Friday, November 5, 2010

string obfuscation

a common practice in pc apps is string encryption. when a program is compiled all literal strings are replaced with function calls with a parameter that retrieve that specific string. the result is when the program is decompiled it is not possible to simply search for strings. string searching makes finding interesting locations in code much easier so preventing this makes cracking more difficult for more people. recently, i came across some string obfuscation. this is the first time i have seen it "in the wild". proguard does not do it because it necessarily requires more work from the processor and may slow things down slightly.

how can we defeat it? it is difficult to analyze dalvik code and perform the encryption process in reverse without damaging other code (at least for me), but it is very simple to merely call the function for every possible string. here is an example: StringDumper.smali

StringDumper will output decrypted strings to the log file. the encrypted strings in my case were held inside StringHolder.smali and were retrieved using getString(I)Ljava/lang/String. there were only 0x15 strings, but if you run into this specific type of encryption there may be more or less.

it is not likely you will see this exact, specific encrypter implementation since i suspect it was developed in-house and is not open source. at least you will have a general idea of how to defeat such protection.

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