[BUUCTF-Reverse] reverse3

file 命令查看文件基本信息：

使用 IDA32 反编译文件，找到主函数（无壳）：

int __cdecl main_0(int argc, const char **argv, const char **envp)
{
  int string_length; // eax
  const char *encrypted_string; // eax
  size_t v5; // eax
  char v7; // [esp+0h] [ebp-188h]
  char v8; // [esp+0h] [ebp-188h]
  signed int j; // [esp+DCh] [ebp-ACh]
  int i; // [esp+E8h] [ebp-A0h]
  signed int v11; // [esp+E8h] [ebp-A0h]
  char Destination[108]; // [esp+F4h] [ebp-94h] BYREF
  char Str[28]; // [esp+160h] [ebp-28h] BYREF
  char v14[8]; // [esp+17Ch] [ebp-Ch] BYREF

  for ( i = 0; i < 100; ++i )
  {
    if ( (unsigned int)i >= 0x64 )
      j____report_rangecheckfailure();
    Destination[i] = 0;
  }
  sub_41132F("please enter the flag:", v7);
  sub_411375("%20s", (char)Str);
  string_length = j_strlen(Str);
  encrypted_string = (const char *)encrypt_function((int)Str, string_length, (int)v14);
  strncpy(Destination, encrypted_string, 0x28u);
  v11 = j_strlen(Destination);
  for ( j = 0; j < v11; ++j )
    Destination[j] += j;
  v5 = j_strlen(Destination);
  if ( !strncmp(Destination, Str2, v5) )
    sub_41132F("rigth flag!\n", v8);
  else
    sub_41132F("wrong flag!\n", v8);
  return 0;
}

关键点在于对 flag 字符串进行了两次处理：

找到与经过两次处理的 flag 字符串相比较的字符串：

点进 encrypt_function：

void *__cdecl sub_411AB0(char *Str, unsigned int string_length, int *a3)
{
  int v4; // [esp+D4h] [ebp-38h]
  int v5; // [esp+D4h] [ebp-38h]
  int v6; // [esp+D4h] [ebp-38h]
  int v7; // [esp+D4h] [ebp-38h]
  int i; // [esp+E0h] [ebp-2Ch]
  unsigned int v9; // [esp+ECh] [ebp-20h]
  int v10; // [esp+ECh] [ebp-20h]
  int v11; // [esp+ECh] [ebp-20h]
  void *v12; // [esp+F8h] [ebp-14h]
  char *v13; // [esp+104h] [ebp-8h]

  if ( !Str || !string_length )
    return 0;
  v9 = string_length / 3;
  if ( (int)(string_length / 3) % 3 )
    ++v9;
  v10 = 4 * v9;
  *a3 = v10;
  v12 = malloc(v10 + 1);
  if ( !v12 )
    return 0;
  j_memset(v12, 0, v10 + 1);
  v13 = Str;
  v11 = string_length;
  v4 = 0;
  while ( v11 > 0 )
  {
    byte_41A144[2] = 0;
    byte_41A144[1] = 0;
    byte_41A144[0] = 0;
    for ( i = 0; i < 3 && v11 >= 1; ++i )
    {
      byte_41A144[i] = *v13;
      --v11;
      ++v13;
    }
    if ( !i )
      break;
    switch ( i )
    {
      case 1:
        *((_BYTE *)v12 + v4) = aAbcdefghijklmn[(int)(unsigned __int8)byte_41A144[0] >> 2];
        v5 = v4 + 1;
        *((_BYTE *)v12 + v5) = aAbcdefghijklmn[((byte_41A144[1] & 0xF0) >> 4) | (16 * (byte_41A144[0] & 3))];
        *((_BYTE *)v12 + ++v5) = aAbcdefghijklmn[64];
        *((_BYTE *)v12 + ++v5) = aAbcdefghijklmn[64];
        v4 = v5 + 1;
        break;
      case 2:
        *((_BYTE *)v12 + v4) = aAbcdefghijklmn[(int)(unsigned __int8)byte_41A144[0] >> 2];
        v6 = v4 + 1;
        *((_BYTE *)v12 + v6) = aAbcdefghijklmn[((byte_41A144[1] & 0xF0) >> 4) | (16 * (byte_41A144[0] & 3))];
        *((_BYTE *)v12 + ++v6) = aAbcdefghijklmn[((byte_41A144[2] & 0xC0) >> 6) | (4 * (byte_41A144[1] & 0xF))];
        *((_BYTE *)v12 + ++v6) = aAbcdefghijklmn[64];
        v4 = v6 + 1;
        break;
      case 3:
        *((_BYTE *)v12 + v4) = aAbcdefghijklmn[(int)(unsigned __int8)byte_41A144[0] >> 2];
        v7 = v4 + 1;
        *((_BYTE *)v12 + v7) = aAbcdefghijklmn[((byte_41A144[1] & 0xF0) >> 4) | (16 * (byte_41A144[0] & 3))];
        *((_BYTE *)v12 + ++v7) = aAbcdefghijklmn[((byte_41A144[2] & 0xC0) >> 6) | (4 * (byte_41A144[1] & 0xF))];
        *((_BYTE *)v12 + ++v7) = aAbcdefghijklmn[byte_41A144[2] & 0x3F];
        v4 = v7 + 1;
        break;
    }
  }
  *((_BYTE *)v12 + v4) = 0;
  return v12;
}

判断是 base64 加密的过程；编写代码如下获得 flag：

import base64

s = "e3nifIH9b_C@n@dH"
flag = ""

for i in range(len(s)):
    flag += chr(ord(s[i]) - i)
print(base64.b64decode(flag))