android-encrypted-archiver/.planning/research/SUMMARY.md

Research Summary

Project: encrypted_archive Researched: 2026-02-24

Stack Decision

Layer	Choice	Rationale
Language (archiver)	Rust	Memory safety, crypto ecosystem, cross-compilation
Encryption	AES-256-CBC + HMAC-SHA256	Only cipher reliably available in busybox openssl + javax.crypto + RustCrypto
Compression	gzip (DEFLATE) via flate2	Native everywhere: Rust, Android GZIPInputStream, busybox gunzip
CLI	clap v4	De facto Rust standard
Binary format	Manual byte I/O	Full control for obfuscation; no recognizable patterns
Kotlin decoder	javax.crypto + java.util.zip	Zero external dependencies, Android SDK built-in
Shell decoder	dd + xxd + openssl + gunzip	Standard busybox applets

Critical constraint: busybox shell is the weakest link. Every technology choice is validated against "can busybox do this?"

Table Stakes Features

1. Multi-file packing/unpacking (texts + APKs)
2. AES-256-CBC encryption with HMAC-SHA256 (encrypt-then-MAC)
3. Gzip compression before encryption
4. Custom magic bytes (not recognizable by binwalk/file/7z)
5. Hardcoded 32-byte key in all decoders
6. Per-file IV (random 16 bytes, stored in cleartext)
7. Round-trip fidelity (byte-identical decompression)
8. Kotlin decoder (primary, Android 13)
9. Shell decoder (fallback, busybox)
10. File metadata (name, sizes, offsets)
11. Data integrity (SHA-256 checksums per file)

Architecture

Three independent implementations unified by a shared format specification:

FORMAT SPEC (shared document)
    |
    +-- Rust Archiver (CLI, Linux/macOS)
    +-- Kotlin Decoder (Android 13, primary)
    +-- Shell Decoder (busybox, fallback)

Rust archiver pipeline: collect → compress → encrypt → format → obfuscate

Key patterns:

• Per-file independence (each file compressed/encrypted separately)
• Encrypt-then-MAC (HMAC over IV || ciphertext)
• Little-endian everywhere
• Format version field for forward compatibility

Top Pitfalls to Prevent

1. busybox openssl cipher availability — test on actual device before format design
2. OpenSSL key derivation mismatch — use -K HEX -iv HEX -nosalt for raw keys
3. Cross-platform crypto incompatibility — golden test vectors mandatory
4. Over-engineering obfuscation — AES encryption IS obfuscation for casual users
5. APKs don't compress — per-file compression flag needed

Recommended Build Order

1. Format spec + busybox feasibility proof — validate constraints first
2. Rust archiver — core pipeline (compress → encrypt → format)
3. Rust test decoder — catch format bugs in same language
4. Kotlin decoder — primary extraction path
5. Shell decoder — busybox fallback
6. Obfuscation hardening + integration testing — binwalk/file/strings testing

Open Questions

• Does target busybox have openssl enc -aes-256-cbc with -K/-iv flags?
• Is xxd available in target busybox? (fallback: od)
• Is gunzip available in target busybox?
• Should HMAC use same key as AES or derived subkey?