C help needed

I've used hex rays to decompile a quiz application to C, it loads an external file with the questions and then decrypts it, just trying to figure out the algo to decrypt the file?

signed int __cdecl sub_4033A0()
{
  FILE *v1; // eax@1
  char *v2; // ecx@15
  char *v3; // ecx@17
  FILE *v4; // eax@25
  FILE *v5; // eax@26
  signed int v6; // [sp+24h] [bp-Ch]@1
  FILE *File; // [sp+20h] [bp-10h]@1
  char DstBuf; // [sp+10h] [bp-20h]@3
  char v9; // [sp+4h] [bp-2Ch]@3
  char v10; // [sp+1Ch] [bp-14h]@3
  char v11; // [sp+18h] [bp-18h]@3
  int v12; // [sp+Ch] [bp-24h]@9
  __int16 v13; // [sp+8h] [bp-28h]@11
  int v14; // [sp+2Ch] [bp-4h]@18
  signed int v15; // [sp+14h] [bp-1Ch]@25
  size_t Count; // [sp+28h] [bp-8h]@30

  v6 = 1;
  memset(Str1, 0, 0x2E4u);
  v1 = fopen("Question\\Question.dat", "rb");
  File = v1;
  if ( !v1 )
  {
    nullsub_2();
    return 0;
  }
  fread(Str1, 1u, 6u, File);
  fread(byte_451E06, 1u, 2u, File);
  fread(byte_451E08, 1u, 0x200u, File);
  fread(&dword_452250, 1u, 4u, File);
  fread(&dword_4520E4, 1u, 4u, File);
  fread(&DstBuf, 1u, 4u, File);
  fread(&v9, 1u, 4u, File);
  fread(&v10, 1u, 4u, File);
  fread(&v11, 1u, 4u, File);
  if ( strncmp(Str1, "JPMBIN", 6u) )
    return 0;
  if ( strncmp(byte_451E06, "12", 2u) )
    return 0;
  if ( dword_452250 > 10 )
    dword_452250 = 10;
  dword_43C91C = dword_452250;
  v12 = 0;
  while ( v12 < dword_452250 )
  {
    fread(&stru_43C6CC[36 * v12], 1u, 0x24u, File);
    dword_452008[5 * v12] = (int)&stru_43C6CC[36 * v12];
    v13 = 0;
    while ( v13 < 36 )
    {
      *(&stru_43C6CC[36 * v12] + v13) = toupper(*(&stru_43C6CC[36 * v12] + v13));
      ++v13;
    }
    if ( strncmp(&stru_43C6CC[36 * v12], "SCIENCE", 7u) )
    {
      if ( !strncmp(&stru_43C6CC[36 * v12], "PEOPLE", 6u) )
      {
        v3 = &stru_43C6CC[36 * v12];
        *(_DWORD *)v3 = dword_43A320;
        *((_DWORD *)v3 + 1) = dword_43A324;
        *((_DWORD *)v3 + 2) = dword_43A328;
        *((_WORD *)v3 + 6) = word_43A32C;
      }
    }
    else
    {
      v2 = &stru_43C6CC[36 * v12];
      *(_DWORD *)v2 = dword_43A338;
      *((_DWORD *)v2 + 1) = dword_43A33C;
      *((_DWORD *)v2 + 2) = dword_43A340;
      *((_WORD *)v2 + 6) = word_43A344;
      *(v2 + 14) = byte_43A346;
    }
    fread(&dword_45200C[5 * v12], 1u, 4u, File);
    v14 = 0;
    while ( v14 < dword_4520E4 && v14 < 6 )
      fread((char *)&stru_452010[5 * v12] + 2 * v14++, 1u, 4u, File);
    ++v12;
  }
  if ( fclose(File) )
    nullsub_2();
  v15 = 1;
  v4 = fopen("Question\\Question.enc", "rb");
  File = v4;
  if ( v4 )
  {
    nullsub_2();
  }
  else
  {
    nullsub_2();
    v15 = 0;
    v5 = fopen("Question\\Question.bin", "rb");
    File = v5;
    if ( !v5 )
    {
      nullsub_2();
      return 0;
    }
    nullsub_2();
  }
  fseek(File, 0, 2);
  Count = ftell(File);
  fseek(File, 0, 0);
  DstBuf = malloc(Count);
  if ( DstBuf )
  {
    if ( v15 )
    {
      fclose(File);
      if ( !sub_42B590((int)DstBuf, "Question\\Question.enc", (int)&unk_42E558) )
      {
        nullsub_2();
        v6 = 0;
      }
    }
    else
    {
      fread(DstBuf, 1u, Count, File);
      fclose(File);
    }
  }
  else
  {
    fclose(File);
    nullsub_2();
    v6 = 0;
  }
  return v6;
}

