进入两个函数，F5，查看c代码

下面就写个反向程序，通过KanXueCTF2019JustForhappy算出key

int __thiscall sub_401890(CWnd *this)
{
struct CString *v1; // ST08_4@1
CWnd *v2; // eax@1
int v3; // eax@1
int result; // eax@2
int v5[26]; // [sp+4Ch] [bp-74h]@7
int i; // [sp+B4h] [bp-Ch]@3
char *Str; // [sp+B8h] [bp-8h]@1
CWnd *v8; // [sp+BCh] [bp-4h]@1

v8 = this;
v1 = (CWnd *)((char *)this + 100);
v2 = CWnd::GetDlgItem(this, 1002);
CWnd::GetWindowTextA(v2, v1);
v3 = sub_401A30((char *)v8 + 100);
Str = CString::GetBuffer((CWnd *)((char *)v8 + 100), v3);
if ( strlen(Str) )
{
 for ( i = 0; Str[i]; ++i )
 {
   if ( Str[i] > 57 || Str[i] < 48 )         // 非数字
   {
     if ( Str[i] > 122 || Str[i] < 97 )      // 非小写字母
     {
       if ( Str[i] > 90 || Str[i] < 65 )     // 非大写字母
         sub_4017B0();
       else
         v5[i] = Str[i] - 29;                // 大写字母减29
     }
     else
     {
       v5[i] = Str[i] - 87;                  // 小写字母减87
     }
   }
   else
   {
     v5[i] = Str[i] - 48;                    // 数字减48
   }
 }
 result = sub_4017F0((int)v5);
}
else
{
 result = CWnd::MessageBoxA(v8, "请输入pass!", 0, 0);
}
return result;
}

int __cdecl sub_4017F0(int a1)
{
int result; // eax@6
char Str1[28]; // [sp+D8h] [bp-24h]@4
int v3; // [sp+F4h] [bp-8h]@1
int v4; // [sp+F8h] [bp-4h]@1

v4 = 0;
v3 = 0;
while ( *(_DWORD *)(a1 + 4 * v4) < 62 && *(_DWORD *)(a1 + 4 * v4) >= 0 )
{
 Str1[v4] = aAbcdefghiabcde[*(_DWORD *)(a1 + 4 * v4)];
 ++v4;
}
Str1[v4] = 0;
if ( !strcmp(Str1, "KanXueCTF2019JustForhappy") )
 result = sub_401770();
else
 result = sub_4017B0();
return result;
}

public static void main(String[] args) {
 String source = "abcdefghiABCDEFGHIJKLMNjklmn0123456789opqrstuvwxyzOPQRSTUVWXYZ";
     String dst = "KanXueCTF2019JustForhappy";

     char[] sA = source.toCharArray();
     char[] dA = dst.toCharArray();

     for (char c : dA) {
         for (int i = 0; i < sA.length; i++) {

             if (c == sA[i]) {
                 if (i + 48 <= 57 && i + 48 >= 48) {
                     System.out.print((char)(i + 48));
                 } else if (i + 87 >= 97 && i + 87 <= 122) {
                     System.out.print((char)(i + 87));
                 } else if (i + 29 >= 65 && i + 29 <= 90) {
                     System.out.print((char)(i + 29));
                 } else {
                     System.out.print("error" + " ");
                 }

             }
         }
     }

 }

1. 先运行程序，随便输入提交，弹出加油提示框
2. 用ida打开exe，查看string，发现KanXueCTF2019JustForhappy、abcdefghiABCDEFGHIJKLMNjklmn0123456789opqrstuvwxyzOPQRSTUVWXYZ等字符串
3. 猜测可能是对比字符串，在function中看到有strcmp，查看graph，发现上面有两个函数调用，分别是sub_401890 -> sub_4017F0 -> strcmp

4. 进入两个函数，F5，查看c代码

   int __thiscall sub_401890(CWnd *this)
   {
   struct CString *v1; // ST08_4@1
   CWnd *v2; // eax@1
   int v3; // eax@1
   int result; // eax@2
   int v5[26]; // [sp+4Ch] [bp-74h]@7
   int i; // [sp+B4h] [bp-Ch]@3
   char *Str; // [sp+B8h] [bp-8h]@1
   CWnd *v8; // [sp+BCh] [bp-4h]@1
   
   v8 = this;
   v1 = (CWnd *)((char *)this + 100);
   v2 = CWnd::GetDlgItem(this, 1002);
   CWnd::GetWindowTextA(v2, v1);
   v3 = sub_401A30((char *)v8 + 100);
   Str = CString::GetBuffer((CWnd *)((char *)v8 + 100), v3);
   if ( strlen(Str) )
   {
    for ( i = 0; Str[i]; ++i )
    {
      if ( Str[i] > 57 || Str[i] < 48 )         // 非数字
      {
        if ( Str[i] > 122 || Str[i] < 97 )      // 非小写字母
        {
          if ( Str[i] > 90 || Str[i] < 65 )     // 非大写字母
            sub_4017B0();
          else
            v5[i] = Str[i] - 29;                // 大写字母减29
        }
        else
        {
          v5[i] = Str[i] - 87;                  // 小写字母减87
        }
      }
      else
      {
        v5[i] = Str[i] - 48;                    // 数字减48
      }
    }
    result = sub_4017F0((int)v5);
   }
   else
   {
    result = CWnd::MessageBoxA(v8, "请输入pass!", 0, 0);
   }
   return result;
   }
   

   int __cdecl sub_4017F0(int a1)
   {
   int result; // eax@6
   char Str1[28]; // [sp+D8h] [bp-24h]@4
   int v3; // [sp+F4h] [bp-8h]@1
   int v4; // [sp+F8h] [bp-4h]@1
   
   v4 = 0;
   v3 = 0;
   while ( *(_DWORD *)(a1 + 4 * v4) < 62 && *(_DWORD *)(a1 + 4 * v4) >= 0 )
   {
    Str1[v4] = aAbcdefghiabcde[*(_DWORD *)(a1 + 4 * v4)];
    ++v4;
   }
   Str1[v4] = 0;
   if ( !strcmp(Str1, "KanXueCTF2019JustForhappy") )
    result = sub_401770();
   else
    result = sub_4017B0();
   return result;
   }
   

5. 分别下断点，通过ida调试，验证猜想，分析之后，流程是先对输入的串进行转换，然后再从abcdefghiABCDEFGHIJKLMNjklmn0123456789opqrstuvwxyzOPQRSTUVWXYZ中按数组取值，最后与KanXueCTF2019JustForhappy比较

6. 下面就写个反向程序，通过KanXueCTF2019JustForhappy算出key

   public static void main(String[] args) {
    String source = "abcdefghiABCDEFGHIJKLMNjklmn0123456789opqrstuvwxyzOPQRSTUVWXYZ";
        String dst = "KanXueCTF2019JustForhappy";
   
        char[] sA = source.toCharArray();
        char[] dA = dst.toCharArray();
   
        for (char c : dA) {
            for (int i = 0; i < sA.length; i++) {
   
                if (c == sA[i]) {
                    if (i + 48 <= 57 && i + 48 >= 48) {
                        System.out.print((char)(i + 48));
                    } else if (i + 87 >= 97 && i + 87 <= 122) {
                        System.out.print((char)(i + 87));
                    } else if (i + 29 >= 65 && i + 29 <= 90) {
                        System.out.print((char)(i + 29));
                    } else {
                        System.out.print("error" + " ");
                    }
   
                }
            }
        }
   
    }
   

7. 最后结果为：j0rXI4bTeustBiIGHeCF70DDM

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