Add signature-derived-key coverage, PID attach, MMKV; fix stale question count
New: references/signature-derived-keys.md (apps that use their own signing certificate as crypto key material: detection greps, why offline extraction is unreliable -- the runtime value is a certificate DER from inside the PKCS#7 chain, not the .RSA container -- plus the hardcode-then-verify procedure); scripts/sig_probe.py (offline candidates via --apk, authoritative read via --live, both exercised); pitfalls.md P27 (rebuilt APK installs and renders but every signed request returns empty/-1 signing params). Updated: dynamic-frida.md (attach by PID when enumerate_processes omits the target; verify runtime/Java-bridge availability before blaming the script); runtime-data.md (MMKV format, parse/write snippet, crc32 and in-memory-overwrite traps); server-api.md (route the signature-derived-key case to the new reference); SKILL.md and README.md (new classification question, corrected question count, index entries).
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#!/usr/bin/env python3
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"""
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sig_probe.py -- find the exact value Android returns for
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`PackageInfo.signatures[0].toCharsString()`.
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Why this exists: apps that use their own signing certificate as a *crypto key* need that
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value hardcoded after a repack. Deriving it offline is unreliable -- it is platform
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dependent, and on modern Android it is a single certificate from the chain, not the whole
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`META-INF/*.RSA` blob. Get it from the device, or cross-check offline candidates.
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Usage
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-----
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# authoritative: read from a running (or launchable) package on a rooted device
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python sig_probe.py --live <package>
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python sig_probe.py --live <package> --serial <adb-serial> --host 127.0.0.1:27042
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# offline: enumerate candidate blobs inside META-INF/*.RSA
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python sig_probe.py --apk <apk>
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Output is plain hex, ready to paste into a `const-string` smali patch.
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Notes
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-----
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* --live needs `frida` (pip install frida) plus a matching frida-server on the device.
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Host and device versions must match; see references/dynamic-frida.md.
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* `--serial` is honoured for adb calls. If several devices are attached, pass it -- Frida's
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USB auto-selection can silently pick the wrong one.
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"""
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import argparse
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import io
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import struct
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import subprocess
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import sys
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import time
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import zipfile
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SIG_EXTS = ('.RSA', '.DSA', '.EC')
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# --------------------------------------------------------------------------- DER helpers
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def _tlv(data, off):
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"""Return (tag, header_len, content_len). Raises on truncation."""
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if off + 2 > len(data):
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raise ValueError('truncated TLV at %d' % off)
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tag = data[off]
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first = data[off + 1]
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if first & 0x80:
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n = first & 0x7F
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if n == 0 or off + 2 + n > len(data):
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raise ValueError('bad length at %d' % off)
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length = int.from_bytes(data[off + 2:off + 2 + n], 'big')
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return tag, 2 + n, length
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return tag, 2, first
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def _children(data, start, end):
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"""Yield (tag, header_len, content_len, content_off) for each child in [start, end)."""
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off = start
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while off < end:
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tag, hlen, clen = _tlv(data, off)
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yield tag, hlen, clen, off + hlen
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off += hlen + clen
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def pkcs7_certificates(blob):
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"""Extract the DER certificate blobs from a PKCS#7 SignedData structure.
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ContentInfo ::= SEQUENCE { contentType OID, content [0] EXPLICIT SignedData }
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SignedData ::= SEQUENCE { version, digestAlgorithms, contentInfo,
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certificates [0] IMPLICIT SET OF Certificate, ... }
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"""
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out = []
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tag, hlen, clen = _tlv(blob, 0)
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if tag != 0x30:
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return out
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content_off = hlen
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content_end = hlen + clen
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# ContentInfo -> [0] EXPLICIT -> SignedData SEQUENCE
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signed_data = None
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for t, h, c, coff in _children(blob, content_off, content_end):
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if t == 0xA0: # content [0] EXPLICIT
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for t2, h2, c2, coff2 in _children(blob, coff, coff + c):
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if t2 == 0x30: # SignedData
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signed_data = (coff2, coff2 + c2)
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break
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if signed_data is None:
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return out
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sd_start, sd_end = signed_data
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# children of SignedData: version, digestAlgorithms, contentInfo, certificates [0], ...
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seen_a0 = 0
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for t, h, c, coff in _children(blob, sd_start, sd_end):
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if t == 0xA0: # certificates [0] IMPLICIT
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seen_a0 += 1
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if seen_a0 > 1:
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break
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for t3, h3, c3, coff3 in _children(blob, coff, coff + c):
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if t3 == 0x30: # a Certificate
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out.append(blob[coff3 - h3:coff3 + c3])
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return out
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def candidates_from_apk(path):
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"""Every plausible hardcode candidate, longest-first, de-duplicated."""
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results = []
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with zipfile.ZipFile(path) as z:
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names = [n for n in z.namelist()
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if n.upper().startswith('META-INF/') and n.upper().endswith(SIG_EXTS)]
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for name in names:
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blob = z.read(name)
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whole = ('whole ' + name, blob)
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results.append(whole)
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for i, cert in enumerate(pkcs7_certificates(blob)):
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results.append(('%s cert[%d]' % (name, i), cert))
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seen, uniq = set(), []
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for label, blob in results:
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if blob not in seen:
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seen.add(blob)
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uniq.append((label, blob))
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return uniq
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# --------------------------------------------------------------------------- live route
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LIVE_JS = r"""
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// Single-shot read. No timers: frida's JS runtime has no setTimeout, and using one
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// turns a "not ready yet" into a silent no-response. The Python side retries instead.
