feat: recover original Kotlin class names from R8-stripped binaries
R8 obfuscates JVM symbols but cannot strip the Kotlin metadata strings —
the Kotlin runtime needs them at runtime for reflection, coroutines, and
data-class features. The original FQNs leak through:
* @DebugMetadata(c = "<real.fqn>") emitted for every coroutine
SuspendLambda (~ every suspend function in modern apps)
* @Metadata(d2 = {"L<real/fqn>;"}) on every Kotlin class
Add scripts/recover-kotlin-names.sh that walks decompiled sources, mines
both annotations, and writes an obf -> real mapping (TSV + JSON + per-real-
package index). On a real-world Kotlin app this recovers ~100 % of
*Repository / *ViewModel / *UseCase / *Impl classes — exactly the classes
worth reading.
Add scripts/lookup-name.sh as a CLI over the mapping with four modes:
search by real-name substring, resolve obf -> real, list a real package,
and an annotated `--grep` that suffixes every hit with the owning real
class. This is a strict upgrade over plain grep against decompiled sources.
Replace the misleading 'use --deobf' tip in call-flow-analysis.md with a
pointer to this technique. --deobf only renames symbols with synthetic
placeholders; metadata recovery returns actual developer-written names.
Document the technique, expected recovery rates, and limitations in
references/kotlin-name-recovery.md, and reference it from SKILL.md as
optional Phase 3.5 (only when Phase 0 reports an obfuscated Kotlin app).
This commit is contained in:
Michał Tajchert
parent
213818fc27
commit
5b63fcb418
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@ -154,6 +154,33 @@ Navigate the decompiled output to understand the app's architecture.
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- Clean Architecture: look for `domain`, `data`, `presentation` packages
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- This informs where to look for network calls in the next phases
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### Phase 3.5: Recover Kotlin Class Names (only for obfuscated Kotlin apps)
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If Phase 0 reported moderate / high obfuscation **and** the app is Kotlin
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(Compose / kotlin_module markers detected), run the metadata recovery
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script before tracing call flows. R8 obfuscates JVM symbols but cannot
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strip Kotlin metadata strings, so original FQNs leak through
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`@DebugMetadata` and `@Metadata.d2`.
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```bash
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bash ${CLAUDE_PLUGIN_ROOT}/skills/android-reverse-engineering/scripts/recover-kotlin-names.sh \
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<output>/sources <output>/mapping
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```
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Then use the lookup helper instead of plain grep — every hit comes
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annotated with the owning class's real name:
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```bash
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bash ${CLAUDE_PLUGIN_ROOT}/skills/android-reverse-engineering/scripts/lookup-name.sh \
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<output>/mapping --grep '"/api/' <output>/sources
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```
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Typical recovery on a real-world Kotlin app: ~100% of `*Repository` /
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`*ViewModel` / `*UseCase` / `*Impl` classes, ~80% of DTOs.
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See `${CLAUDE_PLUGIN_ROOT}/skills/android-reverse-engineering/references/kotlin-name-recovery.md`
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for the full technique and limitations.
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### Phase 4: Trace Call Flows
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Follow execution paths from user-facing entry points down to network calls.
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@ -145,8 +145,9 @@ When code is obfuscated (ProGuard/R8):
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1. **Start from strings**: Search for URLs, error messages, and known constants
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2. **Start from framework classes**: Activities and Fragments are named in the manifest
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3. **Follow library calls**: Retrofit `@GET`/`@POST` annotations are readable even when the interface class name is obfuscated
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4. **Use `--deobf`**: jadx can generate readable replacement names
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4. **Recover original Kotlin names from metadata**: `@DebugMetadata` and `@Metadata.d2` strings preserve the original FQNs even after R8 obfuscation. Run `scripts/recover-kotlin-names.sh` to build an `obf -> real` map (typically recovers 30-50% of classes — and almost 100% of `*Repository` / `*ViewModel` / `*Impl`). See [`kotlin-name-recovery.md`](./kotlin-name-recovery.md). This is the single highest-leverage step on any Kotlin app.
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5. **Cross-reference**: If `class a` calls `Retrofit.create(b.class)`, then `b` is a Retrofit service interface
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6. **`--deobf` is rarely enough on its own**: jadx's `--deobf` renames obfuscated symbols with synthetic placeholders (`p001a`, `C0123Foo`) — useful for disambiguation but it does **not** recover original names. Pair it with the metadata recovery above.
