Minimalist Calculator

A native Android calculator with a hidden, encrypted photo and video vault.

Overview

Minimalist Calculator is a clean native Android app built from scratch in Java. On the surface, it behaves like a simple iPhone-inspired calculator. Behind the calculator interface, it includes a private media vault that unlocks only when the user enters a custom sequence of calculator button taps.

The project focuses on three priorities:

• Simplicity: A lightweight native implementation with no third-party app dependencies.
• Privacy: User-selected media is encrypted locally before being stored.
• Discretion: The vault is accessed through a calculator tap sequence instead of a visible login screen.

Key features

• Minimal calculator UI: Dark, calculator-first interface inspired by the iPhone calculator layout.
• Hidden tap-sequence unlock: The vault opens only after the correct private calculator button sequence is entered.
• First-launch setup: Users create and confirm their tap sequence the first time they open the app.
• No reset from calculator screen: The passcode can only be changed from inside the unlocked vault.
• Encrypted vault storage: Imported images and videos are encrypted before being stored in app-private storage.
• Android Keystore integration: The media encryption key is generated and protected through Android Keystore.
• No broad storage permission: Imports use Android's document picker rather than all-files or external-storage access.
• Screenshot protection: FLAG_SECURE is enabled to reduce screenshots and recent-app previews where supported.
• Backup exclusion: Android backup and data extraction rules exclude app data where supported.
• Small footprint: Native Java + Android SDK only.

How it works

Launch app
   |
   v
Calculator interface
   |
   +-- Normal calculator input
   |
   +-- Correct hidden tap sequence
          |
          v
      Encrypted media vault
          |
          +-- Import photos/videos
          +-- Preview vault items
          +-- Delete vault items
          +-- Change passcode

Security model

This app is designed to reduce casual access to private media on a normal Android device.

Implemented protections:

• AES-GCM encryption at rest: Vault media is encrypted before being written to disk.
• Android Keystore: The encryption key is generated through Android Keystore instead of being hardcoded.
• Private app sandbox: Encrypted vault files are stored in internal app-private storage.
• No broad file permissions: Media selection is handled through the Android system picker.
• Backup disabled: Backup and transfer extraction rules are configured to exclude app data.
• Secure window mode: Screenshots and recent-app thumbnails are blocked where Android supports it.

Security limitations

No mobile app can honestly guarantee absolute protection in every scenario.

Important limitations:

• Rooted or compromised devices: Root access, malware, or a compromised OS can weaken Android sandbox protections.
• Unlocked-device access: Anyone with the unlocked device and the correct tap sequence can access the vault.
• No passcode recovery: Forgetting the tap sequence requires clearing app data, which also removes vault media.
• Preview cache: Media may be temporarily decrypted into app cache for preview.
• Manufacturer differences: Keystore behavior and screenshot blocking can vary across Android versions and OEMs.

Tech stack

Area	Technology
App platform	Native Android
Language	Java
Build system	Gradle
Android Gradle Plugin	8.5.2
Compile SDK	35
Minimum SDK	23
Dependencies	Android SDK only
Encryption	AES-GCM
Key storage	Android Keystore

Project structure

.
├── app/
│   ├── build.gradle
│   └── src/main/
│       ├── AndroidManifest.xml
│       ├── java/com/minimalist/calculator/
│       │   ├── MainActivity.java
│       │   ├── Crypto.java
│       │   └── PasscodeManager.java
│       └── res/
│           ├── drawable/
│           ├── mipmap-*/
│           ├── mipmap-anydpi-v26/
│           ├── values/
│           └── xml/
├── gradle/
├── logo/
├── LICENSE
├── PRIVACY_POLICY.md
├── README.md
├── build.gradle
├── gradlew
├── gradlew.bat
└── settings.gradle

Build locally

Android Studio

1. Clone the repository.
2. Open the project folder in Android Studio.
3. Let Gradle sync.
4. Run the app module on an emulator or physical Android device.

Command line

./gradlew :app:assembleDebug

The generated debug APK will be available at:

app/build/outputs/apk/debug/app-debug.apk

To generate a release Android App Bundle:

./gradlew :app:bundleRelease

The generated release bundle will be available at:

app/build/outputs/bundle/release/app-release.aab

Google Play notes

The Play Store package name is configured as:

com.socialsure.calculator

Before publishing a release:

• Use a signed release bundle: Upload a signed .aab, not a debug APK.
• Protect signing keys: Never commit keystores or signing passwords.
• Test on real devices: Verify calculator behavior, vault unlock, import, preview, delete, and passcode change flows.
• Use accurate store copy: Describe the app as an on-device privacy tool and avoid absolute security claims.
• Complete Data safety carefully: The app handles selected media locally and does not upload media to a backend service.

Privacy policy

The privacy policy is available in this repository:

PRIVACY_POLICY.md

Repository hygiene

The repository excludes local, generated, and sensitive files such as:

• Gradle build outputs
• Android Studio workspace state
• local.properties
• APK/AAB artifacts
• Keystore files
• Signing credentials

Author

Built from scratch by Shayan Mohd.

License

This project is licensed under the MIT License. See LICENSE for details.