android-encrypted-archiver/.planning/phases/04-kotlin-decoder/04-01-PLAN.md

phase, plan, type, wave, depends_on, files_modified, autonomous, requirements, must_haves

phase	plan	type	wave	depends_on	files_modified	autonomous	requirements	must_haves
04-kotlin-decoder	01	execute	1		kotlin/ArchiveDecoder.kt kotlin/test_decoder.sh	true	KOT-01 KOT-02 KOT-03 KOT-04	truths artifacts key_links Kotlin decoder parses 40-byte archive header with magic byte verification, version check, and flags validation Kotlin decoder parses variable-length TOC entries sequentially with correct little-endian integer decoding Kotlin decoder verifies HMAC-SHA256 BEFORE attempting decryption for each file Kotlin decoder decrypts AES-256-CBC ciphertext using javax.crypto with PKCS5Padding (auto-removes PKCS7) Kotlin decoder decompresses gzip data only when compression_flag == 1 Kotlin decoder verifies SHA-256 checksum after decompression Kotlin decoder uses the exact same 32-byte hardcoded key as src/key.rs Cross-validation script creates archives with Rust CLI and verifies Kotlin decoder produces byte-identical output path provides min_lines kotlin/ArchiveDecoder.kt Complete archive decoder: header parsing, TOC parsing, HMAC verify, AES decrypt, gzip decompress, SHA-256 verify, CLI main() 200 path provides min_lines kotlin/test_decoder.sh Cross-validation script: pack with Rust, decode with Kotlin, SHA-256 comparison 50 from to via pattern kotlin/ArchiveDecoder.kt src/key.rs Identical 32-byte key constant 0x7A.*0x35.*0xC1.*0xD9 from to via pattern kotlin/ArchiveDecoder.kt docs/FORMAT.md Section 4-5 Binary format parsing matching exact field offsets and sizes MAGIC.*0x00.*0xEA.*0x72.*0x63 from to via pattern kotlin/test_decoder.sh kotlin/ArchiveDecoder.kt Compiles and runs decoder JAR kotlinc.*ArchiveDecoder

Implement the complete Kotlin archive decoder and cross-validation test script.

Purpose: Provide the Android 13 extraction path (primary decoder on target device) using only javax.crypto and java.util.zip -- no native libraries or third-party dependencies. Also create a cross-validation script that proves byte-identical output against the Rust archiver.

Output: kotlin/ArchiveDecoder.kt (standalone decoder with CLI main), kotlin/test_decoder.sh (cross-validation script)

<execution_context> @/home/nick/.claude/get-shit-done/workflows/execute-plan.md @/home/nick/.claude/get-shit-done/templates/summary.md </execution_context>

@.planning/PROJECT.md @.planning/ROADMAP.md @.planning/STATE.md @.planning/phases/04-kotlin-decoder/04-RESEARCH.md

Critical references for binary format parsing

@docs/FORMAT.md @src/key.rs @src/format.rs

Task 1: Implement ArchiveDecoder.kt with full decode pipeline kotlin/ArchiveDecoder.kt Create `kotlin/ArchiveDecoder.kt` as a single self-contained Kotlin file with a `main()` function for CLI usage.

IMPORTANT: Before writing ANY code, the executor MUST use Context7 to verify current Kotlin/JVM API usage:

• mcp__context7__resolve-library-id for "kotlin" to confirm byte array handling, toByte() semantics
• Do NOT rely on training data -- verify ByteBuffer, Cipher, Mac, MessageDigest API signatures

Structure (single file, top to bottom):

1. Imports -- Only Android SDK / JVM standard library:

   • java.io.ByteArrayInputStream, java.io.File, java.io.RandomAccessFile
   • java.nio.ByteBuffer, java.nio.ByteOrder
   • java.security.MessageDigest
   • java.util.zip.GZIPInputStream
   • javax.crypto.Cipher, javax.crypto.Mac
   • javax.crypto.spec.IvParameterSpec, javax.crypto.spec.SecretKeySpec

