webhook.ex

defmodule Stripe.Webhook do
  @moduledoc """
  Creates a Stripe Event from webhook's payload if signature is valid.
  """

  @default_tolerance 300
  @expected_scheme "v1"

  @doc """
  Verify webhook payload and return a Stripe event.

  `payload` is the raw, unparsed content body sent by Stripe, which can be
  retrieved with `Plug.Conn.read_body/2`. Note that `Plug.Parsers` will read
  and discard the body, so you must implement a [custom body reader][1] if the
  plug is located earlier in the pipeline.

  `signature` is the value of `Stripe-Signature` header, which can be fetched
  with `Plug.Conn.get_req_header/2`.

  `secret` is your webhook endpoint's secret from the Stripe Dashboard.

  `tolerance` is the allowed deviation in seconds from the current system time
  to the timestamp found in `signature`. Defaults to 300 seconds (5 minutes).

  Stripe API reference:
  https://stripe.com/docs/webhooks/signatures#verify-manually

  [1]: https://hexdocs.pm/plug/Plug.Parsers.html#module-custom-body-reader

  ## Example

      case Stripe.Webhook.construct_event(payload, signature, secret) do
        {:ok, %Stripe.Event{} = event} ->
          # Return 200 to Stripe and handle event

        {:error, reason} ->
          # Reject webhook by responding with non-2XX
      end
  """
  @spec construct_event(String.t(), String.t(), String.t(), integer) ::
          {:ok, Stripe.Event.t()} | {:error, any}
  def construct_event(payload, signature_header, secret, tolerance \\ @default_tolerance) do
    case verify_header(payload, signature_header, secret, tolerance) do
      :ok ->
        {:ok, convert_to_event!(payload)}

      error ->
        error
    end
  end

  defp verify_header(payload, signature_header, secret, tolerance) do
    case get_timestamp_and_signatures(signature_header, @expected_scheme) do
      {nil, _} ->
        {:error, "Unable to extract timestamp and signatures from header"}

      {_, []} ->
        {:error, "No signatures found with expected scheme #{@expected_scheme}"}

      {timestamp, signatures} ->
        with {:ok, timestamp} <- check_timestamp(timestamp, tolerance),
             {:ok, _signatures} <- check_signatures(signatures, timestamp, payload, secret) do
          :ok
        else
          {:error, error} -> {:error, error}
        end
    end
  end

  defp get_timestamp_and_signatures(signature_header, scheme) do
    signature_header
    |> String.split(",")
    |> Enum.map(&String.split(&1, "="))
    |> Enum.reduce({nil, []}, fn
      ["t", timestamp], {nil, signatures} ->
        {to_integer(timestamp), signatures}

      [^scheme, signature], {timestamp, signatures} ->
        {timestamp, [signature | signatures]}

      _, acc ->
        acc
    end)
  end

  defp to_integer(timestamp) do
    case Integer.parse(timestamp) do
      {timestamp, _} ->
        timestamp

      :error ->
        nil
    end
  end

  defp check_timestamp(timestamp, tolerance) do
    now = System.system_time(:second)
    tolerance_zone = now - tolerance

    if timestamp < tolerance_zone do
      {:error, "Timestamp outside the tolerance zone (#{now})"}
    else
      {:ok, timestamp}
    end
  end

  defp check_signatures(signatures, timestamp, payload, secret) do
    signed_payload = "#{timestamp}.#{payload}"
    expected_signature = compute_signature(signed_payload, secret)

    if Enum.any?(signatures, &secure_equals?(&1, expected_signature)) do
      {:ok, signatures}
    else
      {:error, "No signatures found matching the expected signature for payload"}
    end
  end

  defp compute_signature(payload, secret) do
    hmac(:sha256, secret, payload)
    |> Base.encode16(case: :lower)
  end

  # TODO: remove when we require OTP 22
  if System.otp_release() >= "22" do
    defp hmac(digest, key, data), do: :crypto.mac(:hmac, digest, key, data)
  else
    defp hmac(digest, key, data), do: :crypto.hmac(digest, key, data)
  end

  defp secure_equals?(input, expected) when byte_size(input) == byte_size(expected) do
    input = String.to_charlist(input)
    expected = String.to_charlist(expected)
    secure_compare(input, expected)
  end

  defp secure_equals?(_, _), do: false

  defp secure_compare(acc \\ 0, input, expected)
  defp secure_compare(acc, [], []), do: acc == 0

  defp secure_compare(acc, [input_codepoint | input], [expected_codepoint | expected]) do
    import Bitwise

    acc
    |> bor(bxor(input_codepoint, expected_codepoint))
    |> secure_compare(input, expected)
  end

  defp convert_to_event!(payload) do
    payload
    |> Stripe.API.json_library().decode!()
    |> Stripe.Converter.convert_result()
  end
end