Help understand decompiled IDA code

Question

I got stuck at yellow lines, dont understand what it's gonna do. Need advice or help, thanks!

File : https://www.sendspace.com/file/3yowjg

Password : 1

void __fastcall sub_A513C0(char *Src, HWND hWnd)
{
  int input_flag; // esi
  HWND v3; // edi
  unsigned int pos; // ebx
  int num_char; // edx
  char *tmp_input_flag; // edi
  char chr; // cl
  int v8; // ecx
  unsigned int v9; // esi
  int encoded_flag; // eax
  HWND v11; // [esp+Ch] [ebp-1D8h]
  unsigned int v12; // [esp+10h] [ebp-1D4h]
  unsigned int tmp_pos; // [esp+14h] [ebp-1D0h]
  char v14[252]; // [esp+1Ch] [ebp-1C8h]
  char Dst; // [esp+118h] [ebp-CCh]
  char v16; // [esp+12Dh] [ebp-B7h]
  CHAR Text[16]; // [esp+17Ch] [ebp-68h]
  __int64 v18; // [esp+18Ch] [ebp-58h]
  int v19; // [esp+194h] [ebp-50h]
  char v20; // [esp+198h] [ebp-4Ch]

  input_flag = Src;
  v3 = hWnd;
  v11 = hWnd;
  strcpy_s(&Dst, 100u, Src);
  pos = 0;
  v12 = 0;
  if ( strlen(input_flag) )
  {
    do
    {
      if ( pos )
        num_char = *(input_flag + pos) - *(input_flag + pos - 1);
      else
        num_char = *input_flag - 65;            // convert to alphabet 0->26
      if ( num_char < 0 )
        num_char = -num_char;
      tmp_pos = 0;
      if ( strlen(input_flag) )                 // lower to upper
                                                // 
      {
        tmp_input_flag = input_flag;
        do
        {
          chr = *tmp_input_flag;
          if ( num_char + *tmp_input_flag > 90 )
            chr -= 26;
          ++tmp_pos;
          v14[tmp_input_flag++ - input_flag] = num_char + chr;
        }
        while ( tmp_pos < strlen(input_flag) );
        pos = v12;
      }
      v8 = input_flag + 1;
      v9 = strlen(input_flag);
      if ( v9 >= 0xFA )
      {
        __report_rangecheckfailure(v8, num_char);
        __debugbreak();
        JUMPOUT(__security_check_cookie);
      }
      v14[v9] = 0;
      ++pos;
      input_flag = v14;
      v12 = pos;
    }
    while ( pos < strlen(v14) );
    v3 = v11;
  }
  encoded_flag = strcmp(input_flag, "NMOVUMUUHUGCMOTOGCNUEY");
  if ( encoded_flag )
    encoded_flag = -(encoded_flag < 0) | 1;
  if ( !encoded_flag && v16 == 69 )
  {
    v19 = 2112115;
    _mm_storeu_si128(Text, _mm_loadu_si128(&xmmword_A521C0));
    v18 = qword_A521D0;
    memset(&v20, 0, 0x48u);
    strcat_s(Text, 0x64u, &Dst);
    if ( MessageBoxA(v3, Text, "Good work!", 0) == 1 )
      PostQuitMessage(0);
  }
}

I believe this code is just getting input_flag and using a Caesar cipher with the first letter of input_flag.

Answer 1

tmp_input_flag is just a pointer to the current char of the whole string which starts with input_flag.

   tmp_input_flag = input_flag;  // set to first chars location
   do
   {
      chr = *tmp_input_flag;     // hold the actual value at pointers location, e.g. 'A'
      if ( num_char + *tmp_input_flag > 90 )  //check to see if outside Upper-Case letters and remedy
         chr -= 26;
      ++tmp_pos;
      v14[tmp_input_flag++ - input_flag] = num_char + chr;  //in first round determine shift modifier to use in rest, except when < 65
   }

Some example cases:

a -> A (32)
l -> L (32)
o -> O (32)
alo->ALO

A -> O (14)
L -> Z (14)
O -> C (12)
ALO->OZC

1 -> D (19)
2 -> E (19)
3 -> F (19)
4 -> G (19)
1234->DEFG

{ -> R (41)
} -> T (41)
| -> S (41)
~ -> U (41)
{}|~->RTSU

Note that if ASCII values less than 65/'A'(line 38) are used the results are not influenced by the same shift value.

_ -> X (7)
= -> P (19)
! -> N (45)
$ -> Q (45)
% -> R (45)
& -> S (45)
_=!$%&->XPNQRS