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[原创]KCTF2023 第五题 争分夺秒
2023-9-12 14:39 7623

[原创]KCTF2023 第五题 争分夺秒

奔跑的阿狸 活跃值
1
2023-9-12 14:39
7623
  1. 程序多解吧？
    两个0x4a数组，len位，程序里是双字节，实际录入4字节
    input base64_decode后的数据结构如下：
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2a 62 53 31 //4字节，第四层的M
4a 00       //arr1长度
37 38             //？？？
arr1            //第七层，大数arr1
f550c4d11be88545a17edf49a600d30212a17760f814f054f252f5fac9e3c915b1195d9f2d52f3bbd2cb5690982c85dfbca0c4102132cff25e4740f3c9c71174802c59a7faf98daebc52
9d a4 72 43 //4字节，第四层的N
4a 00       //arr2长度
33 34         //？？？
arr2            //第七层，大数arr2
54449bc8e9713a6f2de69acceadb2bb55f550294d7f4d887d561c858c12d74aa218e45766d0799391a8c617f5e3fb00fd4995e9e5077100721858261a223fb4773736ce6027275e7a9ec
0B 73 5F 7A //crc32  7A5F730B

  1. 程序中充满着大量恶心的垃圾代码，直接挑选参数的传递函数去分析

  2. 先看弹窗逻辑

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int __usercall sub_4EB910@<eax>(int ebx0@<ebx>, int edi0@<edi>, int a3@<esi>, int a1, int a2)
{
  if ( a1 == 2 )
  {
    lstrlenA(*(LPCSTR *)(a2 + 4));
    if ( sub_47B430(ebx0, edi0, a3, *(_DWORD *)(a2 + 4)) == 1 )
      MessageBoxA(0, "OK!", 0, 0);
    else
      MessageBoxA(0, "Error!", 0, 0);
  }
  return 0;
}
  1. 进入下一层，发现有5层校验
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signed int __usercall sub_47B430@<eax>(int ebx0@<ebx>, int a2@<edi>, int a3@<esi>, int a1)
{
  if ( sub_40A580(ebx0, a2, a3, a1, (char **)&input, &input_len) )  √   //3339 解base64，刚解密出来是正常的
  {
    if ( sub_455F80((int)input, input_len) )    ("4234567890", 10)  Crc32("前len-4位")==后4位  √
    {
      if ( sub_458D90(ebx0, a2, input, input_len - 4, (int)&v2357, (int)&v2356) )   //字符串中有2个长度标记位，校验通过，√
      {
        if ( sub_474170(*v2357, *v2356) )   //对2个dword进行除法校验，满足的情况总共有2*4=8种
        {
          v1378 = v2357 + 8;                                                            //5153  .text:004870AC 这里就要篡改一次数据
          for ( i169 = 0; i169 < v1379; ++i169 )
          {
            v5 = 16;
            *(_BYTE *)(i169 + v1378) ^= *(&v1593 + i169 % 0x10u);
          }
          sub_4E1620(*(_DWORD *)v2357, v2357 + 8, *(unsigned __int16 *)(v2357 + 4));    //5298  将+8数组进行xor操作
          sub_4E1620(*(_DWORD *)v2356, v2356 + 8, *(unsigned __int16 *)(v2356 + 4));    //5453
          v1360 = v2357 + 8;                                                            //5602
          for ( i214 = 0; i214 < v1361; ++i214 )
          {
            v5 = 16;
            *(_BYTE *)(i214 + v1360) ^= *(&v1577 + i214 % 0x10u);       //arr1比arr2多2次xor
          }
          if ( sub_45F640(
                 (int)(v2357 + 2),
                 *((unsigned __int16 *)v2357 + 2),
                 (int)(v2356 + 2),
                 *((unsigned __int16 *)v2356 + 2)) )
          {
            sub_4E81E0(ebx0, a2, v5, v2359);
            result = 1;
          }
          else
            result = 0;
        }
        else
          result = 0;
      }
      else
        result = 0;
    }
    else
      result = 0;
  }
  else
    result = 0;
}
  1. 第一层，sub_40A580，base64解密
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if ( BASE64_table_4FBD18[kk] == a1 )
  break;
v347 = 119;
v232 = -1;
v274 = 104;
  1. 第二层，sub_455F80，crc校验，结果存放在最后4字节
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v207 = sub_453B90(v2, v3, v4, (char *)input, input_len - 4);
if ( v207 == *(_DWORD *)(input_len + input - 4) )
      result = 1;
  1. 第三层，sub_458D90，对字符串长度校验，并取出2个魔法数
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if ( (unsigned int)input_len_jian4 >= 0x10 ) //len-4>=16
  {
    *(_DWORD *)a5 = input;  //1406
    if ( (signed int)*(unsigned __int16 *)(*(_DWORD *)a5 + 4) > 0                                //1547  45B83C
      && *(unsigned __int16 *)(*(_DWORD *)a5 + 4) <= (unsigned int)(input_len_jian4 - 16) )
    {
      *(_DWORD *)a6 = input + *(unsigned __int16 *)(*(_DWORD *)a5 + 4) + 8;     //1655
      if ( (signed int)*(unsigned __int16 *)(*(_DWORD *)a6 + 4) > 0              //1831          45d381
        && *(unsigned __int16 *)(*(_DWORD *)a6 + 4) <= input_len_jian4
                                                     - 16
                                                     - (unsigned int)*(unsigned __int16 *)(*(_DWORD *)a5 + 4) )
      {
        result = 1;
      }
    }
  }
  1. 第四层，sub_474170，对2个魔法书校验
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M = i*0x7D45;
M % 0x346F8717 == 1
M = 3153622A 65C2E941 65C2E941 CEA1F76F
N = j*0xD711;
N % 0x346F8717 == 1
N = 4372A49D B60C0E9C
  1. 第五层，对数组4次xor
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xor(arr1);
          sub_4E1620(*(_DWORD *)v2357, v2357 + 8, *(unsigned __int16 *)(v2357 + 4));    //arr1
          sub_4E1620(*(_DWORD *)v2356, v2356 + 8, *(unsigned __int16 *)(v2356 + 4));    //arr2
xor(arr1);
  1. 第六层，sub_45F640，大数校验
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能跑出数据结果
n = 0x8395475F
m = 0xb5b441af57592c12ec5c722d5b377bfc3d5fecb8ba3daf5b7812ef8fd1255e50913d2e7a6cb48837833074e78dcfec0689d4219c8b98a1c617596a6dfba28c973bebf711efb09844b305
i = 0
while i < 0x99999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999:
    if((m * i + 1) % n == 0):
        print(hex(i))
    i += 1
 