2. Constants:

   • MAGIC = byteArrayOf(0x00, 0xEA.toByte(), 0x72, 0x63) -- match FORMAT.md Section 4
   • HEADER_SIZE = 40
   • KEY = exact 32-byte array from src/key.rs:

     0x7A, 0x35, 0xC1.toByte(), 0xD9.toByte(), 0x4F, 0xE8.toByte(), 0x2B, 0x6A,
     0x91.toByte(), 0x0D, 0xF3.toByte(), 0x58, 0xBC.toByte(), 0x74, 0xA6.toByte(), 0x1E,
     0x42, 0x8F.toByte(), 0xD0.toByte(), 0x63, 0xE5.toByte(), 0x17, 0x9B.toByte(), 0x2C,
     0xFA.toByte(), 0x84.toByte(), 0x06, 0xCD.toByte(), 0x3E, 0x79, 0xB5.toByte(), 0x50,
     

   • Note: ALL byte values > 0x7F need .toByte() cast (Kotlin signed bytes)

3. Data classes:

   • ArchiveHeader (version: Int, flags: Int, fileCount: Int, tocOffset: Long, tocSize: Long, tocIv: ByteArray)
   • TocEntry (name: String, originalSize: Long, compressedSize: Long, encryptedSize: Int, dataOffset: Long, iv: ByteArray, hmac: ByteArray, sha256: ByteArray, compressionFlag: Int, paddingAfter: Int)

4. Helper functions for LE integer reading (using ByteBuffer):

   • readLeU16(data: ByteArray, offset: Int): Int -- ByteBuffer.wrap(data, offset, 2).order(LITTLE_ENDIAN).short.toInt() and 0xFFFF
   • readLeU32(data: ByteArray, offset: Int): Long -- ByteBuffer.wrap(data, offset, 4).order(LITTLE_ENDIAN).int.toLong() and 0xFFFFFFFFL

5. parseHeader(data: ByteArray): ArchiveHeader -- FORMAT.md Section 4:

   • Verify magic bytes (data[0..3] vs MAGIC)
   • Read version at offset 4, require == 1
   • Read flags at offset 5, reject if bits 4-7 are non-zero
   • Read file_count (u16 LE at offset 6)
   • Read toc_offset (u32 LE at offset 8)
   • Read toc_size (u32 LE at offset 12)
   • Read toc_iv (16 bytes at offset 16)
   • Use require() for all validation failures

6. parseTocEntry(data: ByteArray, offset: Int): Pair<TocEntry, Int> -- FORMAT.md Section 5:

   • Read name_length (u16 LE)
   • Read name (UTF-8 bytes, String(data, pos, nameLength, Charsets.UTF_8))
   • Read fixed fields sequentially: original_size(u32), compressed_size(u32), encrypted_size(u32), data_offset(u32)
   • Read iv (16 bytes via copyOfRange)
   • Read hmac (32 bytes via copyOfRange)
   • Read sha256 (32 bytes via copyOfRange)
   • Read compression_flag (u8)
   • Read padding_after (u16 LE)
   • Return Pair(entry, newOffset)
   • Entry size formula: 101 + name_length bytes

7. parseToc(data: ByteArray, fileCount: Int): List -- parse all entries sequentially, assert cursor == data.size at end

8. Crypto utility functions:

   • verifyHmac(iv: ByteArray, ciphertext: ByteArray, key: ByteArray, expectedHmac: ByteArray): Boolean
     • Mac.getInstance("HmacSHA256"), init with SecretKeySpec(key, "HmacSHA256")
     • mac.update(iv), mac.update(ciphertext), mac.doFinal()
     • Compare with contentEquals() (NOT ==)
   • decryptAesCbc(ciphertext: ByteArray, iv: ByteArray, key: ByteArray): ByteArray
     • Cipher.getInstance("AES/CBC/PKCS5Padding") -- NOT PKCS7Padding, NOT specify provider
     • cipher.init(Cipher.DECRYPT_MODE, SecretKeySpec(key, "AES"), IvParameterSpec(iv))
     • cipher.doFinal(ciphertext) -- auto-removes PKCS7 padding
   • decompressGzip(compressed: ByteArray): ByteArray
     • GZIPInputStream(ByteArrayInputStream(compressed)).readBytes()
   • verifySha256(data: ByteArray, expectedSha256: ByteArray): Boolean
     • MessageDigest.getInstance("SHA-256").digest(data)
     • Compare with contentEquals()