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## 8. Tracing a Complete Call Flow: Example
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@ -0,0 +1,108 @@
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# Recovering Original Class Names from Kotlin Metadata
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When R8/ProGuard obfuscates a Kotlin app, JVM symbols are renamed but the
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**Kotlin metadata strings cannot be stripped** — the Kotlin runtime depends
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on them at runtime for reflection, coroutines, and `data class` features.
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Two annotations leak the original fully-qualified names:
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## `@DebugMetadata`
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Generated for nearly every Kotlin coroutine `SuspendLambda` (i.e. almost
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every `suspend` function in a modern app):
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```java
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@DebugMetadata(
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c = "com.example.feature.account.AccountRepositoryImpl$fetch$1",
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f = "AccountRepositoryImpl.kt",
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l = {42, 51},
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m = "invokeSuspend"
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)
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public final class a extends SuspendLambda implements Function2<...> { ... }
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```
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The `c =` field carries the original outer class FQN (with a `$` suffix
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for inner / lambda scopes — strip everything after the first `$` to get the
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declaring class).
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## `@Metadata.d2`
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Every Kotlin class carries a top-level `@Metadata` annotation. The `d2`
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array lists internal class refs in JVM type-descriptor format
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(`Lcom/example/Foo;`):
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```java
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@Metadata(d1 = {"..."},
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d2 = {"...","Lcom/example/feature/account/AccountRepositoryImpl;","..."})
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public final class b implements ... { ... }
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```
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The first non-stdlib descriptor in `d2` is usually the file's primary
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class.
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## How to mine them
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The skill ships two scripts:
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```bash
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# Build a mapping from a decompiled sources directory:
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bash scripts/recover-kotlin-names.sh <output>/sources [mapping-dir]
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# Outputs:
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# <mapping-dir>/mapping.tsv obf_fqn real_fqn file
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# <mapping-dir>/mapping.json same data, JSON
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# <mapping-dir>/by_package/ per-real-package index files
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# Query the mapping:
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bash scripts/lookup-name.sh <mapping-dir> Repository # search
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bash scripts/lookup-name.sh <mapping-dir> -o ab.cd # obf -> real
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bash scripts/lookup-name.sh <mapping-dir> -p com.example.feature # list package
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bash scripts/lookup-name.sh <mapping-dir> --grep '"api/' <output>/sources
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# ^ greps decompiled code and appends '// real.fqn' to each hit
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```
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## What you typically recover
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On a real-world obfuscated Kotlin app the script recovers **30 – 50 % of
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classes** — but more importantly, **almost 100 % of the classes you
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actually want to read**:
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| Class kind | Recovery rate |
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|---------------------------|---------------|
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| `*Repository` / `*Impl` | ~100 % |
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| `*ViewModel` | ~100 % |
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| `*UseCase` / `*Interactor`| ~100 % |
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| Plain `data class` DTOs | ~80 % |
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| Pure-Java helper classes | low (no Kotlin metadata) |
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| Anonymous inner classes | sometimes recovered as the parent FQN |
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## Why `jadx --deobf` is not enough
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`--deobf` renames obfuscated identifiers using internal heuristics, but the
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output is still synthetic (`p001a`, `C0123Foo`). It does **not** recover
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the *original* names. Kotlin metadata recovery is the only reliable way to
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map back to the names the developer actually wrote, and it costs essentially
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nothing — just a regex pass over the decompiled sources.
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Run both: `--deobf` for fields/methods that have no metadata source, plus
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the recovery script for class names.
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## Limitations
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- **Method names and field names** are not recovered. Kotlin metadata only
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preserves class-level FQNs and a few signatures. For method names you
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still need jadx-gui's interactive rename or pattern inference.
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- **Pure-Java classes** carry no `@Metadata`, so they remain obfuscated.
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- **Heavily inlined classes** (`@JvmInline value class`, top-level fun
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files compiled into shared `*Kt.class` synthetic classes) sometimes show
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up under the wrong filename — treat results as a strong hint, not gospel.
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## Reading flow with the mapping
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1. Run `recover-kotlin-names.sh` once after decompiling.
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2. Use `lookup-name.sh --grep '<pattern>' <sources>` instead of plain `grep`
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so every hit comes annotated with the real owning class.
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3. When you hit an obfuscated FQN in code (e.g. `nq.e`), resolve it with
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`lookup-name.sh <mapping-dir> -o nq.e` — you will often see siblings
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(`nq.d`, `nq.f`, ...) that are the same class's split lambdas/inner
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classes, which is useful context.
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@ -0,0 +1,85 @@
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#!/usr/bin/env bash
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# lookup-name.sh — Query the mapping produced by recover-kotlin-names.sh.