n = 0x8395475F
m = 0xb5b441af57592c12ec5c722d5b377bfc3d5fecb8ba3daf5b7812ef8fd1255e50913d2e7a6cb48837833074e78dcfec0689d4219c8b98a1c617596a6dfba28c973bebf711efb09844b305
i = 0
while i < 0x99999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999:
    if((m * i + 1) % n == 0):
        print(hex(i))
    i += 1
  1. 然后再根据结果反推重新计算crc32校验，验证失败！
    OD运行是success，命令行运行是失败，说明有反调试。
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if ( sb001() == 1 )
{
  if ( *(_BYTE *)input & 1 )
  {
    a3 = 0;
    *(_BYTE *)input += (signed int)sb002() % 3 + 1;
  }
  else
  {
    a3 = 0;
    *(_BYTE *)input += (signed int)sb002() % 4 + 4;
  }
}
  1. 然后又重新调整数值，得到flag
    KmJTMUoAAAD1UMTRG+iFRaF+30mmANMCEqF3YPgU8FTyUvX6yePJFbEZXZ8tUvO70stWkJgshd+8oMQQITLP8l5HQPPJxxF0gCxZp/r5ja68Up2kckNKAAAAVESbyOlxOm8t5prM6tsrtV9VApTX9NiH1WHIWMEtdKohjkV2bQeZORqMYX9eP7AP1JlenlB3EAchhYJhoiP7R3NzbOYCcnXnqewe7SYM

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