ZARC

Zero-Knowledge Hardware-Backed Encrypted Vault & Disguised Polyglot Container Engine for Android

An uncompromising, high-security offline media vault and disguised steganographic archive system.
Native Rust Cryptographic Core · libsodium Streaming AEAD · Android Keystore StrongBox · Air-Gapped

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Why Zarc Differs • Disguised Polyglot Archives • Cryptographic Architecture • Vault Capabilities • Repository Structure • Building

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⚡ Why Zarc is NOT a Normal Gallery

Important

Zarc is fundamentally not an ordinary photo gallery or camera-roll viewer.
It does not index your filesystem, it never monitors your DCIM/ directory, and it never leaves unencrypted cleartext on device flash storage.

Most commercial "photo locker" and "calculator vault" applications rely on security theater:

• They rename .jpg extensions to .bin or drop a .nomedia file into a hidden directory.
• They present a PIN screen, but keep raw unencrypted photos sitting directly on flash memory.
• Connecting the phone to a PC over USB, pulling storage via ADB, or using a third-party file manager exposes every single image in cleartext.

How Zarc Operates Differently:

Principle	Traditional "Vaults"	ZARC Architecture
Data at Rest	Raw files renamed or hidden	100% Ciphertext using authenticated XChaCha20-Poly1305 streaming AEAD
Metadata Protection	Filenames and EXIF readable on disk	Sealed inside an encrypted catalog (index.bin); disk contains only random UUIDs
Master Key Storage	Hardcoded passwords or shared prefs	Hardware Security Module (TEE / StrongBox); non-exportable hardware isolation
System Cleaners	Vulnerable to OEM "cleanup" boosters	Stored in Context.noBackupFilesDir; immune to junk cleaners and cloud sync leaks
Network Attack Surface	Sends analytics, ads, and telemetry	Strictly Air-Gapped: android.permission.INTERNET is completely omitted
Data Tampering	Silent corruption or partial reads	Fail-Closed: Any bit-flip, truncation, or block reorder causes immediate auth failure

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🗃️ PROJECT ZARC — Disguised Polyglot Photo Archives

One of Zarc's flagship capabilities is Disguised Polyglot Archives (<custom_name>.jpg or .zarc). This engine allows you to package dozens or hundreds of encrypted private photos inside a single, genuine JPEG picture (with any custom file name you choose) that opens normally in any standard photo viewer.

flowchart TD
    subgraph GenuineCarrier ["1. Outer Disguise (Genuine JPEG Image)"]
        SOI["SOI Marker (0xFF, 0xD8)"] --> JPEG_SCAN["Compressed Scan Data (Visible Cover Photo)"]
        JPEG_SCAN --> EOI["EOI Marker (0xFF, 0xD9)"]
    end

    subgraph EncryptedPayload ["2. Authenticated ZARC Encrypted Binary Payload"]
        HEADER["Fixed Public Header (Magic: 'ZARC' · Version · Flags)"]
        SALT["Argon2id Salt & Key-Wrapping Nonce"]
        DEK["Wrapped Content Key (DEK)"]
        PHOTOS["Sanitized Photos 1..N (XChaCha20-Poly1305 Stream)"]
        MANIFEST["Encrypted JSON Manifest (Filenames · Dimensions · Sizes)"]
        HEADER --> SALT --> DEK --> PHOTOS --> MANIFEST
    end

    subgraph FixedTrailer ["3. 32-Byte Fixed-Length Trailer & Locator"]
        LOCATOR["Payload Offset (u64) · Payload Length (u64)<br/>Header Len (u32) · Version (u16) · Flags (u16) · Magic 'ZARC' (4B)"]
    end

    EOI -.->|"Appended Data"| HEADER
    MANIFEST -.->|"Direct Seek Target"| LOCATOR

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Key Technical Properties:

1. Outer Disguise Compatibility:
   • The file begins with standard JPEG SOI (0xFF, 0xD8) and ends its visible stream with JPEG EOI (0xFF, 0xD9).
   • Stock Android gallery apps, Google Photos, chat apps (WhatsApp, Telegram, Signal), and desktop image viewers open and display the cover photo without errors or warnings.
2. Lossless Metadata Sanitization in Pure Rust:
   • Before packaging, every source image (JPEG, PNG, WebP) is stripped of GPS coordinates, camera serial numbers, EXIF timestamps, IPTC, and XMP metadata at the byte level in native Rust without generational transcoding loss.
3. Argon2id + XChaCha20-Poly1305 AEAD:
   • Archive encryption keys are derived from your passphrase using Argon2id (m_cost: 64 MiB, t_cost: 3 passes, p_cost: 4 lanes), providing hardware resistance against GPU and ASIC cracking rigs.
   • All inner photo bytes, filenames, dimensions, and metadata remain 100% encrypted until unlocked.
4. Deterministic $\mathcal{O}(1)$ Trailer Locator:
   • The extraction engine seeks directly to the last 32 bytes of the file, verifies the trailer magic, and jumps directly to the encrypted payload without scanning the entire file.
5. Interactive Decrypted Gallery & In-Place Inspection:
   • Opening an archive immediately shows the genuine cover photo without needing a password.
   • Entering the passphrase decrypts the inner catalog into an interactive session-cached gallery featuring full-screen pinch-zoom, mini-roll carousel, and selective or bulk export via the Android Storage Access Framework (SAF).

