j:=Length(pass);<<--
j1:=1;<<--
for b:=1 to i do
Begin
if j1>j then j1:=1;<<--
if (ord(s[b])+ord(pass[j1]))>255 then
mmo2.text:=mmo2.text+chr(ord(s[b])+ord(pass[j1])-255)
else
mmo2.text:=mmo2.text+chr(ord(s[b])+ord(pass[j1])) ;
Inc(j1);<<--
end;

unit Unit2;

interface

uses
   Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
   Dialogs, StdCtrls;

type
   TForm1 = class(TForm)
     Edit1: TEdit;
     Button1: TButton;
     Button2: TButton;
     procedure Button1Click(Sender: TObject);
     procedure Button2Click(Sender: TObject);
   private
     { Private declarations }
   public
     { Public declarations }
   end;

const
   StartKey = 471; // Start default key
   MultKey = 62142; // Mult default key
   AddKey = 11719; // Add default key

var
   Form1: TForm1;
   S: string;

implementation

{$R *.dfm}

function Encrypt(const InString: string; StartKey, MultKey, AddKey: Integer):
   string;
var
   I: Byte;
begin
   Result := '';
   for I := 1 to Length(InString) do
   begin
     Result := Result + CHAR(Byte(InString[I]) xor (StartKey shr 8));
     StartKey := (Byte(Result[I]) + StartKey) * MultKey + AddKey;
   end;
end;

function Decrypt(const InString: string; StartKey, MultKey, AddKey: Integer):
   string;
var
   I: Byte;
begin
   Result := '';
   for I := 1 to Length(InString) do
   begin
     Result := Result + CHAR(Byte(InString[I]) xor (StartKey shr 8));
     StartKey := (Byte(InString[I]) + StartKey) * MultKey + AddKey;
   end;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
S:=Encrypt(Edit1.Text,StartKey, MultKey, AddKey);
Caption:=S;
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
Caption:=Decrypt(S,StartKey, MultKey, AddKey);
end;

end.