Encrypt and decrypt a Unicode string

Question

Can someone give me the code to encrypt and decrypt a Unicode strings in delphi firemonkey Mobile? I've tried everything with xor with other libraries , and nothing. There are always characters that are not recognized as the euro symbol € . If someone could help me , would be appreciated.

Edit: Thank you Hans, but always I have the same problem with stringstream . This code works perfectly in windows , but ios gives me this error : "No mapping for the Unicode character exists in the target multibyte code page"

unit UMain;

interface

uses
  System.SysUtils, System.Types, System.UITypes, System.Classes, System.Variants,
  FMX.Types, FMX.Controls, FMX.Forms, FMX.Graphics, FMX.Dialogs, ElAES,
  FMX.StdCtrls, FMX.Layouts, FMX.Memo, Math;

type
  TForm2 = class(TForm)
    ToolBar1: TToolBar;
    Label1: TLabel;
    Label2: TLabel;
    Memo1: TMemo;
    Layout1: TLayout;
    Button1: TButton;
    Button3: TButton;
    procedure Button1Click(Sender: TObject);
    procedure Button3Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;
const
  PASSWORD = '1234';
var
  Form2: TForm2;

implementation

{$R *.fmx}
{$R *.iPhone.fmx IOS}
function StringToHex(S: string): string;
var
    i: integer;

begin
  Result := '';

  // Go throught every single characters, and convert them
  // to hexadecimal...
    for i := 1 to Length( S ) do
    Result := Result + IntToHex( Ord( S[i] ), 2 );
end;

function HexToString(S: string): string;
var
    i: integer;

begin
  Result := '';

  // Go throught every single hexadecimal characters, and convert
  // them to ASCII characters...
  for i := 1 to Length( S ) do
  begin
    // Only process chunk of 2 digit Hexadecimal...
    if ((i mod 2) = 1) then
        Result := Result + Chr( StrToInt( '0x' + Copy( S, i, 2 )));
  end;
end;


procedure TForm2.Button1Click(Sender: TObject);
var
  Source: TStringStream;
  Dest: TStringStream;
  Key: TAESKey128;
begin
try

  Source := TStringStream.Create( Memo1.Text );
  Dest   := TStringStream.Create('');
  FillChar( Key, SizeOf(Key), 0 );
  Move( PChar(PASSWORD)^, Key, Min( SizeOf( Key ), Length( PASSWORD )));
  EncryptAESStreamECB( Source, 0, Key, Dest );
  //Memo1.Lines.BeginUpdate;
  Memo1.Text := Dest.DataString;
  //Memo1.Lines.EndUpdate;
  Label2.Text := 'Texto Encriptado';
  Source.Free;
  Dest.Free;
except on E: Exception do
  begin
    ShowMessage(e.ToString);
    Source.Free;
    Dest.Free;
    Memo1.Text :='';
  end;
end;

end;


procedure TForm2.Button3Click(Sender: TObject);
var
  Source: TStringStream;
  Dest: TStringStream;
  Key: TAESKey128;
  Size: integer;
begin
try
  Source := TStringStream.Create(Trim(Memo1.Text) );
  Dest   := TStringStream.Create('');
  Size := Source.Size;
  Source.ReadBuffer(Size, SizeOf(Size));
  FillChar(Key, SizeOf(Key), 0);
  Move(PChar(PASSWORD)^, Key, Min(SizeOf(Key), Length(PASSWORD)));
  Source.Position := 0;
  DecryptAESStreamECB(Source, Source.Size - Source.Position, Key, Dest);
  Memo1.Text := Trim(Dest.DataString);
  Label2.Text := 'Texto Original';
  Source.Free;
  Dest.Free;
except on E: Exception do
  begin
    ShowMessage(e.ToString);
    Source.Free;
    Dest.Free;
    Memo1.Text :='';
  end;
end;

end;

end.

I've also tried to create stringstream with this:

Source := TStringStream.Create(Trim(Memo1.Text) , TEncoding.Unicode) ;

and sometimes works well and sometimes gives me the following error:"Los surrogate char without a preceding high surrogate char at index: 8. chaeck that the string is encoded properly. Any ideas?

Answer 1

Use standardized libraries instead of trying to make your own encryption solution. There are for example several implementation of AES encryption available for Delphi (e.g. Eldos which is included in the NativeXML library). Write your string (MyString) to a stream and encrypt it:

var
  lSourceStream: TStringStream;
  lDestinationStream: TMemoryStream;
begin
  lSourceStream := TStringStream.Create(MyString);
  lDestinationStream := TMemoryStream.Create;
  AESencrypt(lSourceStream,lDestinationStream);
  lDestinationStream.SaveToFile(<filename>);
end;