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🔒 Cryptographic Architecture

flowchart TD
    subgraph Keystore ["Hardware Security Module (TEE / StrongBox)"]
        KEK["AES-256-GCM Master Key<br/>Non-Exportable · Hardware Keymaster"]
    end

    subgraph Memory ["Ephemeral Volatile Memory (Zeroized on Lock)"]
        VMK["Vault Master Key (VMK)<br/>256-bit CSPRNG · Ephemeral"]
        GATE["Auth-Bound Keystore Key<br/>AES-256-GCM · BIOMETRIC_STRONG"]
        PVK["Private Vault Subkey<br/>Independent Random 256-bit"]
        DEK["Per-Object DEK<br/>256-bit Key per Media Item"]
    end

    subgraph FlashStorage ["Encrypted Storage (noBackupFilesDir)"]
        INDEX["Encrypted Catalog (index.bin)<br/>XChaCha20-Poly1305 Sealed"]
        BLOB["Encrypted Object Blobs<br/>Streaming AEAD · 64 KB Chunks · UUIDs"]
    end

    KEK -->|"Unwraps at launch"| VMK
    GATE -->|"Wraps on biometric unlock"| PVK
    VMK -->|"Encrypts Catalog"| INDEX
    VMK -->|"Wraps DEK (Main Vault)"| DEK
    PVK -->|"Wraps DEK (Private Vault)"| DEK
    DEK -->|"Encrypts Media Stream"| BLOB

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Cryptographic Parameters

Component	Specification	Description
Symmetric Cipher	crypto_secretstream_xchacha20poly1305	192-bit nonce, 256-bit key, streaming authenticated encryption with IETF draft compliance
Chunking & Streaming	64 KiB Chunks with Poly1305 MAC	Incremental authentication tags (TAG_MESSAGE / TAG_FINAL) prevent truncation attacks
Key Wrapping	crypto_aead_xchacha20poly1305_ietf	Authenticated envelope encryption with contextual Additional Authenticated Data (AAD)
Key Derivation (KDF)	Argon2id	64 MiB memory cost, 3 iterations, 4 parallel lanes for maximum password defense
Subkey Derivation	BLAKE2b Subkey KDF	Legacy private areas only (zarc.private.v1); new private areas use an independent random subkey
Memory Sanitization	Rust zeroize & JVM sweeps	Keys and sensitive plaintext buffers are explicitly overwritten with zeroes upon lock

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📱 Vault Capabilities In-Depth

1. High-Performance Encrypted Roll

• Engineered for 10,000+ Items: Paginated memory windowing with LRU thumbnail caching ensures fluid 120Hz scrolling across large libraries without Out-Of-Memory crashes.
• Precision Fast Scrollbar: Custom timeline thumb tracker mapped 1:1 to library dates, providing buttery-smooth navigation.
• Responsive 2–6 Column Grid: Dynamic pinch-to-zoom grid transitions smoothly between 2 and 6 columns.
• Batch Selection: Header-level and item-level checkmarks paired with a quick-action bottom bar (Favorite, Delete, More).

2. Immersive Viewer & Dual-Pane Split View

• Multi-Touch Gesture Zoom: Fluid pinch-to-zoom (1×–6×) starting right from 1.0x with double-tap snap and panning.
• Interactive Bottom Filmstrip: Sleek horizontal thumbnail carousel (Samsung Gallery style) for seamless browsing.
• Dual-Pane Split Screen: Compare two photos or videos side-by-side with independent zoom, pan, and a zero-gap hairline divider.
• Technical Inspector: Modal displaying exact uncompressed size, dimensions, capture timestamp, MIME format, and encryption metadata.

3. Concealed Biometric Private Vault & Stealth PIN

• Zero-Trace Secret Entry: Completely absent from standard navigation bars. Accessible solely via a long-press directly on the title header text ("Photos", "Videos", or "All").
• Hardware Biometric Gate: Unlocks via Android BiometricPrompt with a strong-biometric CryptoObject; the private subkey is released only after the platform authorises the Keystore wrap/unwrap operation.
• Dynamic Time-Based App Stealth PIN: Optional stealth launch lock requiring a dynamic mathematical time formula: $$\text{PIN} = \text{Current Minute } (MM) + \text{Reversed 24h Hour } (\text{rev}(HH))$$ (Example: at 14:09, Minute is 09, Hour is 14 $\rightarrow$ reversed 41, producing PIN 0941).
• Instant Relock & Key Purge: Minimizing or switching away from Zarc immediately zeroes the private keys in RAM, clears every in-memory media cache, deletes decrypted archive sessions and relocks the vault. FLAG_SECURE blocks screenshots and recent-task previews app-wide.

