Offline QR Payment: Mobile Fuel Stations in Brazil

About the Project

For a network of mobile fuel stations in rural Brazil, we developed a QR payment system that works without a permanent internet connection.

Problem

Mobile fuel stations serve remote areas where:

• No stable internet — 2G connection or none at all
• Cash is inconvenient — security, accounting, collection
• Terminals don't work — require online connection
• Customers prefer QR — don't want to carry cash

Client Requirements

• Accept payment without internet
• Sync when connection appears
• Work on basic Android devices
• Accounting integration

Our Solution

Offline-First Architecture

Workflow: Customer → QR code → Terminal (offline) → Local DB → Synchronization → Central server

How It Works

1. QR generation — customer shows code from app
2. Scanning — terminal reads and verifies signature
3. Local transaction — record in encrypted DB
4. Fuel dispensing — confirmation on screen
5. Synchronization — when internet appears

Offline Payment Security

• Cryptographic signature — QR contains signed data
• Transaction limits — offline operation amount limits
• QR expiration — automatic invalidation
• Blacklists — synchronization on connection

Mobile Applications

For Customers (iOS + Android)

• Payment QR code generation
• Transaction history
• Balance and top-up
• Nearby station search

For Station Operators (Android)

• QR code scanner
• Transaction log
• Data synchronization
• Shift reports

Results

Metric	Before	After
Payment method	100% cash	70% QR
Transaction time	2-3 min	15 sec
Accounting errors	8%	<1%
Cash collection	daily	weekly

Business Impact

• Collection reduction: $24,000/year
• Theft reduction: $18,000/year
• Time savings: +20% customers per day
• Customer loyalty: +35% repeat visits

Technical Features

Synchronization

• Priority transaction queue
• Conflict resolution
• Retry logic with exponential backoff
• Data compression for 2G

Security

• AES-256 local DB encryption
• RSA QR code signing
• Certificate pinning
• Replay attack protection

Reliability

• Works offline up to 7 days
• Automatic recovery
• Audit logging
• Backup systems

Technologies

• Customer app: React Native
• Station terminal: Android (Kotlin)
• Backend: Laravel (PHP)
• Database: PostgreSQL + SQLite (local)
• Synchronization: Custom protocol

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