modexp := proc(message::nonnegint,
               expkey::nonnegint, modkey::nonnegint)
   description `modular exponentiation of message`;
   local result, wrk, rest, shift:
   result := 0:
   wrk := message:
   shift := 1:
   while wrk > 0 do
      rest := wrk mod modkey:
      result := (rest&^expkey mod modkey)*shift + result:
      wrk := (wrk - rest)/modkey:
      shift := shift*modkey:
   end do:
   return result:
end proc: