Solving Binary 300 From the Defcon 2011 Quals Using AndBug

Binary 300 was provided in the form of a memory dump, a few encrypted files, and a classes.dex. For this walkthrough, we will use AndBug and ApkTool for analysis, and python for implementing a cipher I wouldn't use to protect HBGary's address book.. Let alone my phone pr0n.

The classes.dex had an invalid checksum and version (666); updating the version to 035 and regenerating the checksum let us use dexdump to dump Dalvik pseudocode from classes.dex.

Immediately, it became obvious that this was "LokPixLite," an image-encryption app in the Android Market; we downloaded LPL from the market, compared the dex files to verify our assumption using AndroGuard, and rebuilt the dex file using "apktool d -d" and "apktool b -d" before installing into an emulator.

We then used the emulator to encrypt a few reference images with a known password, then went to the decompiled source, we see there's a rat's nest of obfuscated methods in class "g" that are packed full of references to "XOR" and "SHA1". What we need to sort this out is some context.. A trace of call flow with arguments will do nicely:

$ ./andbug trace -p com.closecrowd.lokpixlite com.closecrowd.lokpixlite.g

This command instructs AndBug to connect to LokPixLite, and use JDWP to produce METHOD_ENTRY events for every method of "g"; we then decrypt one of our images by submitting the password:

[::] setting hooks
[::] hooked com.closecrowd.lokpixlite.g
[::] hooks set
[::] thread <1> main com.closecrowd.lokpixlite.g.d()Ljava/lang/Boolean;:0
     this = <obj Lcom/closecrowd/lokpixlite/g; #c1406db458>
[::] thread <1> main com.closecrowd.lokpixlite.g.b()V:0
     this = <obj Lcom/closecrowd/lokpixlite/g; #c1406db458>
[::] thread <1> main com.closecrowd.lokpixlite.g.d()Ljava/lang/Boolean;:0
     this = <obj Lcom/closecrowd/lokpixlite/g; #c1406db458>
[::] thread <10> password com.closecrowd.lokpixlite.g.a(Ljava/lang/String;)V:0
     this = <obj Lcom/closecrowd/lokpixlite/g; #c1406db458>
     p1 = test
[::] thread <10> password com.closecrowd.lokpixlite.g.e()V:0
     this = <obj Lcom/closecrowd/lokpixlite/g; #c1406db458>
[::] thread <10> password com.closecrowd.lokpixlite.g.b(Ljava/lang/String;)[B:0
     p0 = test
[::] thread <1> main com.closecrowd.lokpixlite.g.d()Ljava/lang/Boolean;:0
     this = <obj Lcom/closecrowd/lokpixlite/g; #c1406db458>
[::] thread <10> loading com.closecrowd.lokpixlite.g.a()V:0
     this = <obj Lcom/closecrowd/lokpixlite/g; #c1406db458>
[::] thread <10> loading com.closecrowd.lokpixlite.g.a()V:0
     this = <obj Lcom/closecrowd/lokpixlite/g; #c1406db458>
[::] thread <10> loading com.closecrowd.lokpixlite.g.a()V:0
     this = <obj Lcom/closecrowd/lokpixlite/g; #c1406db458>
[::] thread <10> loading com.closecrowd.lokpixlite.g.a([BIZ)[B:0
     this = <obj Lcom/closecrowd/lokpixlite/g; #c1406db458>
     p2 = 16
     p1 = 830009846960
     p3 = False
[::] thread <10> loading com.closecrowd.lokpixlite.g.b([BIZ)[B:0
     this = <obj Lcom/closecrowd/lokpixlite/g; #c1406db458>
     p2 = 16
     p1 = 830009846960
     p3 = False

We can see our password, "test" is submitted to b(Ljava.lang.String;) to get a [B (byte array) back, which is then stashed in a global variable. Later, we see a byte array with the encrypted data passed to b([BIZ]), which returns our plaintext image.

At this point, we have found the two key functions in the cipher -- the conversion of a password to a key, and applying the key to the ciphertext. We flip back to static analysis, at this point with our context data to analyze the code. The method teases apart pretty easily with context, resulting in the following python implementation:

def decrypt(data, key):
    mask = sha1sum(key)[:8]
    return strxor(data[16:], cycle(mask))

After that, it is simply a round of using strings to find all the passwords in the heap dump, and applying them in turn until we get a valid JPEG decode. From there, the password's easy to read if you squint, and it's on to the next level.