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#
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# Modes:
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# lookup-name.sh <mapping-dir> <substring> search by real-FQN substring
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# lookup-name.sh <mapping-dir> -o <obf> resolve obf -> real
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# lookup-name.sh <mapping-dir> -p <pkg> list a real package
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# lookup-name.sh <mapping-dir> --grep <regex> <sources-dir>
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# grep decompiled sources and annotate each hit with the real class name
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set -euo pipefail
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usage() {
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cat <<EOF
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Usage: lookup-name.sh <mapping-dir> <query>
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lookup-name.sh <mapping-dir> -o <obf-fqn>
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lookup-name.sh <mapping-dir> -p <real-package-substring>
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lookup-name.sh <mapping-dir> --grep <regex> <sources-dir>
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<mapping-dir> is the directory produced by recover-kotlin-names.sh
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(must contain mapping.json).
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EOF
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exit 0
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}
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[[ $# -lt 2 ]] && usage
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DIR="$1"; shift
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[[ ! -f "$DIR/mapping.json" ]] && { echo "no mapping.json in $DIR" >&2; exit 1; }
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python3 - "$DIR" "$@" <<'PY'
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import json, os, re, sys, subprocess
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DIR = sys.argv[1]
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args = sys.argv[2:]
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MAP = json.load(open(os.path.join(DIR, "mapping.json")))
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REV = {}
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for o, r in MAP.items():
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REV.setdefault(r, []).append(o)
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def search(q):
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ql = q.lower()
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for r in sorted(REV):
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if ql in r.lower():
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print(r)
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for o in sorted(REV[r]):
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print(f" {o}")
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def by_obf(o):
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if o not in MAP:
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print(f"no mapping for {o}", file=sys.stderr); sys.exit(1)
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print(f"{o} -> {MAP[o]}")
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sibs = [s for s in REV[MAP[o]] if s != o]
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for s in sorted(sibs):
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print(f" sibling: {s}")
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def by_pkg(p):
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pl = p.lower()
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for r in sorted(REV):
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if pl in r.rsplit(".", 1)[0].lower():
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print(r)
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for o in sorted(REV[r]):
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print(f" {o}")
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def grep_annot(pattern, sources):
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res = subprocess.run(
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["grep", "-rEn", "--include=*.java", pattern, sources],
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capture_output=True, text=True)
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for line in res.stdout.splitlines():
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try:
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path, lineno, content = line.split(":", 2)
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except ValueError:
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continue
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rel = os.path.relpath(path, sources)
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obf = rel.replace(os.sep, ".")[:-5]
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suffix = f" // {MAP[obf]}" if obf in MAP else ""
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print(f"{rel}:{lineno}:{content}{suffix}")
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if args[0] == "-o" and len(args) == 2:
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by_obf(args[1])
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elif args[0] == "-p" and len(args) == 2:
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by_pkg(args[1])
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elif args[0] == "--grep" and len(args) == 3:
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grep_annot(args[1], args[2])
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else:
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search(" ".join(args))
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PY
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@ -0,0 +1,140 @@
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#!/usr/bin/env bash
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# recover-kotlin-names.sh — Rebuild a (obfuscated -> real) class-name map
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# from Kotlin metadata strings left in decompiled sources.
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#
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# R8 obfuscates JVM symbols but cannot strip the Kotlin metadata strings —
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# the Kotlin runtime (reflection, coroutines) needs them at runtime. Two
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# annotations carry the original FQN:
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#
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# * @DebugMetadata(c = "<full.qualified.Name>", f = "<File.kt>", ...)
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# emitted for almost every `suspend` function (every coroutine
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# SuspendLambda).
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#
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# * @Metadata(... d2 = {"...L<pkg/Class>;..."} ...) listing internal
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# class refs of the file.
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#
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# Typical recovery on a real-world app: 30-50 % of classes regain their real
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# names — usually 100 % of the *Repository / *ViewModel / *UseCase / *Impl
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# classes you actually want to read.
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set -euo pipefail
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usage() {
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cat <<EOF
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Usage: recover-kotlin-names.sh <decompiled-sources-dir> [output-dir]
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Walks every *.java under <decompiled-sources-dir>, mines @DebugMetadata
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and @Metadata annotations, and writes:
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<output-dir>/mapping.tsv tab-separated obf_fqn <TAB> real_fqn <TAB> file
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<output-dir>/mapping.json same data as JSON { obf_fqn: real_fqn, ... }
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<output-dir>/by_package/ one file per real package, listing
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real_fqn <TAB> obf_fqn <TAB> file
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If [output-dir] is omitted, files are written next to the sources dir.
