Make captcha (kocaptcha) stateless

Also rename seconds_retained to seconds_valid since that's how it is
now. Put it down from 180 to 20 seconds. The answer data is now
stored in an encrypted text transfered to the client and back, so no
ETS is needed

config/config.exs

@@ -12,7 +12,7 @@
 
 config :pleroma, Pleroma.Captcha,
   enabled: false,
-  seconds_retained: 180,
+  seconds_valid: 20,
   method: Pleroma.Captcha.Kocaptcha
 
 config :pleroma, Pleroma.Captcha.Kocaptcha, endpoint: "https://captcha.kotobank.ch"

docs/config.md

@@ -168,7 +168,7 @@ Web Push Notifications configuration. You can use the mix task `mix web_push.gen
 ## Pleroma.Captcha
 * `enabled`: Whether the captcha should be shown on registration
 * `method`: The method/service to use for captcha
-* `seconds_retained`: The time in seconds for which the captcha is valid (stored in the cache)
+* `seconds_valid`: The time in seconds for which the captcha is valid
 
 ### Pleroma.Captcha.Kocaptcha
 Kocaptcha is a very simple captcha service with a single API endpoint,

lib/pleroma/captcha/captcha.ex

@@ -1,8 +1,6 @@
 defmodule Pleroma.Captcha do
   use GenServer
 
-  @ets_options [:ordered_set, :private, :named_table, {:read_concurrency, true}]
-
   @doc false
   def start_link() do
     GenServer.start_link(__MODULE__, [], name: __MODULE__)
@@ -10,14 +8,6 @@ def start_link() do
 
   @doc false
   def init(_) do
-    # Create a ETS table to store captchas
-    ets_name = Module.concat(method(), Ets)
-    ^ets_name = :ets.new(Module.concat(method(), Ets), @ets_options)
-
-    # Clean up old captchas every few minutes
-    seconds_retained = Pleroma.Config.get!([__MODULE__, :seconds_retained])
-    Process.send_after(self(), :cleanup, 1000 * seconds_retained)
-
     {:ok, nil}
   end
 
@@ -31,8 +21,8 @@ def new() do
   @doc """
   Ask the configured captcha service to validate the captcha
   """
-  def validate(token, captcha) do
-    GenServer.call(__MODULE__, {:validate, token, captcha})
+  def validate(token, captcha, answer_data) do
+    GenServer.call(__MODULE__, {:validate, token, captcha, answer_data})
   end
 
   @doc false
@@ -47,19 +37,8 @@ def handle_call(:new, _from, state) do
   end
 
   @doc false
-  def handle_call({:validate, token, captcha}, _from, state) do
-    {:reply, method().validate(token, captcha), state}
-  end
-
-  @doc false
-  def handle_info(:cleanup, state) do
-    :ok = method().cleanup()
-
-    seconds_retained = Pleroma.Config.get!([__MODULE__, :seconds_retained])
-    # Schedule the next clenup
-    Process.send_after(self(), :cleanup, 1000 * seconds_retained)
-
-    {:noreply, state}
+  def handle_call({:validate, token, captcha, answer_data}, _from, state) do
+    {:reply, method().validate(token, captcha, answer_data), state}
   end
 
   defp method, do: Pleroma.Config.get!([__MODULE__, :method])

lib/pleroma/captcha/captcha_service.ex

@@ -14,15 +14,15 @@ defmodule Pleroma.Captcha.Service do
   Arguments:
   * `token` the captcha is associated with
   * `captcha` solution of the captcha to validate
+  * `answer_data` is the data needed to validate the answer (presumably encrypted)
 
   Returns:
 
   `true` if captcha is valid, `false` if not
   """
-  @callback validate(token :: String.t(), captcha :: String.t()) :: boolean
-
-  @doc """
-  This function is called periodically to clean up old captchas
-  """
-  @callback cleanup() :: :ok
+  @callback validate(
+              token :: String.t(),
+              captcha :: String.t(),
+              answer_data :: String.t()
+            ) :: :ok | {:error, String.t()}
 end

lib/pleroma/captcha/kocaptcha.ex

@@ -1,11 +1,11 @@
 defmodule Pleroma.Captcha.Kocaptcha do
+  alias Plug.Crypto.KeyGenerator
+  alias Plug.Crypto.MessageEncryptor
   alias Calendar.DateTime
 
   alias Pleroma.Captcha.Service
   @behaviour Service
 
-  @ets __MODULE__.Ets
-
   @impl Service
   def new() do
     endpoint = Pleroma.Config.get!([__MODULE__, :endpoint])
@@ -18,50 +18,56 @@ def new() do
         json_resp = Poison.decode!(res.body)
 
         token = json_resp["token"]
+        answer_md5 = json_resp["md5"]
 
-        true =
-          :ets.insert(
-            @ets,
-            {token, json_resp["md5"], DateTime.now_utc() |> DateTime.Format.unix()}
-          )
+        secret_key_base = Pleroma.Config.get!([Pleroma.Web.Endpoint, :secret_key_base])
 