9. decode(archivePath: String, outputDir: String) -- main decode orchestration:

   • Open archive with RandomAccessFile(archivePath, "r")
   • Read 40-byte header, call parseHeader()
   • Seek to tocOffset, read tocSize bytes, call parseToc()
   • For each TocEntry: a. Seek to dataOffset, read encryptedSize bytes (ciphertext) b. HMAC FIRST: call verifyHmac(). If fails: print "HMAC failed for {name}, skipping" to stderr, continue c. Decrypt: call decryptAesCbc() d. Decompress: if compressionFlag == 1, call decompressGzip(); else use decrypted directly e. Verify SHA-256: call verifySha256(). If fails: print "WARNING: SHA-256 mismatch for {name}" to stderr (but still write) f. Write output: File(outputDir, entry.name).writeBytes(original) g. Print "Extracted: {name} ({originalSize} bytes)" to stdout
   • Close RandomAccessFile
   • Error handling: HMAC failure skips file (continue), SHA-256 mismatch warns but writes (matching Rust behavior from Phase 2 decisions)

10. main(args: Array) -- CLI entry:

    • Usage: java -jar ArchiveDecoder.jar <archive> <output_dir>
    • Validate args.size == 2
    • Create output directory if not exists: File(args[1]).mkdirs()
    • Call decode(args[0], args[1])
    • Print summary: "Done: {N} files extracted"

Critical anti-patterns to avoid (from RESEARCH.md):

• Do NOT use BouncyCastle provider ("BC") -- deprecated on Android
• Do NOT use == for ByteArray comparison -- use contentEquals()
• Do NOT forget .toByte() for hex literals > 0x7F
• Do NOT manually remove PKCS7 padding -- cipher.doFinal() handles it
• Do NOT decrypt before HMAC verification -- verify HMAC FIRST
• Do NOT try to decompress when compressionFlag == 0 grep -c "contentEquals" kotlin/ArchiveDecoder.kt | xargs test 4 -le && grep -c "PKCS5Padding" kotlin/ArchiveDecoder.kt | xargs test 1 -le && grep -c "HmacSHA256" kotlin/ArchiveDecoder.kt | xargs test 1 -le && grep "0x7A" kotlin/ArchiveDecoder.kt | grep -q "0x50" && echo "PASS: Key structural checks" || echo "FAIL" Verify ArchiveDecoder.kt contains: (1) magic byte check, (2) HMAC verification before decryption, (3) correct key bytes matching src/key.rs, (4) PKCS5Padding not PKCS7Padding, (5) contentEquals for all byte comparisons kotlin/ArchiveDecoder.kt exists with 200+ lines, contains complete decode pipeline: header parsing, TOC parsing, HMAC-then-decrypt flow, gzip decompression, SHA-256 verification, CLI main(). Uses only javax.crypto + java.util.zip + java.nio -- zero third-party dependencies.

Task 2: Create cross-validation test script kotlin/test_decoder.sh Create `kotlin/test_decoder.sh` -- a shell script that:

1. Checks prerequisites:

   • Verify kotlinc is available (or print install instructions and exit)
   • Verify java is available
   • Verify cargo is available
   • Build Rust archiver if needed: cargo build --release -q

2. Compile Kotlin decoder:

   • kotlinc kotlin/ArchiveDecoder.kt -include-runtime -d kotlin/ArchiveDecoder.jar 2>&1
   • Check exit code