4. 30-Day Encrypted Recycle Bin

• Retention Countdown: Soft-deleted files track remaining retention days (29d left, today).
• Automatic Permanent Shredding: Media past 30 days is automatically overwritten and purged from disk.
• Full Preview & Restore: Preview binned photos full-screen and restore them to the active vault with a single tap.

5. Modern Midnight OLED Architecture & Design

• True OLED Black Background: Optimized for battery efficiency and high-contrast viewing on AMOLED screens.
• Hairline Tactile Borders: 1dp subtle gradient boundaries eliminate visual noise.
• Offline Typography: Bundled Google Fonts Andika rendered completely offline with zero web font fetching.
• Tactile Haptic Feedback: Subtle vibration cues for navigation, selection toggles, long presses, and gestures.

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🔍 Supported Media Formats

Category	Supported Formats	Deliberately Excluded Formats
Main Vault Photos	JPEG (.jpg, .jpeg), PNG (.png), WebP (.webp), HEIC (.heic), GIF (.gif), BMP	Camera RAW (.dng, .cr2, .nef), TIFF, SVG vectors, PSD
Main Vault Videos	MP4 (.mp4), MKV (.mkv), MOV (.mov), WebM (.webm), 3GP (.3gp)	AVI, WMV, FLV, TS, ProRes (formats without hardware decoders)
Disguised Archive Cover	JPEG only (0xFFD8 ... 0xFFD9)	PNG, WebP, GIF (cannot function as polyglot outer carrier)
Disguised Archive Content	JPEG, PNG, WebP	Formats without byte-level metadata parsers are rejected to prevent metadata leaks

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🏛️ Repository Structure

Directory / Module	Role & Technologies
android/	Android Studio Gradle project (Kotlin, Jetpack Compose, Material3, Coroutines)
android/app/src/main/java/com/zarc/vault/archive/	Disguised archive builder, parser, session decryption cache, and SAF exporter
android/app/src/main/java/com/zarc/vault/core/	JNI native bridge and bindings to the Rust cryptographic core
android/app/src/main/java/com/zarc/vault/gallery/	Paginated gallery engine, catalog serialization, and private vault gate
android/app/src/main/java/com/zarc/vault/keystore/	Hardware-backed Android Keystore StrongBox / TEE key management
android/app/src/main/java/com/zarc/vault/ui/	Jetpack Compose UI screens, media viewer, split screen, theme, and haptics
crates/zarc-core/	Pure Rust cryptographic core (container parser, streaming AEAD, metadata sanitizer)
crates/zarc-jni/	C-ABI JNI layer exposing Rust cryptographic primitives to Kotlin
fuzz/	Continuous fuzzing harnesses (libFuzzer / cargo-fuzz) for container parsing and crypto bounds
third_party/libsodium/	Vendored libsodium 1.0.22 with SHA-256 verification
tools/	Cross-compilation automation scripts (build-android.ps1, cargo-fuzz)
docs/	Binary format specification (FORMAT_SPEC.md) and technical security documentation

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🛡️ Continuous Fuzzing & Security Auditing

The native parsing engine includes dedicated fuzz testing targets powered by cargo-fuzz and libFuzzer:

# Fuzz the 152-byte container header and trailer locator
cargo fuzz run parse_container

# Fuzz the streaming AEAD chunk decryptor and boundary handling
cargo fuzz run decrypt_stream

# Fuzz key wrapping bounds and authenticated AAD verification
cargo fuzz run wrap_key

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🛠️ Building and Verification

Prerequisites

• Android SDK: API 36 Platform (minimum SDK 24)
• Android NDK: Version 27.1+ (android-ndk-r27c recommended)
• Rust: 1.85+ stable with target toolchains:

  rustup target add aarch64-linux-android armv7-linux-androideabi i686-linux-android x86_64-linux-android

• JDK: OpenJDK 21
• Gradle: 9.6.1 (wrapper included)

Build Commands

# 1. Compile native Rust cryptographic libraries for all 4 Android ABIs
powershell -NoProfile -ExecutionPolicy Bypass -File tools\build-android.ps1

# 2. Compile and package the release APK
cd android
.\gradlew assembleRelease

# 3. The signed, minified release APK will be generated at:
# android/app/build/outputs/apk/release/app-release.apk

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📄 License

This project is licensed under the Apache License 2.0 — see the LICENSE file for details.
Vendored libsodium is distributed under the ISC License.