Encrypt large string in Delphi

Question

i'm using this method to encrypt small string's for a long time and work just fine.. when i try to pass a large string (lenght > 500) it dont work someone can tell me why?

`function Encrypt(const Text: string): string;
var
  i: Byte;
  Key: Word;
  strTemp: string;
const
  KEY  = 7519;
  KEY1 = 03001;
  KEY2 = 002279;
begin
  Key := KEY;
  SetLength(strTemp, Length(Text));
  Result := '';
  for i := 1 to Length(Text) do
  begin
    strTemp[i] := Char(byte(Text[I]) xor (Key shr 8));
    Result := Result + IntToHex(Byte(strTemp[i]), 2);
    Key := (Byte(strTemp[i]) + KEY) * KEY1 + KEY2 ;
  end;
end;`

i tried to pass a large value in base64 with approximately 5000 characters, the result after encryption is approximately 250 characters..

i'm trying to encrypt a base64 text.. base64 extracted from a .jpeg

example:

    var vText: AnsiString;
    vText := 'iVBORw0KGgoAAAANSUhEUgAAB4AAAAQ4CAYAAADo08FDAAAgAElEQVR4XuydUbbjOJIss5bR+19oz' + '1Fla1KpS4kgCcLdI+z9XpBhbgGRfIisnn/+85///PcX/w8DGMAABsoa+Oeff8pmew1WMWelTJWyPPZdt' +'TzP31LVXO8PwS45jz788XLUmMd6+ubRh8oU//0vRwZp/aVnnzvWxU3FnNUyVcpTKcvz6VExzFgtU6U8l' +'bJUHGp/e09U693eO5G/YwADGOhggAFwhy6TEQMYaGegy0FyxZyVMpHF99FTqTcMfsf2';

Result = 6EA0B289D3DB602BC0EFCC2F2B38A54FF5916CE39FF43E4F7CACFFF7BB372D2E5485038DE606514C0BC943D55B9246C8

Answer 1

1. Your program cannot compile as you show it in this example, because of

   var
     Key: Word;
   const
     KEY  = 7519;
   

   You tagged this question Delphi, and by that identifiers are case insensitive: KEY cannot be defined since Key already exists. This might be possible in Pascal alone, tho. So please check the example you gave us against your actual code.
2. Your loop variable is not big enough:

   var
     i: Byte;
   begin
     for i := 1 to Length(Text) do
   

   Should Length(Text) be 304, then the loop will run 48 iterations only, because Byte can at max hold 256 values. Should a loop end condition have a higher value than the variable is able to ever hold, the maximum amount of the variable is subtracted from that end condition. Which is 304 minus 256 equalling 48.
3. Why do you need a temporary String when you only need one single character? And even then you only treat is as Byte anyway, so why not using that?

   var
     strTemp: string;
   begin
     SetLength(strTemp, Length(Text));
     for i := 1 to Length(Text) do
     begin
       strTemp[i] := Char(...);
       Result := Result + IntToHex(Byte(strTemp[i]), 2);
       Key := (Byte(strTemp[i]) ...)...;
   

   This can be written a lot cleaner as:

   var
     byteTemp: Byte;
   begin
     for i := 1 to Length(Text) do
     begin
       byteTemp := Byte(Text[I]) xor (Key shr 8);
       Result := Result + IntToHex(byteTemp, 2);
       Key := (byteTemp + KEY0) * KEY1 + KEY2;
   

4. Never encrypt text - encrypt bytes instead. The declaration

   function Encrypt(const Text: string): string;
   

   will differ as per Delphi version. Why not making it at least AnsiString? Or even better: TBytes or Array of Byte? When dealing with any kind of String you can never be certain how many bytes one character needs. Using String in Delphi 2009 and above uses more than 1 byte per character, so with Byte(String[i]) you're walking on thin ice.