Add webhooks support

README.md

@@ -138,6 +138,19 @@ Using the code request parameter, you make the following call:
 resp[:access_token]
 ```
 
+# Webhooks
+
+Stripe uses webhooks to notify your web app with events. `Stripe.Webhook`
+provides `construct_event/3` to authenticate the request and convert the
+payload to a `Stripe.Event` struct.
+
+```ex
+payload = # HTTP content body (e.g. from Plug.Conn.read_body/3)
+signature = # 'Stripe-Signature' HTTP header (e.g. from Plug.Conn.get_req_header/2)
+secret = # Provided by Stripe
+{:ok, %Stripe.Event{}} = Stripe.Webhook.construct_event(payload, signature, secret)
+```
+
 ## Contributing
 
 Feedback, feature requests, and fixes are welcomed and encouraged.  Please make

lib/stripe/util.ex

@@ -58,4 +58,35 @@ defmodule Stripe.Util do
 
     Module.concat("Stripe", module_name)
   end
+
+  @doc """
+  Compare two strings in constant-time, useful against timing attacks.
+
+  Returns as soon as possible if string lengths don't match.
+
+  iex> Stripe.Util.secure_equals?("abcdef", "abcdef")
+  true
+
+  iex> Stripe.Util.secure_equals?("abcdef", "123456")
+  false
+
+  iex> Stripe.Util.secure_equals?("abcdef", "abc")
+  false
+  """
+  @spec secure_equals?(String.t, String.t) :: boolean
+  def secure_equals?(input, expected) when byte_size(input) == byte_size(expected) do
+    input = String.to_charlist(input)
+    expected = String.to_charlist(expected)
+    do_secure_equals?(input, expected)
+  end
+  def secure_equals?(_, _), do: false
+
+  defp do_secure_equals?(acc \\ 0, input, expected)
+  defp do_secure_equals?(acc, [], []), do: acc == 0
+  defp do_secure_equals?(acc, [input_char | input], [expected_char | expected]) do
+    import Bitwise
+    acc
+    |> bor(input_char ^^^ expected_char)
+    |> do_secure_equals?(input, expected)
+  end
 end

lib/stripe/webhook.ex

@@ -0,0 +1,112 @@
+defmodule Stripe.Webhook do
+  @moduledoc """
+  Creates a Stripe Event from webhook's payload if signature is valid.
+
+  Use `construct_event/3` to verify the authenticity of a webhook request and
+  convert its payload into a `Stripe.Event` struct.
+
+    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
+  """
+  alias Stripe.Util
+
+  @type webhook_error :: :invalid_payload | :invalid_signature | :missing_signature | :missing_timestamp | :expired_timestamp | :invalid_timestamp
+
+  @default_tolerance 300
+  @expected_scheme "v1"
+
+  @spec construct_event(String.t, String.t, String.t, integer) :: {:ok, Stripe.Event.t} | {:error, webhook_error}
+  def construct_event(payload, signature_header, secret, tolerance \\ @default_tolerance) do
+    case verify_signature(payload, signature_header, secret, tolerance) do
+      :ok ->
+        case convert_to_event(payload) do
+          %Stripe.Event{} = event ->
+            {:ok, event}
+          _ ->
+            {:error, :invalid_payload}
+        end
+
+      error ->
+        error
+    end
+  end
+
+  defp verify_signature(payload, signature_header, secret, tolerance) do
+    with {:ok, timestamp} <- get_timestamp(signature_header, tolerance),
+         {:ok, signatures} <- get_signatures(signature_header) do
+      signed_payload = "#{timestamp}.#{payload}"
+      expected_signature = compute_signature(signed_payload, secret)
+      if Enum.any?(signatures, & Util.secure_equals?(&1, expected_signature)) do
+        :ok
+      else
+        {:error, :invalid_signature}
+      end
+    end
+  end
+
+  defp get_timestamp(signature_header, tolerance) do
+    timestamp =
+      signature_header
+      |> String.split(",")
+      |> Enum.find_value(:not_found, fn
+        "t=" <> timestamp ->
+          timestamp
+        _ ->
+          false
+      end)
+
+    with {:ok, timestamp} <- parse_timestamp(timestamp) do
+      if timestamp < (System.system_time(:seconds) - tolerance) do
+        {:error, :expired_timestamp}
+      else
+        {:ok, timestamp}
+      end
+    end
+  end
+
+  defp parse_timestamp(:not_found), do: {:error, :missing_timestamp}
+  defp parse_timestamp(timestamp) do
