#!/usr/bin/env python3
"""Checks the dex inside an APK the way Dalvik's dexopt does at install time: format version,
size, Adler-32, SHA-1, no post-035 sections. usage: check_dex.py app.apk"""
import hashlib
import struct
import sys
import zipfile
import zlib

d = zipfile.ZipFile(sys.argv[1]).read("classes.dex")
problems = []
if d[:8] != b"dex\n035\x00":
    problems.append("dex version %r (Android 2.3 needs 035)" % d[:8])
if struct.unpack("<I", d[32:36])[0] != len(d):
    problems.append("file_size field wrong")
if zlib.adler32(d[12:]) & 0xFFFFFFFF != struct.unpack("<I", d[8:12])[0]:
    problems.append("Adler-32 checksum wrong")
if hashlib.sha1(d[32:]).digest() != d[12:32]:
    problems.append("SHA-1 signature wrong")
map_off = struct.unpack("<I", d[52:56])[0]
for i in range(struct.unpack("<I", d[map_off:map_off + 4])[0]):
    t = struct.unpack("<H", d[map_off + 4 + i * 12:map_off + 6 + i * 12])[0]
    if t in (7, 8):
        problems.append("contains call_site/method_handle sections (needs Android 8+)")
for p in problems:
    print("DEX PROBLEM:", p)
print("classes.dex: %d bytes, %s" % (len(d), "OK" if not problems else "REJECTED"))
sys.exit(1 if problems else 0)