<- REVERSING

Shattered Tablet

4 minutos de lectura

Se nos proporciona un binario llamado tablet:

$ file tablet
tablet: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, BuildID[sha1]=71ad3ff9f7e5fbf0edc75446337a0a68deb7ecd6, for GNU/Linux 3.2.0, not stripped

Descompilación

Si lo abrimos en Ghidra, veremos esta función main:

/* WARNING: Could not reconcile some variable overlaps */

undefined8 main(void)

{
  undefined8 local_48;
  undefined8 local_40;
  undefined8 local_38;
  undefined8 local_30;
  undefined8 local_28;
  undefined8 local_20;
  undefined8 local_18;
  undefined8 local_10;
  
  local_48 = 0;
  local_40 = 0;
  local_38 = 0;
  local_30 = 0;
  local_28 = 0;
  local_20 = 0;
  local_18 = 0;
  local_10 = 0;
  printf("Hmmmm... I think the tablet says: ");
  fgets((char *)&local_48,0x40,stdin);
  if (((((local_28._2_1_ == '4') && (local_38._4_1_ == '3')) && (local_28._4_1_ == 'r')) &&
      ((((local_48._1_1_ == 'T' && (local_38._5_1_ == 'v')) &&
        ((local_48._6_1_ == '0' && ((local_28._7_1_ == '}' && (local_28._6_1_ == 'd')))))) &&
       (local_30._7_1_ == 'r')))) &&
     ((((((local_30._5_1_ == '3' && ((char)local_40 == '3')) && (local_38._6_1_ == 'e')) &&
        ((local_28._3_1_ == '1' && (local_48._5_1_ == 'r')))) &&
       (((char)local_48 == 'H' && (((char)local_28 == '3' && (local_38._2_1_ == '.')))))) &&
      (((((local_40._5_1_ == '4' &&
          (((((local_48._3_1_ == '{' && (local_40._2_1_ == '_')) && ((char)local_38 == '.')) &&
            ((local_48._4_1_ == 'b' && (local_48._7_1_ == 'k')))) && (local_40._7_1_ == 't')))) &&
         (((local_40._6_1_ == 'r' && (local_38._3_1_ == 'n')) &&
          ((local_30._1_1_ == 't' &&
           (((local_38._1_1_ == '.' && (local_40._1_1_ == 'n')) && (local_30._6_1_ == '_')))))))) &&  
        (((local_30._2_1_ == '0' && ((char)local_30 == '_')) && (local_40._4_1_ == 'p')))) &&
       ((((local_38._7_1_ == 'r' && (local_30._4_1_ == 'b')) &&
         ((local_28._1_1_ == 'p' &&
          (((local_48._2_1_ == 'B' && (local_30._3_1_ == '_')) && (local_40._3_1_ == '4')))))) &&
        (local_28._5_1_ == '3')))))))) {
    puts("Yes! That\'s right!");
  }
  else {
    puts("No... not that");
  }
  return 0;
}

Parece un poco extraño, pero podemos decirle a Ghidra que local_48 es en realidad de tipo char[64] y cambiar el nombre de la variable a data (además, formatear un poco el código):

int main() {
  char data[64];

  data._0_8_ = 0;
  data._8_8_ = 0;
  data._16_8_ = 0;
  data._24_8_ = 0;
  data._32_8_ = 0;
  data._40_8_ = 0;
  data._48_8_ = 0;
  data._56_8_ = 0;

  printf("Hmmmm... I think the tablet says: ");  
  fgets(data, 0x40, stdin);

  if (
      (data[34] == '4') &&
      (data[20] == '3') &&
      (data[36] == 'r') &&
      (data[1]  == 'T') &&
      (data[21] == 'v') &&
      (data[6]  == '0') &&
      (data[39] == '}') &&
      (data[38] == 'd') &&
      (data[31] == 'r') &&
      (data[29] == '3') &&
      (data[8]  == '3') &&
      (data[22] == 'e') &&
      (data[35] == '1') &&
      (data[5]  == 'r') &&
      (data[0]  == 'H') &&
      (data[32] == '3') &&
      (data[18] == '.') &&
      (data[13] == '4') &&
      (data[3]  == '{') &&
      (data[10] == '_') &&
      (data[16] == '.') &&
      (data[4]  == 'b') &&
      (data[7]  == 'k') &&
      (data[15] == 't') &&
      (data[14] == 'r') &&
      (data[19] == 'n') &&
      (data[25] == 't') &&
      (data[17] == '.') &&
      (data[9]  == 'n') &&
      (data[30] == '_') &&
      (data[26] == '0') &&
      (data[24] == '_') &&
      (data[12] == 'p') &&
      (data[23] == 'r') &&
      (data[28] == 'b') &&
      (data[33] == 'p') &&
      (data[2]  == 'B') &&
      (data[27] == '_') &&
      (data[11] == '4') &&
      (data[37] == '3')
    ) {
    puts("Yes! That\'s right!");
  } else {
    puts("No... not that");
  }

  return 0;
}

Básicamente, el programa verifica cada carácter de la lista, así que podemos ordenar los caracteres y descubrir la string esperada:

      (data[0]  == 'H') &&  
      (data[1]  == 'T') &&
      (data[2]  == 'B') &&
      (data[3]  == '{') &&
      (data[4]  == 'b') &&
      (data[5]  == 'r') &&
      (data[6]  == '0') &&
      (data[7]  == 'k') &&
      (data[8]  == '3') &&
      (data[9]  == 'n') &&
      (data[10] == '_') &&
      (data[11] == '4') &&
      (data[12] == 'p') &&
      (data[13] == '4') &&
      (data[14] == 'r') &&
      (data[15] == 't') &&
      (data[16] == '.') &&
      (data[17] == '.') &&
      (data[18] == '.') &&
      (data[19] == 'n') &&
      (data[20] == '3') &&
      (data[21] == 'v') &&
      (data[22] == 'e') &&
      (data[23] == 'r') &&
      (data[24] == '_') &&
      (data[25] == 't') &&
      (data[26] == '0') &&
      (data[27] == '_') &&
      (data[28] == 'b') &&
      (data[29] == '3') &&
      (data[30] == '_') &&
      (data[31] == 'r') &&
      (data[32] == '3') &&
      (data[33] == 'p') &&
      (data[34] == '4') &&
      (data[35] == '1') &&
      (data[36] == 'r') &&
      (data[37] == '3') &&
      (data[38] == 'd') &&
      (data[39] == '}')

Flag

Y aquí tenemos la flag:

HTB{br0k3n_4p4rt...n3ver_t0_b3_r3p41r3d}