A customer of mine asked whether it is possible to protect his software from reverse engineering. I didn't found any C/C++ compiler which was able to produce obfuscated code making it hard to reverse engineer and complicate the use of such tools as Hex-Rays Decompiler, so I made a little attempt to hack Tiny C compiler's codegenerator.
I patched it so it produces a lot of random noise code between effective code. Of course, resulting code will work much slower. But in real life, we can obfuscate only critical parts of code containing algorithms we don't want to be easily leaked. Of course, it is virtually impossible to protect any code from reverse engineering, but it is possible to make it much more difficult.
Example: simple function:
int a (int a, int b)
{
return a + b * 4;
};
On output...
a proc near
var_CD500B = byte ptr -0CD500Bh
arg_0 = dword ptr 8
arg_4 = dword ptr 0Ch
arg_1D364BDE = byte ptr 1D364BE6h
nop
push ebp
mov ebp, esp
sub esp, 0
nop
xor eax, ebx
mov eax, 99B7A34Ah
mov eax, 0EC06E7ACh
lea edx, [esi+63h]
mov ebx, [ebp+arg_0]
and ebx, ebx
loc_800001F:
lea ebx, [ebp+arg_1D364BDE]
mov ebx, 9EF81F3Eh
lea eax, [ebx+3Eh]
lea ecx, [esi]
mov eax, 0FD6D5D47h
sub ebx, edx
lea ecx, [ebp+var_CD500B]
lea ecx, [eax]
mov eax, [ebp+arg_4] ; *
shl eax, 2 ; *
mov ecx, eax
adc ecx, edx
mov ecx, [ebp+arg_0]
adc ecx, ecx
sub edx, ecx
sub edx, eax
lea ebx, [esp+ecx*8]
mov ecx, 29262C66h
mov ebx, 0CC18D2C4h
mov ebx, 0FDB56490h
mov ecx, 9E709D5Eh
mov ecx, 73805EBFh
mov ecx, eax
or ecx, eax
mov ebx, 7339AD0Eh
mov edx, 2CA8725Ah
lea edx, [edi+esi*8]
mov ebx, 87684A89h
mov ebx, 52A74759h
xor edx, edx
jnz short loc_800001F
mov ebx, 0CCA90613h
sub ecx, eax
mov ecx, 0C6699FDh
mov ebx, 0A8B272A1h
mov ebx, eax
sbb ebx, ebx
mov ecx, [ebp+arg_0] ; *
add ecx, eax ; *
or edx, ebx
mov edx, 47257B14h
mov edx, ecx
add edx, edx
mov eax, 9E3E878Ah
mov ebx, 0DAB5E429h
mov edx, 0ABFDB94Eh
adc eax, ebx
add edx, ebx
lea edx, [ebx+75A1EF29h]
or edx, edx
mov eax, ecx ; *
jmp $+5
leave
pop ebx
jmp ebx
a endp
(effective code marked with asterisk)
One funny thing is that now the compiler uses random number generator. Almost all good computer programs contain at least one random-number generator. (fortune file in plan 9 OS).
Here is also my crackme I created for testing. It was eventually reversed, though.
For those who are interested:
Patch for Tiny C version 0.9.25
Tiny C 0.9.25 patched win32 executables. Unfortunately, there were complaints about win32 executables, someone said antivirus detected malware in these. So the files archived with a password: 'tcc'.
Update: comp.compilers thread