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Java.perform(function () {
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var pkg = PKG_NAME;
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try {
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var app = Java.use('android.app.ActivityThread').currentApplication();
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if (app === null) {
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send({ ok: false, retry: true, error: 'Application not initialized yet' });
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return;
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}
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var ctx = app.getApplicationContext();
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var pm = ctx.getPackageManager();
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var pi = pm.getPackageInfo(pkg, 64);
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var arr = pi.signatures.value;
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var out = [];
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for (var i = 0; i < arr.length; i++) {
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var s = arr[i].toCharsString();
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var blen = -1;
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try { blen = arr[i].toByteArray().length; } catch (e) {}
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out.push({ i: i, hex: s, len: s.length, bytes: blen });
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}
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send({ ok: true, signatures: out });
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} catch (e) {
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send({ ok: false, error: String(e) });
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}
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});
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"""
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def run_live(pkg, serial, host, timeout):
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try:
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import frida
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except ImportError:
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print('[!] frida not installed: pip install frida', file=sys.stderr)
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return 2
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try:
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dev = frida.get_device_manager().add_remote_device(host)
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except Exception as exc: # noqa: BLE001
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print('[!] cannot reach frida-server at %s: %s' % (host, exc), file=sys.stderr)
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print(' is it running, and is the port forwarded? '
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'adb %sforward tcp:%s tcp:27042'
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% (('-s %s ' % serial) if serial else '',
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host.rsplit(':', 1)[-1]), file=sys.stderr)
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return 2
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# A running process is preferred; fall back to spawning so package data is readable.
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pid, spawned = None, False
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for p in dev.enumerate_processes():
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if p.name == pkg:
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pid = p.pid
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break
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if pid is None:
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try:
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pid = dev.spawn([pkg])
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spawned = True
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except Exception as exc: # noqa: BLE001
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print('[!] package not running and could not be spawned: %s' % exc, file=sys.stderr)
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print(' launch it once, then re-run', file=sys.stderr)
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return 2
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result = {}
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def on_msg(msg, _data):
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if msg.get('type') == 'error':
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result['_js_error'] = msg.get('description') or str(msg)
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else:
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result.update(msg.get('payload') or {})
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try:
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session = dev.attach(pid)
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if spawned:
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dev.resume(pid)
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time.sleep(1.0)
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# Retry at the Python level: right after a spawn the Application object may not
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# exist yet, and the script reports that as retryable rather than failing.
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deadline = time.time() + timeout
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while time.time() < deadline:
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result.clear()
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# V8 is required: on some frida builds the default runtime has no Java bridge,
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# and the failure looks like "no response" rather than a clear error.
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script = session.create_script(
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LIVE_JS.replace('PKG_NAME', repr(pkg).replace("'", '"')), runtime='v8')
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script.on('message', on_msg)
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script.load()
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t0 = time.time()
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while not result and time.time() - t0 < 1.5:
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time.sleep(0.1)
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if result.get('ok') or not result.get('retry'):
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break
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time.sleep(0.4)
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except Exception as exc: # noqa: BLE001
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print('[!] injection failed: %s' % exc, file=sys.stderr)
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return 2
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if not result.get('ok'):
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detail = result.get('error') or result.get('_js_error') or 'no response'
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print('[!] probe failed: %s' % detail, file=sys.stderr)
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return 2
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for sig in result['signatures']:
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print('signature[%d] toCharsString length = %d toByteArray bytes = %d'
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% (sig['i'], sig['len'], sig['bytes']))
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print(sig['hex'])
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print()
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return 0
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# --------------------------------------------------------------------------- main
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def main():
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ap = argparse.ArgumentParser(
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description='Find the exact signatures[0].toCharsString() value (for hardcoding '
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'into a repacked APK).',
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formatter_class=argparse.RawDescriptionHelpFormatter,
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epilog=__doc__)
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g = ap.add_mutually_exclusive_group(required=True)
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g.add_argument('--apk', help='APK to inspect offline (enumerates candidates)')
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g.add_argument('--live', metavar='PKG', help='package name; read the value from the device')
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ap.add_argument('--serial', help='adb serial (when several devices are attached)')
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ap.add_argument('--host', default='127.0.0.1:27042',
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help='frida-server endpoint for --live (default %(default)s)')
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ap.add_argument('--timeout', type=float, default=20.0,
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help='seconds to wait for the live probe (default %(default)s)')
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args = ap.parse_args()
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if args.live:
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return run_live(args.live, args.serial, args.host, args.timeout)
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cands = candidates_from_apk(args.apk)
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if not cands:
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print('[!] no META-INF/*.RSA|*.DSA|*.EC in %s (is it signed?)' % args.apk,
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file=sys.stderr)
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return 1
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print('Candidates in %s' % args.apk)
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print('(modern Android returns ONE certificate from the chain, not the whole .RSA --')
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print(' use --live to know which; otherwise build one variant per candidate)\n')
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for label, blob in cands:
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print('%-28s %6d bytes %5d hex chars' % (label, len(blob), len(blob) * 2))
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print(blob.hex())
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print()
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return 0
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if __name__ == '__main__':
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sys.exit(main())
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