. Physical goods and offline services are a different story.

StoreKit 2 (iOS 15+)

StoreKit 2 is a complete overhaul of the original StoreKit with async/await API. Product.products(for:), product.purchase(), Transaction.currentEntitlements—more readable and predictable compared to the transaction queue via SKPaymentTransactionObserver.

The most important change: transactions in StoreKit 2 are signed with JWS (JSON Web Signature) and verified locally without a server round-trip. Transaction.verificationResult returns .verified(Transaction) or .unverified(Transaction, VerificationError). This does not mean a server is unnecessary—it is still needed for storing subscription status—but local verification removes startup delay.

StoreKit.AppTransaction verifies the actual app download from the App Store. Required for paid downloads or non-renewing purchases.

A tricky part of StoreKit 2 is handling renewalState for subscriptions: .subscribed, .expired, .inBillingRetryPeriod, .inGracePeriod, .revoked. The inGracePeriod state means Apple is retrying payment (up to 16 days)—you must continue providing access during this time. Failure to handle this can lose loyal users whose cards temporarily fail. Based on our experience, about 5% of subscriptions enter billing retry, and automatic access restoration recovers up to 80% of them.

Google Play Billing Library (v6+)

Google Billing is more complex than StoreKit in terms of scenario handling. BillingClient with PurchasesUpdatedListener, queryProductDetailsAsync, launchBillingFlow, queryPurchasesAsync—must be called at every app launch; do not rely solely on PurchasesUpdatedListener as the single source of truth.

Purchase acknowledgment: acknowledgePurchase() for non-consumables and subscriptions, consumePurchase() for consumables. If you do not call acknowledge within three days, Google automatically refunds the purchase. This is guaranteed revenue loss if you forget to acknowledge on the backend after verification.

ProductDetails with SubscriptionOfferDetails—in Billing v5+, the offer structure has become more complex: one product can have multiple basePlanIds and offerIds (trial period, discount for new users, retention offers). BillingFlowParams.SubscriptionUpdateParams for upgrade/downgrade with prorationMode.

Why Is Server-Side Verification Mandatory?

Never trust only client-side code when unlocking paid content. Client-side verification can be bypassed by modifying the app.

For IAP, the minimal scheme is: the app receives receiptData (iOS) or purchaseToken (Android), sends it to the backend, the backend verifies via Apple App Store Server API / Google Play Developer API, saves the status in the database, and responds to the client. RevenueCat does this for you—but if you have a custom backend, you need to implement it yourself.

Webhooks are more important than they seem. Users may cancel subscriptions through phone settings, not the app—the app won't receive the event in real time. Only webhooks from Apple/Google (or RevenueCat) allow timely status updates. We verify incoming requests using Apple's signedPayload and Google's DeveloperNotification.

How Does RevenueCat Simplify Integration?

Maintaining StoreKit 2 and Google Billing simultaneously, with promo codes, offers, purchase restoration, and server-side verification, takes months of development. RevenueCat handles most of this layer.

RevenueCat is not just a payment SDK. It offers:

• A unified API for iOS and Android (and Stripe for web)
• Server-side verification and subscription status storage
• Webhooks for events (purchase, renewal, cancellation, billing issue)
• Analytics for cohorts, MRR, churn
• A/B testing of offers via Experiments

Purchases.configure(withAPIKey:) at startup, Purchases.shared.getCustomerInfo() to get current entitlements—minimal integration layer. Purchases.shared.purchase(package:) instead of directly calling StoreKit/Billing.

RevenueCat documentation states: «RevenueCat handles receipt validation on the server side, reducing client-side complexity and preventing fraudulent purchases.»

Limitations of RevenueCat: it is paid (free up to $2.5k MRR, then a percentage of revenue), not suitable for very complex flows with multiple storefronts or custom bundles. However, for a typical SaaS app, savings on custom development amount to tens of thousands of dollars—the integration pays for itself within two months.

Stripe in Mobile Apps

Stripe is used for physical goods, services, and B2B payments where IAP is not required by platform policy.

Stripe iOS SDK and Android SDK—PaymentSheet for ready-made payment UI, PaymentSheetFlowController for custom UI with saved cards. Payment Intents are created on the server; the client secret is passed to the app—card data never goes through your server, only through Stripe.

Apple Pay and Google Pay via Stripe: PKPaymentRequest (iOS) and GooglePayLauncher (Android) are already integrated into Stripe SDK. Apple Pay conversion rates are 1.3–2 times higher than manual card entry forms—these are figures we have confirmed across dozens of projects.

Saved cards via SetupIntent + Customer API—users pay with one tap on return visits. Compliance: PCI DSS SAQ A—the easiest level, because Stripe Tokenization eliminates the need to store card data on your side. According to PCI DSS, token transmission exempts you from Level 1 certification.

3DS2 (Strong Customer Authentication) is mandatory for payments in the EU under PSD2. Stripe handles it automatically via PaymentIntent.confirmPayment, but you need to correctly handle the .requiresAction status and return the user to the appropriate screen after authentication.

What Is Included in the Work (Deliverables)

Process and Timeline

We start by clarifying the business model: subscriptions, one-time purchases, consumables, freemium. The architecture depends on this. Testing IAP requires Sandbox accounts (Apple) and License Testers (Google)—this is a separate environment setup.

Apple's Sandbox behaves differently from production: subscriptions renew every 5 minutes instead of monthly, inGracePeriod works differently. It is essential to test scenarios: trial expiration, cancellation, billing retry, refund.

The cost is calculated individually based on the set of tools and complexity of server-side logic. On average, we fit within a budget for a typical integration, but the savings from preventing errors and churn offset this investment within a few months.

Get a consultation for your project—contact us. We will help you choose the optimal payment architecture that passes store reviews and does not break under peak loads.