IOLI 0x04

0x04

  1. [0x080483d0]> pdd@main
  2. /* r2dec pseudo code output */
  3. /* ./crackme0x04 @ 0x8048509 */
  4. #include <stdint.h>
  6. int32_t main (void) {
  7.     int32_t var_78h;
  8.     int32_t var_4h;
  9.     eax = 0;
  10.     eax += 0xf;
  11.     eax += 0xf;
  12.     eax >>= 4;
  13.     eax <<= 4;
  14.     printf ("IOLI Crackme Level 0x04\n");
  15.     printf ("Password: ");
  16.     eax = &var_78h;
  17.     scanf (0x8048682, eax);
  18.     eax = &var_78h;
  19.     check (eax);
  20.     eax = 0;
  21.     return eax;
  22. }

Let’s enter check.

  1. #include <stdint.h>
  3. int32_t check (char * s) {
  4.     char * var_dh;
  5.     uint32_t var_ch;
  6.     uint32_t var_8h;
  7.     int32_t var_4h;
  8.     char * format;
  9.     int32_t var_sp_8h;
  10.     var_8h = 0;
  11.     var_ch = 0;
  12.     do {
  13.         eax = s;
  14.         eax = strlen (eax);
  15.         if (var_ch >= eax) {
  16.             goto label_0;
  17.         }
  18.         eax = var_ch;
  19.         eax += s;
  20.         eax = *(eax);
  21.         var_dh = al;
  22.         eax = &var_4h;
  23.         eax = &var_dh;
  24.         sscanf (eax, eax, 0x8048638);
  25.         edx = var_4h;
  26.         eax = &var_8h;
  27.         *(eax) += edx;
  28.         if (var_8h == 0xf) {
  29.             printf ("Password OK!\n");
  30.             exit (0);
  31.         }
  32.         eax = &var_ch;
  33.         *(eax)++;
  34.     } while (1);
  35. label_0:
  36.     printf ("Password Incorrect!\n");
  37.     return eax;
  38. }

manually analyze with both the assembly and pseudo code we can simply write down the C-like code to describe this function:

  1. #include <stdint.h>
  2. int32_t check(char *s)
  3. {
  4.     var_ch = 0;
  5.     var_8h = 0;
  6.     for (var_ch = 0; var_ch < strlen(s); ++var_ch)
  7.     {
  8.         var_dh = s[var_ch];
  9.         sscanf(&var_dh, %d, &var_4h);            // read from string[var_ch], store to var_4h
  10.         var_8h += var_4h;
  11.         if(var_8h == 0xf)
  12.             printf("Password OK\n");
  13.     }
  14.     printf("Password Incorrect!\n");
  15.     return 0;
  16. }

In short, it calculates the Digit Sum of a number (add a number digit by digit. for example, 96 => 9 + 6 = 15) :

  1. ./crackme0x04
  2. IOLI Crackme Level 0x04
  3. Password: 12345
  4. Password OK!
  6. ./crackme0x04
  7. IOLI Crackme Level 0x04
  8. Password: 96
  9. Password OK!