怎样给文件加密最安全？

<%@ language = vbscript%>
<%

response.expires = 0

p = "abcdefg"
set testpwd = new cpassword
testpwd.enpwd p
testpwd.unpwd testpwd.showpwd(true)

k = testpwd.showpwd(true)
l = testpwd.showpwd(false)
set testpwd = nothing

with response
.write "原文:" & p & "<br>"
.write "加密后:" & k & "<br>"
.write "解密后:" & l & "<br>"
end with


class cpassword
private cenpwd,cunpwd

private function my_hex(my_hex_source)
  my_hex = hex(my_hex_source)
  if len(my_hex) = 1 then
   my_hex = "0" & my_hex
  end if
end function

private function my_unhex(my_unhex_source)
  if len(my_unhex_source) = 2 then
   temp_value = my_hexreplace(left(my_unhex_source,1))
  end if
  my_unhex = temp_value * 16 + my_hexreplace(right(my_unhex_source,1))
end function

private function my_hexreplace(my_hexreplace_source)
  if asc(my_hexreplace_source) > 64 then
   my_hexreplace = asc(ucase(my_hexreplace_source)) - 55
  else
   my_hexreplace = asc(my_hexreplace_source) - 48
  end if
end function

public property get showpwd(enp)

' 输出.
  if enp then
   showpwd = cenpwd
  else
   showpwd = cunpwd
  end if
end property

sub enpwd(enpwd_source)
  if enpwd_source = "" then
   cenpwd = 0
   exit sub
  else
   randomize
   key = rnd * 256
   while key = 0
    key = rnd * 256
   wend
   for i=1 to len(enpwd_source)
    temp_string = key xor asc(mid(enpwd_source,i,1))
    crc = crc + temp_string
    cenpwd = cenpwd + my_hex(temp_string)
   next
   cenpwd = my_hex(key) + strreverse(cenpwd) + my_hex(crc mod 256)
  end if
end sub

sub unpwd(unpwd_source)
  if len(unpwd_source) < 6 or len(unpwd_source) mod 2 = 1 then
   cunpwd = 0
   exit sub
  end if
  old_key = my_unhex(left(unpwd_source,2))
  old_crc = my_unhex(right(unpwd_source,2))
  old_string = strreverse(mid(unpwd_source,3,len(unpwd_source)-4))
  for i=1 to len(old_string) - 1 step 2
   temp_value = my_unhex(mid(old_string,i,2))
   crc_temp = crc_temp + temp_value
   temp_string = temp_string + chr(temp_value xor old_key)
  next
  if old_crc <> (crc_temp mod 256) then
   cunpwd = 403
   exit sub
  end if
  cunpwd = temp_string
end sub
end class

%>