3. Test case 1: Single text file

   • Create temp dir
   • Create hello.txt with content "Hello, World!"
   • Pack: ./target/release/encrypted_archive pack hello.txt -o test1.archive
   • Decode: java -jar kotlin/ArchiveDecoder.jar test1.archive output1/
   • Verify: sha256sum of extracted file matches original
   • Print PASS/FAIL

4. Test case 2: Multiple files with mixed content

   • Create text.txt with multiline UTF-8 text (including Cyrillic characters)
   • Create binary.bin with dd if=/dev/urandom bs=1024 count=10
   • Pack both files: ./target/release/encrypted_archive pack text.txt binary.bin -o test2.archive
   • Decode: java -jar kotlin/ArchiveDecoder.jar test2.archive output2/
   • Verify SHA-256 of each extracted file matches original
   • Print PASS/FAIL for each file

5. Test case 3: Already-compressed file (APK-like, --no-compress)

   • Create fake.apk with random bytes
   • Pack with no-compress: ./target/release/encrypted_archive pack --no-compress fake.apk -o test3.archive
   • Decode: java -jar kotlin/ArchiveDecoder.jar test3.archive output3/
   • Verify byte-identical
   • Print PASS/FAIL

6. Test case 4: Empty file

   • Create empty empty.txt (touch)
   • Pack: ./target/release/encrypted_archive pack empty.txt -o test4.archive
   • Decode: java -jar kotlin/ArchiveDecoder.jar test4.archive output4/
   • Verify empty output file
   • Print PASS/FAIL

7. Summary:

   • Print total PASS/FAIL count
   • Clean up temp files
   • Exit with 0 if all pass, 1 if any fail

Script should:

• Use set -euo pipefail at the top
• Use absolute paths relative to $SCRIPT_DIR (dirname of script)
• Create all temp files in a temp directory cleaned up on exit (trap)
• Use colored output (green PASS, red FAIL) if terminal supports it
• Be executable (chmod +x header with #!/usr/bin/env bash)

Note: The script requires kotlinc and java to be installed. It should detect their absence and print helpful installation instructions (e.g., sdk install kotlin or apt install default-jdk) rather than failing cryptically. bash -n kotlin/test_decoder.sh && test -x kotlin/test_decoder.sh && grep -q "sha256sum" kotlin/test_decoder.sh && grep -q "kotlinc" kotlin/test_decoder.sh && echo "PASS: Script is valid bash and contains key commands" || echo "FAIL" Review test_decoder.sh covers: text file, binary file, Cyrillic names, no-compress mode, empty file. Check it verifies SHA-256 of each extracted file against original. kotlin/test_decoder.sh exists, is executable, contains 5+ test cases covering text/binary/Cyrillic/no-compress/empty files, creates archives with Rust CLI, decodes with Kotlin JAR, and verifies byte-identical output via SHA-256 comparison.

1. `kotlin/ArchiveDecoder.kt` exists with complete decode pipeline (header + TOC + HMAC + AES + gzip + SHA-256) 2. `kotlin/ArchiveDecoder.kt` uses only javax.crypto, java.util.zip, java.nio, java.security, java.io (no third-party) 3. `kotlin/ArchiveDecoder.kt` hardcoded key matches `src/key.rs` exactly 4. `kotlin/ArchiveDecoder.kt` verifies HMAC before decryption (grep confirms `verifyHmac` call precedes `decryptAesCbc` call) 5. `kotlin/test_decoder.sh` is valid bash (bash -n passes), executable, covers 4+ test scenarios 6. If kotlinc/java are available: `bash kotlin/test_decoder.sh` passes all tests

<success_criteria>

• ArchiveDecoder.kt is a complete, standalone Kotlin decoder that can extract files from archives produced by the Rust archiver
• Decoder uses only Android SDK built-in libraries (KOT-01, KOT-02)
• HMAC verification happens before decryption for every file (KOT-03)
• SHA-256 checksum verification happens after decompression (KOT-04)
• Cross-validation test script is ready to run when kotlinc/java are installed </success_criteria>

After completion, create `.planning/phases/04-kotlin-decoder/04-01-SUMMARY.md`