SecureWallet-iOS

SecureWallet-iOS is a fintech-grade iOS project demonstrating a secure wallet and payout pipeline built with Clean Architecture, MVVM, UIKit and strict Test-Driven Development (TDD).

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💥 What This Project Demonstrates

• Designing financial systems where **correctness is prioritised **
• Implementing encryption-at-rest with proper key management
• Building deterministic, replay-based state systems
• Applying modern UIKit architecture (diffable + compositional layout)
• Writing testable, auditable, production-grade iOS code

🏗 Architecture Overview

The project is divided into three primary modules:

SecureWalletApp     → iOS Application 
SecureWalletDomain  → Pure business logic (no frameworks)
SecureWalletData    → Infrastructure & implementations

🔄 Wallet Execution Flow (State → Persistence → UI)

User Action (Add / Spend)
   ↓
ViewController
   ↓
ViewModel (state orchestration)
   ↓
WalletService (use case boundary)
   ↓
Wallet (domain logic, validation)
   ↓
WalletStore (persistence boundary)
   ↓
Core Data (encrypted payload)
   ↓
Wallet reconstructed via replay
   ↓
@Published state updated
   ↓
Diffable snapshot applied
   ↓
UI reflects new state

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🧠 Domain Layer (Single Source of Truth)

The SecureWalletDomain module contains: The domain module contains all financial rules and invariants.

Core Components

CoinAmount

• Immutable value object storing milliCoins
• Prevents negative values
• Safe arithmetic operations

LedgerEntry

• Immutable entity
• Credit / Debit direction
• Timestamped

Wallet (Aggregate Root)

• Owns append-only ledger
• Balance is derived, never stored
• Enforces all financial invariants

WalletRecord

• Deterministic persistence representation

WalletStore

• Protocol boundary for persistence

Financial Invariants Enforced

• Balance is never stored directly
• Balance can never be negative
• Debit greater than balance throws
• Debit equal to balance succeeds
• Failed debit does not mutate state
• Ledger entries are append-only
• Duplicate entry IDs are ignored (idempotency)
• Wallet state is reconstructed via deterministic replay

The domain is fully unit tested and framework-agnostic.

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📱 iOS Engineering Highlights (UIKit)

This project intentionally uses UIKit to demonstrate production-level iOS engineering skills.

🧩 Modern Collection View Architecture

• UICollectionViewCompositionalLayout

  • Section-based layout (balance + transaction feed)
  • Dynamic sizing using .estimated
  • Clean separation via WalletLayoutFactory

• UICollectionViewDiffableDataSource

  • Snapshot-driven UI updates
  • Eliminates manual index management
  • Ensures UI consistency with state

• Supplementary Views (Headers)

  • Date-grouped transaction sections
  • Configured via supplementaryViewProvider

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🔄 State-Driven UI (Combine)

• ViewModel exposes state via:

  • @Published balance
  • @Published transactions

• UI binding:

Publishers.CombineLatest(viewModel.$balance, viewModel.$transactions)

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📱 App Layer (MVVM)

The App layer is responsible for rendering state and handling user interaction.

• ViewModels:

  • Expose state via @Published
  • Contain no business logic
  • Orchestrate use cases via services

• ViewControllers:

  • Bind to ViewModel using Combine
  • Apply diffable snapshots
  • Handle UI composition only

UI is fully state-driven. No business rules exist outside the domain.

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💾 Persistence Layer (Core Data + Encrypted Ledger)

The project uses Core Data as a secure persistence engine, storing wallet state as an encrypted payload rather than plain relational data.

🧠 Design Principle

Persistence stores encrypted history. Domain reconstructs truth.

• Wallet is not stored directly
• Full state is serialized into a WalletRecord
• Stored as a single encrypted blob
• Domain rebuilds wallet via deterministic replay

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🧱 Storage Model

Core Data entity:

• WalletEntity
  • id: UUID
  • encryptedPayload: Data

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🔐 Save Flow (Secure & Atomic)

1. Convert Wallet → WalletRecord
2. Encode using JSONEncoder
3. Encrypt using AES-GCM
4. Store encrypted payload in Core Data

Wallet (Domain)
   ↓
WalletRecord
   ↓
JSON Encode
   ↓
AES-GCM Encrypt 🔐
   ↓
Core Data (encryptedPayload)

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🔐 Security & Cryptography

The project implements encryption-at-rest using Apple’s CryptoKit.

Key Characteristics

• Wallet data is never stored in plaintext
• Encryption is enforced at the persistence boundary
• AES-256-GCM is used for authenticated encryption
• Keys are generated and stored securely using Keychain
• Decryption failures are treated as data corruption

Flow

Wallet → JSON → AES-GCM Encrypt → Core Data
Core Data → AES-GCM Decrypt → WalletRecord → Domain

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🧪 Testing Strategy (TDD)

• Tests are written before implementation
• Domain logic is 100% unit tested
• Infrastructure is tested with mocks
• No Apple system APIs are used directly in tests

Test targets:

• SecureWalletDomainTests
• SecureWalletDataTests
• SecureWalletTests

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📸 Wallet UI (UIKit, Compositional Layout, Diffable Data Source)

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SecureWallet-iOS focuses on engineering rigor rather than feature breadth.
It is designed to demonstrate how financial systems should be built on iOS: deterministic, testable, and auditable.