-        %{type: :kocaptcha, token: token, url: endpoint <> json_resp["url"]}
-    end
-  end
-
-  @impl Service
-  def validate(token, captcha) do
-    with false <- is_nil(captcha),
-         [{^token, saved_md5, _}] <- :ets.lookup(@ets, token),
-         true <- :crypto.hash(:md5, captcha) |> Base.encode16() == String.upcase(saved_md5) do
-      # Clear the saved value
-      :ets.delete(@ets, token)
-
-      true
-    else
-      _ -> false
-    end
-  end
-
-  @impl Service
-  def cleanup() do
-    seconds_retained = Pleroma.Config.get!([Pleroma.Captcha, :seconds_retained])
-    # If the time in ETS is less than current_time - seconds_retained, then the time has
-    # already passed
-    delete_after =
-      DateTime.subtract!(DateTime.now_utc(), seconds_retained) |> DateTime.Format.unix()
-
-    :ets.select_delete(
-      @ets,
-      [
-        {
-          {:_, :_, :"$1"},
-          [{:<, :"$1", {:const, delete_after}}],
-          [true]
+        # This make salt a little different for two keys
+        secret = KeyGenerator.generate(secret_key_base, token <> "_encrypt")
+        sign_secret = KeyGenerator.generate(secret_key_base, token <> "_sign")
+        # Basicallty copy what Phoenix.Token does here, add the time to
+        # the actual data and make it a binary to then encrypt it
+        encrypted_captcha_answer =
+          %{
+            at: DateTime.now_utc(),
+            answer_md5: answer_md5
           }
-      ]
-    )
+          |> :erlang.term_to_binary()
+          |> MessageEncryptor.encrypt(secret, sign_secret)
 
-    :ok
+        %{
+          type: :kocaptcha,
+          token: token,
+          url: endpoint <> json_resp["url"],
+          answer_data: encrypted_captcha_answer
+        }
+    end
+  end
+
+  @impl Service
+  def validate(token, captcha, answer_data) do
+    secret_key_base = Pleroma.Config.get!([Pleroma.Web.Endpoint, :secret_key_base])
+    secret = KeyGenerator.generate(secret_key_base, token <> "_encrypt")
+    sign_secret = KeyGenerator.generate(secret_key_base, token <> "_sign")
+
+    # If the time found is less than (current_time - seconds_valid), then the time has already passed.
+    # Later we check that the time found is more than the presumed invalidatation time, that means
+    # that the data is still valid and the captcha can be checked
+    seconds_valid = Pleroma.Config.get!([Pleroma.Captcha, :seconds_valid])
+    valid_if_after = DateTime.subtract!(DateTime.now_utc(), seconds_valid)
+
+    with {:ok, data} <- MessageEncryptor.decrypt(answer_data, secret, sign_secret),
+         %{at: at, answer_md5: answer_md5} <- :erlang.binary_to_term(data) do
+      if DateTime.after?(at, valid_if_after) do
+        if not is_nil(captcha) and
+             :crypto.hash(:md5, captcha) |> Base.encode16() == String.upcase(answer_md5),
+           do: :ok,
+           else: {:error, "Invalid CAPTCHA"}
+      else
+        {:error, "CAPTCHA expired"}
+      end
+    else
+      _ -> {:error, "Invalid answer data"}
+    end
   end
 end

lib/pleroma/web/twitter_api/twitter_api.ex

@@ -136,22 +136,28 @@ def register_user(params) do
       password: params["password"],
       password_confirmation: params["confirm"],
       captcha_solution: params["captcha_solution"],
-      captcha_token: params["captcha_token"]
+      captcha_token: params["captcha_token"],
+      captcha_answer_data: params["captcha_answer_data"]
     }
 
     captcha_enabled = Pleroma.Config.get([Pleroma.Captcha, :enabled])
     # true if captcha is disabled or enabled and valid, false otherwise
     captcha_ok =
       if !captcha_enabled do
-        true
+        :ok
       else
-        Pleroma.Captcha.validate(params[:captcha_token], params[:captcha_solution])
+        Pleroma.Captcha.validate(
+          params[:captcha_token],
+          params[:captcha_solution],
+          params[:captcha_answer_data]
+        )
       end
 
     # Captcha invalid
-    if not captcha_ok do
+    if captcha_ok != :ok do
+      {:error, error} = captcha_ok
       # I have no idea how this error handling works
-      {:error, %{error: Jason.encode!(%{captcha: ["Invalid CAPTCHA"]})}}
+      {:error, %{error: Jason.encode!(%{captcha: [error]})}}
     else
       registrations_open = Pleroma.Config.get([:instance, :registrations_open])
 

test/captcha_test.exs

@@ -25,16 +25,18 @@ defmodule Pleroma.CaptchaTest do
     end
 
     test "new and validate" do
-      assert Kocaptcha.new() == %{
-               type: :kocaptcha,
-               token: "afa1815e14e29355e6c8f6b143a39fa2",
-               url: "https://captcha.kotobank.ch/captchas/afa1815e14e29355e6c8f6b143a39fa2.png"
-             }
+      new = Kocaptcha.new()
+      assert new[:type] == :kocaptcha
+      assert new[:token] == "afa1815e14e29355e6c8f6b143a39fa2"
+
+      assert new[:url] ==
+               "https://captcha.kotobank.ch/captchas/afa1815e14e29355e6c8f6b143a39fa2.png"
 
       assert Kocaptcha.validate(
-               "afa1815e14e29355e6c8f6b143a39fa2",
-               "7oEy8c"
-             )
+               new[:token],
+               "7oEy8c",
+               new[:answer_data]
+             ) == :ok
     end
   end
 end

test/support/captcha_mock.ex

@@ -6,8 +6,5 @@ defmodule Pleroma.Captcha.Mock do
   def new(), do: %{type: :mock}
 
   @impl Service
-  def validate(_token, _captcha), do: true
-
-  @impl Service
-  def cleanup(), do: :ok
+  def validate(_token, _captcha, _data), do: :ok
 end