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diff --git a/lib/pleroma/web/plugs/mapped_signature_to_identity_plug.ex b/lib/pleroma/web/plugs/mapped_signature_to_identity_plug.ex
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+++ b/lib/pleroma/web/plugs/mapped_signature_to_identity_plug.ex
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+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2021 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Web.Plugs.MappedSignatureToIdentityPlug do
+  alias Pleroma.Helpers.AuthHelper
+  alias Pleroma.Signature
+  alias Pleroma.User
+  alias Pleroma.Web.ActivityPub.Utils
+
+  import Plug.Conn
+  require Logger
+
+  def init(options), do: options
+
+  def call(%{assigns: %{user: %User{}}} = conn, _opts), do: conn
+
+  # if this has payload make sure it is signed by the same actor that made it
+  def call(%{assigns: %{valid_signature: true}, params: %{"actor" => actor}} = conn, _opts) do
+    with actor_id <- Utils.get_ap_id(actor),
+         {:user, %User{} = user} <- {:user, user_from_key_id(conn)},
+         {:user_match, true} <- {:user_match, user.ap_id == actor_id} do
+      conn
+      |> assign(:user, user)
+      |> AuthHelper.skip_oauth()
+    else
+      {:user_match, false} ->
+        Logger.debug("Failed to map identity from signature (payload actor mismatch)")
+        Logger.debug("key_id=#{inspect(key_id_from_conn(conn))}, actor=#{inspect(actor)}")
+        assign(conn, :valid_signature, false)
+
+      # remove me once testsuite uses mapped capabilities instead of what we do now
+      {:user, nil} ->
+        Logger.debug("Failed to map identity from signature (lookup failure)")
+        Logger.debug("key_id=#{inspect(key_id_from_conn(conn))}, actor=#{actor}")
+        conn
+    end
+  end
+
+  # no payload, probably a signed fetch
+  def call(%{assigns: %{valid_signature: true}} = conn, _opts) do
+    with %User{} = user <- user_from_key_id(conn) do
+      conn
+      |> assign(:user, user)
+      |> AuthHelper.skip_oauth()
+    else
+      _ ->
+        Logger.debug("Failed to map identity from signature (no payload actor mismatch)")
+        Logger.debug("key_id=#{inspect(key_id_from_conn(conn))}")
+        assign(conn, :valid_signature, false)
+    end
+  end
+
+  # no signature at all
+  def call(conn, _opts), do: conn
+
+  defp key_id_from_conn(conn) do
+    with %{"keyId" => key_id} <- HTTPSignatures.signature_for_conn(conn),
+         {:ok, ap_id} <- Signature.key_id_to_actor_id(key_id) do
+      ap_id
+    else
+      _ ->
+        nil
+    end
+  end
+
+  defp user_from_key_id(conn) do
+    with key_actor_id when is_binary(key_actor_id) <- key_id_from_conn(conn),
+         {:ok, %User{} = user} <- User.get_or_fetch_by_ap_id(key_actor_id) do
+      user
+    else
+      _ ->
+        nil
+    end
+  end
+end