/* ./crackme0x03 @ 0x804846e */
#include <stdint.h>
int32_t test (int32_t arg_8h, uint32_t arg_ch) {
eax = arg_8h;
if (eax != arg_ch) {
shift ("Lqydolg#Sdvvzrug$");
} else {
shift ("Sdvvzrug#RN$$$#=,");
}
return eax;
}
can also reverse shift() to satisfy curiosity.
[0x08048360]> pdf@sym.shift
; CODE (CALL) XREF 0x08048491 (sym.test)
; CODE (CALL) XREF 0x08048483 (sym.test)
/ function: sym.shift (90)
| 0x08048414 sym.shift:
| 0x08048414 55 push ebp
| 0x08048415 89e5 mov ebp, esp
| 0x08048417 81ec98000000 sub esp, 0x98
| 0x0804841d c7458400000000 mov dword [ebp-0x7c], 0x0 ; this seems to be a counter
| . ; CODE (JMP) XREF 0x0804844e (sym.shift)
/ loc: loc.08048424 (74)
| . 0x08048424 loc.08048424:
| .--> 0x08048424 8b4508 mov eax, [ebp+0x8] ; ebp+0x8 = strlen(chain)
| | 0x08048427 890424 mov [esp], eax
| | 0x0804842a e811ffffff call dword imp.strlen
| | ; imp.strlen()
| | 0x0804842f 394584 cmp [ebp-0x7c], eax
| |,=< 0x08048432 731c jae loc.08048450
| || 0x08048434 8d4588 lea eax, [ebp-0x78]
| || 0x08048437 89c2 mov edx, eax
| || 0x08048439 035584 add edx, [ebp-0x7c]
| || 0x0804843c 8b4584 mov eax, [ebp-0x7c]
| || 0x0804843f 034508 add eax, [ebp+0x8]
| || 0x08048442 0fb600 movzx eax, byte [eax]
| || 0x08048445 2c03 sub al, 0x3
| || 0x08048447 8802 mov [edx], al
| || 0x08048449 8d4584 lea eax, [ebp-0x7c]
| || 0x0804844c ff00 inc dword [eax]
| `==< 0x0804844e ebd4 jmp loc.08048424
| | ; CODE (JMP) XREF 0x08048432 (sym.shift)
/ loc: loc.08048450 (30)
| | 0x08048450 loc.08048450:
| `-> 0x08048450 8d4588 lea eax, [ebp-0x78]
| 0x08048453 034584 add eax, [ebp-0x7c]
| 0x08048456 c60000 mov byte [eax], 0x0
| 0x08048459 8d4588 lea eax, [ebp-0x78]
| 0x0804845c 89442404 mov [esp+0x4], eax
| 0x08048460 c70424e8850408 mov dword [esp], 0x80485e8
| 0x08048467 e8e4feffff call dword imp.printf
| ; imp.printf()
| 0x0804846c c9 leave
\ 0x0804846d c3 ret
; ------------
you can read the assembly code and find the decryption is actually a "sub al, 0x3". we can write a python script for it:
print(''.join([chr(ord(i)-0x3) for i in 'SdvvzrugRN$$$']))
print(''.join([chr(ord(i)-0x3) for i in 'LqydolgSdvvzrug$']))
the easier way is to run the decryption code, that means debug it or emulate it. I used radare2 ESIL emulator but it got stuck when executed call dword imp.strlen. And I can't find the usage of hooking function / skip instruction in radare2. The following is an example to show u how to emulate ESIL.
[0x08048414]> s 0x08048445 # the 'sub al, 0x03'
[0x08048445]> aei # init VM
[0x08048445]> aeim # init memory
[0x08048445]> aeip # init ip
[0x08048445]> aer eax=0x41 # set eax=0x41 -- 'A'
[0x08048445]> aer # show current value of regs
oeax = 0x00000000
eax = 0x00000041
ebx = 0x00000000
ecx = 0x00000000
edx = 0x00000000
esi = 0x00000000
edi = 0x00000000
esp = 0x00178000
ebp = 0x00178000
eip = 0x08048445
eflags = 0x00000000
[0x08048445]> V # enter Visual mode
# 'p' or 'P' to change visual mode
# I prefer the [xaDvc] mode
# use 's' to step in and 'S' to step over
[0x08048442 [xaDvc]0 0% 265 ./crackme0x03]> diq;?0;f t.. @ sym.shift+46 # 0x8048442
dead at 0x00000000
- offset - 0 1 2 3 4 5 6 7 8 9 A B C D E F 0123456789ABCDEF
0x00178000 0000 0000 0000 0000 0000 0000 0000 0000 ................
0x00178010 0000 0000 0000 0000 0000 0000 0000 0000 ................
0x00178020 0000 0000 0000 0000 0000 0000 0000 0000 ................
0x00178030 0000 0000 0000 0000 0000 0000 0000 0000 ................
oeax 0x00000000 eax 0x00000041 ebx 0x00000000 ecx 0x00000000
edx 0x00000000 esi 0x00000000 edi 0x00000000 esp 0x00178000
ebp 0x00178000 eip 0x08048445 eflags 0x00000000
: 0x08048442 0fb600 movzx eax, byte [eax]
: ;-- eip:
: 0x08048445 2c03 sub al, 3
: 0x08048447 8802 mov byte [edx], al
: 0x08048449 8d4584 lea eax, [var_7ch]
: 0x0804844c ff00 inc dword [eax]
:=< 0x0804844e ebd4 jmp 0x8048424
; CODE XREF from sym.shift @ 0x8048432
0x08048450 8d4588 lea eax, [var_78h]
By the way, u can also open the file and use write data command to decrypt data.
r2 -w ./crackme0x03
[0x08048360]> aaa
[0x08048360]> fs strings
[0x08048360]> f
0x080485ec 18 str.Lqydolg_Sdvvzrug
0x080485fe 18 str.Sdvvzrug_RN
0x08048610 25 str.IOLI_Crackme_Level_0x03
0x08048629 11 str.Password:
[0x08048360]> s str.Lqydolg_Sdvvzrug
[0x080485ec]> wos 0x03 @ str.Lqydolg_Sdvvzrug!0x11
[0x080485ec]> px
- offset - 0 1 2 3 4 5 6 7 8 9 A B C D E F 0123456789ABCDEF
0x080485ec 496e 7661 6c69 6420 5061 7373 776f 7264 Invalid Password
0x080485fc 2100 5364 7676 7a72 7567 2352 4e24 2424 !.Sdvvzrug#RN$$$
0x0804860c 233d 2c00 494f 4c49 2043 7261 636b 6d65 #=,.IOLI Crackme
0x0804861c 204c 6576 656c 2030 7830 330a 0050 6173 Level 0x03..Pas