Bitecode

Description

Written by: ItzSomebody

Keith went crazy and told me to work on the compiled form of Java instead of the source code. Unfortunately, all decompilers I've tried crash on attempting to decompile. Can you help out?

bite.class

I tried to run this, but didn't work well:

# java bite
Error: LinkageError occurred while loading main class bite
	java.lang.ClassFormatError: Illegal class name "java/lang/System" in class file bite

I using this website to decompile it!

Using CFX as Decompiler because JDCore didn't work

Copied all into bite.java

I investigate the code quite long because its messy

First, var0 is our input also arguments:

public static void main(String[] var0) {
	...
	...

These line is interesting:

var3_3 = var0[0].toCharArray();
var4_4 = var3_3.length;
if ((var4_4 ^ 927739485 ^ 927739457) != 0) {
    (System)null;
    java.lang.System.out.println("Nah");
    return;
}

var4_4 is our input length, these lines means if our input line xor 927739485 xor 927739457 == 0 then it will not return

So our input length should be 927739485 XOR 927739457 which is 28:

>>> 927739485 ^ 927739457
28

Next it declare v0 = var3_3 and do XOR condition:

v0 = var3_3;
if ((v0[0] ^ 189074585) - 189074673 != 0)
{
    do {
        if (var2_2 == 0) {
            (System)null;
            java.lang.System.out.println("Nah");
        }
        break;
    } while (true);
}

These lines basically means the first letter of our input must be 189074585 XOR 189074673

Calculate it with python:

>>> 189074585 ^  189074673
104
>>> chr(104) # ascii value
'h'

Yeah! Must be it, first character is h

Next few lines are almost the same comparison:

if ((v0[1] ^ -227215135) - -227215214 != 0)
...
if ((v0[2] ^ 19240864) - 19240899 == 0)
...
if ((v0[3] ^ 245881291) - 245881279 == 0)
...
...
...

I wrote a python script auto calculate all the condition and combine all characters:

import re
text = open("bite.java").read()
text = re.findall("v0\[[0-9]+\] \^ [0-9-]+\) \- [0-9-]+",text)
v0 = [0 for i in range(28)]
for i,t in enumerate(text):
	exec("v0[%i] = chr(" % (i) + t.replace(") -",' ^') + ")")
print ''.join(v0)

Flag

hsctf{wH04_u_r_2_pr0_4_th1$}