Как зашифровать строку символов заданным криптографическим ключом?

procedure TMainForm.btn1Click(Sender: TObject);
var
  S,key,K:string;
  L, Lk,i,j:Integer;
begin
  S:=edt1.Text;
  key:=edt2.Text;
  L:=Length(S);
  Lk:=Length(key);
 
  if Lk>=L then begin
  ShowMessage('Ошибка!');
  edt1.Clear;
  edt2.Clear;
  end;
 
  if L<Lk then begin
  K:='';
  for i:=1 to L do
  for j:=1 to Lk do
  K[i]:=chr(ord(S[i]) xor ord(key[j]));
  edt3.Text:=K;
  end;
  end;

var
  s: string;
 
procedure Code(var text: string; password: string;
  decode: boolean);
var
  i, PasswordLength: integer;
  sign: shortint;
begin
  PasswordLength := length(password);
  if PasswordLength = 0 then Exit;
  if decode
    then sign := -1
    else sign := 1;
  for i := 1 to Length(text) do
    text[i] := chr(ord(text[i]) + sign *
      ord(password[i mod PasswordLength + 1]));
end;
 
procedure TForm1.Button1Click(Sender: TObject);
begin
  s := Memo1.Text;
  code(s, Edit1.Text, false);
  Memo1.Text := s;
end;
 
procedure TForm1.Button2Click(Sender: TObject);
begin
  code(s, Edit1.Text, true);
  Memo1.Text := s;
end;

unit MainU;
 
interface
 
uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;
 
type
  TForm1 = class(TForm)
    Label1: TLabel;
    Edit1: TEdit;
    Label2: TLabel;
    Edit2: TEdit;
    Button1: TButton;
    Button2: TButton;
    Label3: TLabel;
    Edit3: TEdit;
    Button3: TButton;
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
    procedure Button3Click(Sender: TObject);
    procedure FormCreate(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;
 
var
  Form1: TForm1;
qstr,qkey:ansistring;
 
implementation
 
{$R *.dfm}
 
function qencode(s,Key:ansistring):ansistring;
var
i,j,L,Lk:integer;
begin
L:=length(s);
Lk:=length(Key);
if Lk>=L then
begin
showmessage('Длина криптографического ключа должна быть меньше длины строки!');
result:='';
exit;
end;
j:=1;
for i:=1 to L do
begin
if j>Lk then
j:=1;
s[i]:=chr(ord(s[i]) xor ord(Key[j]));
j:=j+1;
end;
result:=s;
end;
 
procedure TForm1.Button1Click(Sender: TObject);
var
i:integer;
s:ansistring;
begin
qkey:=edit2.Text;
s:=qencode(edit1.Text,qkey);
if s<>'' then
begin
qstr:=s;
for i:=1 to length(s) do
if s[i]<' ' then
s[i]:=' ';
edit1.Clear;
edit3.Text:=s;
end;
end;
 
procedure TForm1.Button2Click(Sender: TObject);
begin
if (qstr<>'')and(qkey<>'') then
edit1.Text:=qencode(qstr,qkey);
end;
 
procedure TForm1.Button3Click(Sender: TObject);
begin
close;
end;
 
procedure TForm1.FormCreate(Sender: TObject);
begin
qstr:='';
qkey:='';
end;
 
end.