constant_time_compare

compare.ex

defmodule Compare do
  use Bitwise

  @type bytes :: [char]
  @spec constant_time(bytes, bytes) :: boolean
  def constant_time(x, y) when length(x) != length(y), do: false
  def constant_time(x, y),                             do: do_constant_time(x, y, 0)

  defp do_constant_time([], _, acc),          do: acc == 0
  defp do_constant_time([x|xs], [y|ys], acc), do: do_constant_time(xs, ys, bor(acc, bxor(x,y)))
end

compare2.ex

defmodule Compare2 do
  use Bitwise

  @type bytes :: [char]
  @spec constant_time(bytes, bytes) :: boolean
  def constant_time(x, y) when length(x) != length(y),    do: false
  def constant_time(x, y) when is_list(x) and is_list(y), do: do_constant_time(x, y)

  defp do_constant_time(x, y) do
    (Enum.zip(x, y) |> Enum.reduce(0, &compare_bytes/2)) == 0
  end
  defp compare_bytes({x, y}, acc), do: bor(acc, bxor(x, y))
end