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EOF
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exit 0
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}
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[[ $# -lt 1 || "$1" == "-h" || "$1" == "--help" ]] && usage
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SRC="$1"
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OUT="${2:-$(dirname "$SRC")/mapping}"
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[[ ! -d "$SRC" ]] && { echo "not a directory: $SRC" >&2; exit 1; }
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mkdir -p "$OUT/by_package"
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python3 - "$SRC" "$OUT" <<'PY'
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import os, re, sys, json
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from collections import defaultdict
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SRC, OUT = sys.argv[1], sys.argv[2]
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# @DebugMetadata(c = "com.foo.Bar$Inner$1", ...)
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RE_DEBUG = re.compile(r'@DebugMetadata\([^)]*?c\s*=\s*"([^"]+)"', re.S)
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# @Metadata(... d2 = { "...Lcom/foo/Bar;..." ...} )
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RE_DTWO = re.compile(r'@Metadata\([^)]*?d2\s*=\s*\{([^}]*)\}', re.S)
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RE_LCLASS = re.compile(r'L([A-Za-z][\w/$]+);')
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# jadx sometimes emits this comment for renamed classes
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RE_RENAMED = re.compile(r'/\*\s*renamed from:\s*([\w.$]+)\s*\*/')
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# Skip third-party / framework trees — their names are already real.
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SKIP_PREFIXES = (
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"kotlin.", "kotlinx.", "androidx.", "android.", "java.", "javax.",
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"com.google.", "com.facebook.", "com.appsflyer.", "com.datadog.",
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"io.ktor.", "io.sentry.", "io.realm.", "okhttp3.", "okio.",
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"com.squareup.", "com.bumptech.", "com.airbnb.", "com.payu.",
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"com.storyteller.", "zendesk.", "io.intercom.", "com.microsoft.",
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"com.tinder.", "com.hotjar.", "com.amplitude.", "com.segment.",
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"com.mixpanel.", "com.onesignal.", "com.stripe.", "com.braintreepayments.",
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"retrofit2.", "dagger.", "javax.inject.", "org.jetbrains.",
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)
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mapping = {}
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file_real = {}
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counts = defaultdict(int)
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for dp, _, files in os.walk(SRC):
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for f in files:
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if not f.endswith(".java"):
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continue
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path = os.path.join(dp, f)
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rel = os.path.relpath(path, SRC)
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obf = rel[:-5].replace(os.sep, ".")
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if obf.startswith(SKIP_PREFIXES):
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continue
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try:
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text = open(path, "r", errors="replace").read()
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except OSError:
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continue
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real = None
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m = RE_DEBUG.search(text)
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if m:
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real = m.group(1).split("$", 1)[0]
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counts["debug_meta"] += 1
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if not real:
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m = RE_DTWO.search(text)
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if m:
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for lm in RE_LCLASS.finditer(m.group(1)):
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cand = lm.group(1).replace("/", ".").split("$", 1)[0]
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if "." in cand and not cand.startswith(("kotlin.", "java.", "android")):
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real = cand
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counts["d2"] += 1
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break
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if not real:
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m = RE_RENAMED.search(text)
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if m:
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real = m.group(1)
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counts["renamed"] += 1
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if real:
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mapping[obf] = real
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file_real[obf] = path
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with open(os.path.join(OUT, "mapping.tsv"), "w") as f:
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f.write("obf_fqn\treal_fqn\tfile\n")
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for k in sorted(mapping):
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f.write(f"{k}\t{mapping[k]}\t{file_real[k]}\n")
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with open(os.path.join(OUT, "mapping.json"), "w") as f:
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json.dump(mapping, f, indent=2, sort_keys=True)
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by_pkg = defaultdict(list)
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for obf, real in mapping.items():
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pkg = real.rsplit(".", 1)[0] if "." in real else "(default)"
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by_pkg[pkg].append((real, obf, file_real[obf]))
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for pkg, rows in by_pkg.items():
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safe = pkg.replace(".", "_") or "default"
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with open(os.path.join(OUT, "by_package", f"{safe}.txt"), "w") as f:
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for real, obf, p in sorted(rows):
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f.write(f"{real}\t{obf}\t{p}\n")
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print(f"Recovered {len(mapping)} class names")
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for k, v in counts.items():
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print(f" via {k}: {v}")
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print(f"Real packages: {len(by_pkg)}")
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print(f"Wrote {OUT}/mapping.tsv, mapping.json, by_package/")
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PY
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