Переполнение стека

procedure TForm1.Button3Click(Sender: TObject);
var
  buf: string;
  Enc,Dec : TFileStream;
  FileName, buff : String;
  i,n,d,j: LongInt;
begin
  if OpenDialog1.InitialDir = '' then
    OpenDialog1.InitialDir := ExtractFilePath( Application.ExeName )
  ;
  Filename := OpenDialog1.InitialDir;
  if OpenDialog1.Execute then Filename := OpenDialog1.FileName;
 
  try
    Dec:=TFileStream.Create(Filename, fmOpenRead);
    Dec.Seek(0,soFromBeginning);
    i := Dec.Size;
    SetLength(FMes, i div SizeOf(Integer));
    Dec.Read(Pointer(FMes)^,i);
  finally
    Dec.Free;
  end;
    d := StrToIntDef(Edit2.Text,0);
  buf := Decrypt(FMes, d);
 
 
  if SaveDialog1.InitialDir = '' then
    SaveDialog1.InitialDir := ExtractFilePath( Application.ExeName )
  ;
  Filename := SaveDialog1.InitialDir;
  if SaveDialog1.Execute then Filename:= SaveDialog1.FileName;
 
  try
    Enc := TFileStream.Create(Filename, fmCreate);
    Enc.Seek(0,soFromBeginning);
    i := Length(buf) * SizeOf(Char);
    Enc.Write(Pointer(buf)^,i);
    Enc.Size := i;
  finally
    Enc.Free;
  end;
end;

function Decrypt(Arr: array of Integer; d:LongInt): string;
var
  res,j:LongInt;
begin
  SetLength(Result, Length(Arr));
  for i := 0 to Length(Arr) - 1 do
  begin
    res:=1;
    for j:=1 to d do
      res:=(res*arr[i]) mod n;
    result[i + 1] := chr(res);
  end;
end;