Saturday, January 26, 2013

string decryption with dex2jar

i have been getting a lot of questions about string decryption lately, so let's talk.

let's say you have an app and notice encrypted strings. strings are an easy way to get a basic idea of what code is doing so naturally you want to decrypt them. but how? there are many different ways to encrypt strings and then decrypt at runtime but in practices there are some assumptions we can make in decreasing order of likelihood.

1. the encryption must be reversible. the strings must be decrypted at run time somehow. this is good but we can assume even more.

2. the process is automated. when Alice wants to release her app she puts the source code through an automated modification process which iterates over every string literal, encrypts it and replaces it with a call to a decryption method with the encrypted string as a parameter.

3. decryption is the same or nearly the same for each string. there is only one decryption method.

4. the type signature of the method is:
static String decryptMethod(String)

while these assumptions hold, it is not very difficult to create a general technique by which we can decrypt all of the strings of an app in place. the real question is do you want to do it at the java or smali level? if you primarily look at decompiled code you can work at the java level. and you're in luck, such a tool already exists in dex2jar:

there is a wiki article about it here: but it is unfinished. you can at least get a visual for what the decompiled code will look like before and after. if you're a good person, you will update the wiki. i leave that as a task for some good reader.

the tool is currently incorrectly spelled as d2j-decrpyt-string.(sh|bat). it takes at least two parameters and sometimes needs three. they are:
  1. method name, -mn : in our case, decryptMethod
  2. method owner, -mo : let's say com.alice.utils
  3. class path, -cp : if decryptMethod makes use of any framework api, you will need to give the path to a framework.jar from the android.sdk
d2j-decrpyt-string -mn decryptMethod -mo com.alice.utils -cp ~/android/sdk/platforms/android-4/framework.jar

doing this at the smali level requires access to a dalvik vm, so in that regard it is trickier, but there are many emulators and you can even use your phone. here's how the process can work:
  • pull out all of the strings and put into a file
  • write some java code, unless you're comfortable with smali, to open the file and iterate over each line and call the decryption method on each string.
  • compile java bytecode and convert to dalvik executable with dx from the android sdk
  • run the code on a dalvik vm