+    case Integer.parse(timestamp) do
+      {timestamp, _} ->
+        {:ok, timestamp}
+      :error ->
+        {:error, :invalid_timestamp}
+    end
+  end
+
+  defp get_signatures(signature_header, scheme \\ @expected_scheme) do
+    signatures =
+      signature_header
+      |> String.split(",")
+      |> Enum.reduce([], fn item, acc ->
+        case String.split(item, "=") do
+          [^scheme, value] ->
+            [value | acc]
+          _ ->
+            acc
+        end
+      end)
+
+    case signatures do
+      [] ->
+        {:error, :missing_signature}
+      signatures ->
+        {:ok, signatures}
+    end
+  end
+
+  defp compute_signature(payload, secret) do
+    :crypto.hmac(:sha256, secret, payload)
+    |> Base.encode16(case: :lower)
+  end
+
+  defp convert_to_event(payload) do
+    payload
+    |> Poison.decode!()
+    |> Stripe.Converter.convert_result()
+  end
+end

test/stripe/util_test.exs

@@ -21,4 +21,19 @@ defmodule Stripe.UtilTest do
       assert object_name_to_module("token") == Stripe.Token
     end
   end
+
+  describe "secure_equals?/2" do
+    test "identical strings should equal" do
+      assert secure_equals?("abcdef", "abcdef") == true
+    end
+
+    test "non-identical strings should not equal" do
+      assert secure_equals?("abcdef", "123456") == false
+    end
+
+    test "strings of different lengths should not equal" do
+      assert secure_equals?("abcdef", "abc") == false
+      assert secure_equals?("abc", "abcdef") == false
+    end
+  end
 end

test/stripe/webhook_test.exs

@@ -0,0 +1,67 @@
+defmodule Stripe.WebhookTest do
+  use ExUnit.Case
+
+  import Stripe.Webhook
+
+  @valid_payload ~S({"object": "event"})
+  @invalid_payload "{}"
+
+  @valid_scheme "v1"
+  @invalid_scheme "v0"
+
+  @secret "secret"
+
+  defp generate_signature(timestamp, payload, secret \\ @secret) do
+    :crypto.hmac(:sha256, secret, "#{timestamp}.#{payload}")
+    |> Base.encode16(case: :lower)
+  end
+
+  defp create_signature_header(timestamp, scheme, signature) do
+    "t=#{timestamp},#{scheme}=#{signature}"
+  end
+
+  test "payload with a valid signature should return event" do
+    timestamp = System.system_time(:seconds)
+    payload = @valid_payload
+    signature = generate_signature(timestamp, payload)
+    signature_header = create_signature_header(timestamp, @valid_scheme, signature)
+
+    assert {:ok, %Stripe.Event{}} = construct_event(payload, signature_header, @secret)
+  end
+
+  test "payload with an invalid signature should fail" do
+    timestamp = System.system_time(:seconds)
+    payload = @valid_payload
+    signature = generate_signature(timestamp, "random")
+    signature_header = create_signature_header(timestamp, @valid_scheme, signature)
+
+    assert construct_event(payload, signature_header, @secret) == {:error, :invalid_signature}
+  end
+
+  test "payload with wrong secret should fail" do
+    timestamp = System.system_time(:seconds)
+    payload = @valid_payload
+    signature = generate_signature(timestamp, payload, "wrong")
+    signature_header = create_signature_header(timestamp, @valid_scheme, signature)
+
+    assert construct_event(payload, signature_header, @secret) == {:error, :invalid_signature}
+  end
+
+  test "payload with missing signature scheme should fail" do
+    timestamp = System.system_time(:seconds)
+    payload = @valid_payload
+    signature = generate_signature(timestamp, payload)
+    signature_header = create_signature_header(timestamp, @invalid_scheme, signature)
+
+    assert construct_event(payload, signature_header, @secret) == {:error, :missing_signature}
+  end
+
+  test "invalid payload with a valid signature should fail" do
+    timestamp = System.system_time(:seconds)
+    payload = @invalid_payload
+    signature = generate_signature(timestamp, payload)
+    signature_header = create_signature_header(timestamp, @valid_scheme, signature)
+
+    assert construct_event(payload, signature_header, @secret) == {:error, :invalid_payload}
+  end
+end