//----- (00403850) --------------------------------------------------------
signed int __cdecl sub_403850()
{
  signed int v1; // [sp+0h] [bp-4h]@1

  v1 = 0;
  if ( DstBuf )
  {
    free(DstBuf);
    v1 = 1;
  }
  return v1;
}

//----- (00403890) --------------------------------------------------------
__int16 __cdecl sub_403890(int a1, __int16 a2, __int16 a3, __int16 a4)
{
  __int16 result; // ax@7
  int v5; // edx@11
  int v6; // eax@12
  int v7; // ecx@13
  int v8; // edx@15
  int v9; // ecx@16
  void *v10; // [sp+4h] [bp-Ch]@8
  __int16 v11; // [sp+8h] [bp-8h]@8
  int v12; // [sp+Ch] [bp-4h]@12
  signed int v13; // [sp+0h] [bp-10h]@16

  if ( a2 >= 0
    && a2 < dword_43C91C
    && a3 >= 0
    && a3 <= 6
    && a4 < (signed int)*((_WORD *)&stru_452010[5 * a2] + a3)
    && a4 >= 0 )
  {
    v10 = (char *)DstBuf + dword_45200C[5 * a2];
    v11 = 0;
    while ( v11 < a3 )
      a4 += *((_WORD *)&stru_452010[5 * a2] + v11++);
    do
    {
      v11 = *(_BYTE *)v10;
      v5 = v11;
      v10 = (char *)v10 + 1;
      if ( !v11 )
      {
        v10 = (char *)v10 - 1;
        v12 = ((signed int)((v5 & 0x1FFFF) + v10 - DstBuf) >> 17) + 1;
        v6 = (((signed int)((v5 & 0x1FFFF) + v10 - DstBuf) >> 17) + 1) << 17;
        v10 = (char *)DstBuf + v6;
        v11 = *((_BYTE *)DstBuf + v6);
        v10 = DstBuf + v6 + 1;
      }
      v10 = (char *)v10 + v11;
      v7 = a4--;
    }
    while ( v7 );
    v10 = (char *)v10 - v11;
    Str = (char *)&unk_452100;
    while ( 1 )
    {
      v8 = v11--;
      if ( !v8 )
        break;
      v13 = *(_BYTE *)v10;
      v9 = (int)((char *)v10 + 1);
      v10 = (char *)v10 + 1;
      sub_403AA0(v9, v13);
    }
    *Str = 13;
    Str = (char *)&unk_452100;
    v11 = 0;
    while ( v11 < 5 )
    {
      *(_DWORD *)(a1 + 4 * v11) = Str;
      Str = strchr(Str, 13);
      if ( !Str )
        return 0;
      *Str++ = 0;
      ++v11;
    }
    result = 1;
  }
  else
  {
    result = 0;
  }
  return result;
}

//----- (00403AA0) --------------------------------------------------------
char *__thiscall sub_403AA0(int this, signed int a2)
{
  char *result; // eax@1
  int v3; // [sp+0h] [bp-4h]@1

  v3 = this;
  result = (char *)(Str - (char *)&unk_452100);
  if ( (signed int)(Str - (char *)&unk_452100) <= 335 )
  {
    if ( a2 >= 39 )
    {
      sub_403AA0(byte_451E08[2 * a2]);
      result = (char *)sub_403AA0(byte_451E09[2 * a2]);
    }
    else
    {
      v3 = a2 - 1;
      if ( (unsigned int)(a2 - 1) > 5 )
      {
        result = (char *)(dword_43A1FC + a2);
        *Str++ = (_BYTE)dword_43A1FC + (_BYTE)a2;
        dword_43A1FC = 89;
      }
      else
      {
        result = (char *)v3;
        switch ( v3 )
        {
          case 0:
            dword_43A1FC -= 32;
            break;
          case 1:
            result = (char *)sub_42B2A0(Str, (int)off_43A1F4);
            Str += 8;
            break;
          case 2:
            dword_43A1FC -= 64;
            break;
          case 3:
            result = Str;
            *Str++ = 32;
            break;
          case 4:
            *Str = 13;
            result = Str++ + 1;
            break;
          case 5:
            *Str++ = 63;
            break;
        }
      }
    }
  }
  else
  {
    dword_43C918 = 1;
  }
  return result;
}

Code (markup):

Any help at all much appreciated, I can upload the entire C file if something's missing