CDKey.asm - skullsecurity.org

cdkey1

segment .data
  cdkey db  "1111111111111"
segment .text

  global  _start
  _start:

   mov     ecx, cdkey
   mov     eax, 3                   ; eax = 3
   mov     esi, ecx                 ; esi = ecx, ecx = 13 digit starcraft cd key entered
   xor     ecx, ecx                 ; ecx = 0

  Top:
   movsx   edx, byte [ecx+esi]      ; move sign extension retains sign flag of 1 byte at address [ecx+esi] to edx.
   sub     edx, 30h                 ; convert ascii char in form of hex to actual integer number. "0" is 0x30 - 0x30 = 0 integer
   lea     edi, [eax+eax]           ; load effective address allows complex calculation e.g. [eax*ebx+5] faster than add/mul
                                    ; and can store result in register other than source register e.g. (eax in case of mul)
                                    ; LEA instruction calculates [eax+eax] or more like [eax]+[eax] operation and store value 
                                    ; result in edi = 6 whereas;
                                    ; MOV will calculate [eax+eax] which is [6] and tries to look for value in memory address 0x00000006
                                    ; to store in edi, end up as invalid address and crash.
   xor     edx, edi                 ; xor byteswap calculation and store result in edx
   add     eax, edx                 ; accumulate xor result to eax
   inc     ecx                      ; increment counter ecx
   cmp     ecx, 0Ch                 ; once 12 digits processed, move to next intructions.
                                    ; not sure why it doesnt process all 13 digits.
   jl      short Top                ; otherwise, go back to Top:

   xor     edx, edx                 ; clear out edx in preparation of div instruction which may have remainder
                                    ; and will be put in edx if there is a remainder.
   mov     ecx, 0Ah                 ; move 10 to ecx, act as divisor.
   div     ecx                      ; divide eax / ecx, quotient stored in eax, remainder stored in edx

   movsx   eax, byte [esi+0Ch]      ; move last digit value to eax. cdkey[-1]
   add     edx, 30h                 ; convert int value(1) back to ascii value(0x31)
   cmp     eax, edx                 ; compare if 13th digit  == remainder, if true we exit with 1 in eax
   jnz     bottom                   ; if not true, exit to bottom with 0 in eax:

   mov     eax, 1
   ret

 bottom:
   xor     eax, eax
   ret

I tried to interpret it in python:

#! /usr/bin/env python

import subprocess as sub

sub.Popen('clear').communicate()[0]
cdkey = raw_input('enter cd key: ')
eax, ecx, edx = 3, 0, 0

with open('cdkey.txt','w+') as fobj:
  for i in cdkey[:-1]:
    eax += int(i) ^ (eax+eax)
    ecx += 1
    fobj.write('eax = {}\necx = {}\n\n'.format(hex(eax),ecx))

if (cdkey[-1]) == str(eax%10):
  print('CDKey is valid.')
else:
  print('CDKey not valid.')
print 'eax = ',hex(eax)
print 'remainder = ', eax%10
print 'last digit of cdkey = ', cdkey[-1]