feat: apk-reverse Agent Skill

Progressive-disclosure skill for Android APK reverse engineering: recon, ad removal, membership limits, dex surgical patching, repack/sign, runtime data, Frida, environment setup, and a hard-won failure catalogue.
2026-09-19 21:25:35 +08:00
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+# Repack and sign
+
+Rebuilding is the step where otherwise-correct patches die. Two mistakes account for nearly all of it: **stripping `META-INF/`** and **recompressing entries that must stay stored**.
+
+## The rules
+
+**1. Strip only signature artifacts, never the whole directory.**
+
+`META-INF/` contains ServiceLoader registrations the app needs at runtime. Details and the exact failure in `references/pitfalls.md` P1.
+
+```python
+SIG_EXT = ('.SF', '.RSA', '.DSA', '.EC')
+def is_signature_entry(name):
+    if not name.upper().startswith('META-INF/'):
+        return False
+    rest = name[len('META-INF/'):]
+    if '/' in rest:                 # keep services/, androidx/, native-image/ ...
+        return False
+    up = rest.upper()
+    return up == 'MANIFEST.MF' or up.endswith(SIG_EXT)
+```
+
+**2. `AndroidManifest.xml` and `resources.arsc` must stay uncompressed (STORED).**
+
+Recompressing them produces builds that fail to install or misbehave.
+
+**3. Keep the APK's zip entry metadata.** Preserve `compress_type`, `external_attr`, `date_time` for entries you copy through.
+
+**4. Changing any dex changes nothing about resources.** If you only edited dex, do not touch `res/`, `assets/`, or `lib/`.
+
+**5. Signing creates new `MANIFEST.MF`/`*.SF`/`*.RSA`.** That is expected — the check is that **no signature artifact from the ORIGINAL** survives, and **no non-signature entry was lost**.
+
+## Pipeline
+
+`scripts/repack.py` implements all of the above. Conceptually:
+
+```
+open original (zip)
+  for each entry:
+    skip entries that are signature artifacts (per rule 1)
+    skip entries being replaced
+    copy through, preserving compression + attributes
+  write replacement entries
+close
+sign (v1 + v2 + v3)
+verify
+```
+
+Usage:
+
+```bash
+python scripts/repack.py --apk original.apk --dexdir extracted_dex_dir --out out.apk
+# or replace specific dex files:
+python scripts/repack.py --apk original.apk --dexdir extracted_dex_dir \
+  --dex "classes7.dex=patched/classes7.dex" \
+  --dex "classes8.dex=patched/classes8.dex" \
+  --out out.apk
+```
+
+`--dexdir` supplies the untouched dex files; `--dex name=path` overrides specific ones.
+
+## Signing
+
+Two viable routes.
+
+**Route A — `uber-apk-signer` (handy, wraps zipalign + apksigner):**
+```bash
+java -jar uber-apk-signer.jar --apks in.apk --ks ks.jks \
+  --ksAlias <alias> --ksPass <pass> --ksKeyPass <pass> -o out_dir
+```
+Known quirk: `-o` and `--overwrite` are mutually exclusive; passing both errors out. Also, it **silently skips already-signed APKs** (`0 processed`) — so de-sign first.
+
+**Route B — `zipalign` + `apksigner` directly (most explicit):**
+```bash
+zipalign -p -f 4 unsigned.apk aligned.apk
+apksigner sign --ks ks.jks --ks-pass pass:<pass> --key-pass pass:<pass> \
+  --v1-signing-enabled true --v2-signing-enabled true --v3-signing-enabled true \
+  --out signed.apk aligned.apk
+apksigner verify --print-certs -v signed.apk
+```
+
+Enable **v1 + v2 + v3**. v1 is needed for older Android; v2/v3 for modern verification.
+
+Generate a keystore once:
+```bash
+keytool -genkeypair -v -keystore ks.jks -alias <alias> \
+  -keyalg RSA -keysize 2048 -validity 36500 \
+  -storepass <pass> -keypass <pass> \
+  -dname "CN=<name>, OU=dev, O=dev, L=NA, ST=NA, C=NA"
+```
+
+## Installing over an existing app
+
+| Situation | Command |
+|---|---|
+| Same signing key as installed version | `pm install -r` — keeps app data (login state, caches) |
+| Different signing key | `pm uninstall` first, then install. **App data is lost.** |
+| OEM installer refuses (`INSTALL_FAILED_*`, vendor restrictions) | push the APK and install via root: `su -c 'pm install -r -t -d /data/local/tmp/app.apk'` |
+| Downgrade needed | add `-d` |
+| Test-only flag needed | add `-t` |
+
+Useful detail: keeping the same keystore across builds lets you iterate with `-r` and **preserve a logged-in session**, which matters a lot when the feature you are testing needs auth.
+
+## Post-install / post-upgrade hazards
+
+- **Data directory uid mismatch.** After reinstall the app uid increments; a restored `/data/user/0/<pkg>` directory owned by the old uid is unreadable. Symptom: crash in a DB-init path (`Cannot open database`). Fix:
+  ```
+  su -c "chown -R <uid>:<uid> /data/user/0/<pkg>"
+  su -c "restorecon -R /data/user/0/<pkg>"
+  ```
+  Get `<uid>` from `dumpsys package <pkg> | grep userId=`.
+- **Restoring a data backup can itself cause this.** Prefer `cp -f` over an existing file (preserves owner) rather than deleting and re-extracting.
+
+## Verify the build, not just the signature
+
+Signature verification proves the file is well-formed. It does **not** prove the app works. Always:
+
+1. `apksigner verify` (or equivalent) passes.
+2. Install succeeds.
+3. Launch succeeds; process stays alive.
+4. `logcat` shows no `FATAL EXCEPTION` / `VerifyError` / `IncompatibleClassChangeError` / `uncaughtException`.
+5. `META-INF/services/*` count matches the original.
+6. Unchanged dex files are byte-identical to the original (compare hashes).
+
+Build a **control** at least once: same pipeline, zero patches. If the control fails, your pipeline or environment is at fault, not your patch (`references/pitfalls.md` P9).