Unable to validate data

hi i am using 'system.web.security' name space .encrypting some data in one page and redirecting the control to another page in other site(not in same site).While redirecting sending the encrypted data in query string to that page and ther i am trying to decrypt it.But i am getting the error like
"Unable to validate data"
code i used for encryption and decryption as follows
 public string encrypt(string pwd)
    {
        FormsAuthenticationTicket ftk = new FormsAuthenticationTicket(pwd, true, 500);
       return FormsAuthentication.Encrypt(ftk);
       
    }


    public string decrypt(string pwd)
    {
       FormsAuthenticationTicket Ftk = FormsAuthentication.Decrypt(pwd);
       ...