1. Executive Overview

Notes (branded as Notes by StrangeInfinity) is an original, production-quality, offline-first Android note-taking application designed with inspiration from Google Keep and Samsung Notes. Engineered strictly around user sovereignty, it guarantees zero tracking, client-side AES-256-GCM encryption, and local-first persistence.

Zero Internet Permissions 100% Offline Vault AES-256-GCM + PBKDF2 Kotlin 2.0.21

2. Technology Stack

Layer Technology Version Role
Platform Android Min SDK 26 (Android 8.0), Target SDK 34 (Android 14) Target runtime ecosystem
Language Kotlin 2.0.21 Strict typing, coroutines, and Flow
UI Toolkit Jetpack Compose & Material 3 BOM 2024.10.00 Declarative reactive UI, animations
Persistence Android Room 2.6.1 SQLite schema, relational queries, atomic transactions
Preferences Jetpack DataStore 1.1.1 Type-safe user preferences persistence
Navigation Navigation Compose 2.8.3 Type-safe single-activity screen routing
Cryptography Standard Java/Android Provider JVM Standard AES/GCM/NoPadding, PBKDF2WithHmacSHA256
File Access Storage Access Framework (SAF) Android Standard Zero broad storage permissions needed

3. Clean Architecture & Modular Structure

The project follows strict Unidirectional Data Flow (UDF) and Clean Architecture patterns: 

com.strangeinfinity.notes/
├── MainActivity.kt               # Single root Activity, edge-to-edge system insets
├── NotesApplication.kt          # Application lifecycle container & dependency provider
├── core/
│   ├── model/                  # Domain models (Note, Folder, Tag, Checklist, Drawing)
│   ├── database/               # Room entities, DAOs, NotesDatabase singleton
│   ├── security/               # BackupCrypto (AES-256-GCM, PBKDF2, tamper detection)
│   ├── backup/                 # BackupManager, schemas, staging, transactional restore
│   ├── data/                   # NotesRepository, PreferencesRepository, AttachmentManager
│   └── designsystem/           # Color palettes, high-contrast typography, components
├── feature/
│   ├── home/                   # Note grid/list, debounced search, multi-select, filter chips
│   ├── editor/                 # Note editor, formatting bar, checklist mode, autosave
│   ├── drawing/                # Handwriting & sketch canvas, smoothing, stroke controls
│   ├── folders/                # Folder management, custom color picker, safe cascade
│   ├── trash/                  # Soft deletion, note restoration, permanent deletion
│   ├── backup/                 # Encrypted/Plain JSON backup & restore with safe preview
│   └── settings/               # App theme (Light/Dark/System), layout, sorting, storage
└── ui/
    └── navigation/             # Type-safe screen navigation and routes

4. Client-Side Cryptographic Specifications

Password-protected backups are encrypted on-device before writing to external storage using authenticated AES-256-GCM:

Parameter Specification Security Rationale
Cipher Algorithm AES/GCM/NoPadding Authenticated encryption provides confidentiality and integrity in a single pass.
Key Length 256 bits (32 bytes) Exceeds current NIST cryptographic standards.
Key Derivation PBKDF2WithHmacSHA256 Strong key derivation resistant to brute-force acceleration.
Salt Size 16 bytes (128 bits) Generated freshly via SecureRandom per backup archive.
Iteration Count 100,000 iterations High computational work factor defeating rainbow table dictionaries.
IV / Nonce Size 12 bytes (96 bits) Standard recommended GCM nonce length; never reused.
Auth Tag Length 128 bits (16 bytes) Guarantees that bit-flipping attacks cause decryption failure.

5. Versioned Backup Envelope Schema

All notes, folders, checklists, and vector drawings are marshaled into a clean, human-inspectable JSON envelope when unencrypted, or encapsulated in a binary cryptographic container: 

{
  "version": 1,
  "appVersion": "1.0.0",
  "exportedAt": 1729684800000,
  "isEncrypted": true,
  "cryptoMetadata": {
    "algorithm": "AES-256-GCM",
    "kdf": "PBKDF2WithHmacSHA256",
    "iterations": 100000,
    "saltBase64": "vF8aR3g2sK4x7...",
    "ivBase64": "d98g2aL1m5p0..."
  },
  "payload": "<Base64-encoded encrypted ciphertext with appended 128-bit GCM tag>"
}

6. Vector Handwriting & Canvas Engine

Rather than saving low-resolution raster bitmaps, strokes in Notes are captured as raw floating-point vector paths:

  • Bézier Interpolation: Drag gestures calculate midpoints and interpolate smooth quadratic Bézier curves to prevent jagged stroke artifacts.
  • Lossless Vector Scaling: Drawings scale cleanly across small phone screens and ultra-high-resolution 12-inch tablets without pixelation.
  • Stroke History Stack: Complete undo and redo capability, color changes, and customizable stroke thickness.

7. Building from Source Code

Prerequisites: JDK 17 or 21, Android SDK 34, Git. 

# 1. Clone repository
git clone https://github.com/dev-hints/Notes.git
cd Notes

# 2. Build Debug APK
./gradlew assembleDebug

# 3. Output artifact location:
# app/build/outputs/apk/debug/app-debug.apk

8. Installation & ADB Deployment

Install directly via standalone APK or Android Debug Bridge:

Download Notes-release.apk (12 MB) 
# Install directly to USB-connected Android device:
adb install -r Notes-release.apk

# Launch MainActivity:
adb shell am start -n com.strangeinfinity.notes/.MainActivity