CryptographicEraser

CryptographicEraser is a lightweight Android application designed to securely delete files by encrypting them in place before removal, and optionally overwriting free space with cryptographically secure random data. Developed as part of a Bachelor’s thesis in Computer Science at FernUniversität in Hagen, this tool ensures that deleted data cannot be recovered—even on modern flash media where traditional shredding techniques are less effective.

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Motivation

Mobile devices increasingly store sensitive personal and business data—from photos and documents to authentication tokens and private messages. Simply deleting a file does not guarantee that its content can’t be recovered: on flash-based storage (e.g., internal eMMC or removable SD cards), wear-leveling and block remapping can leave remnants of “deleted” data intact.

To mitigate this risk, CryptographicEraser:

1. Encrypts files in place using AES (GCM for smaller files, CTR for larger) with a strong password-based key derivation (PBKDF2-HMAC-SHA256).
2. Deletes the encrypted file, leaving uniformly random ciphertext on disk.
3. Optionally wipes free space by writing and deleting dummy files filled with random data—twice—ensuring that any leftover blocks are overwritten.

This approach leverages cryptographic irrecoverability (ciphertext indistinguishability) as the first defense, followed by a free-space wipe as a second layer, providing a high degree of confidence that sensitive content cannot be reconstructed.

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Key Features

• In-Place Encryption & Deletion
  Files are read into memory, encrypted with AES, then overwritten at the original location—no temporary .enc files remain.

• Adaptive Cipher Selection

  • AES-GCM for files up to 20 MiB (offers authenticated encryption).
  • AES-CTR for larger files (stream cipher with minimal memory overhead).

• Password-Based Key Derivation
  Uses PBKDF2 with HMAC-SHA256 and 100 000 iterations to derive a 256-bit AES key from the user’s passphrase, resisting brute-force attacks.

• Optional Free-Space Wipe
  Overwrites all available free space in the app’s sandbox directory with 1 MiB random-data files (twice), then deletes them.

• Minimal Permissions
  Only requires full file-access permission (MANAGE_EXTERNAL_STORAGE on Android 11+, or WRITE_EXTERNAL_STORAGE on older OS versions) for operation within the app’s own sandbox.

• Lightweight, Open-Source
  No background services. All code runs in a single Activity + Fragment architecture. Easily auditable and modifiable.

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Usage

1. Install the APK on your Android device.
2. Grant the storage permission when prompted.
3. Select “Shred File”, choose one or more files within the app’s sandbox view, and enter your passphrase.
4. The app will encrypt & delete each file, then ask whether you’d like to wipe free space.
5. If you agree, the app will overwrite free space with random data, then clean up.

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Architecture & Implementation

• MainActivity

  • Displays storage statistics.
  • Launches FileExplorer (Fragment) for file selection.
  • Orchestrates the shred-and-wipe workflow via CryptoEraseController.

• CryptoEraseController

  • Prompts for passphrase.
  • Calls CryptoUtils.encryptFileInPlace() for each selected file.
  • Deletes the encrypted file.
  • Optionally invokes WipeUtils to overwrite free space.

• CryptoUtils

  • Implements in-place encryption without temporary files.
  • Logs key events (password length, file sizes, salt/IV values) to Logcat for debugging.

• WipeUtils

  • Provides both silent and feedback-driven free-space overwriting routines.
  • Writes 1 MiB dummy files until the volume is full (caught via exception), repeating twice.

• FileExplorer

  • Simple RecyclerView-based file browser rooted at filesDir.
  • Allows directory navigation and per-file “CryptoShred” buttons.

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Bachelor Thesis Context

CryptographicEraser was developed as part of a Bachelor’s thesis in Computer Science at FernUniversität in Hagen. The thesis investigates:

• Modern challenges in data sanitization on flash-based storage, focusing on mobile devices.
• Comparative security of encryption-based shredding vs. overwriting strategies.
• Usability considerations for mobile secure-delete tools.

The full thesis document, including design rationale, and security analysis, is available (in German) upon request.

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License

This project is released under the MIT License. Contributions and improvements are welcome!

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FernUniversität in Hagen | Bachelor’s Thesis in Computer Science
CryptographicEraser © 2025 by Fabian Kozlowski