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diff --git a/lib/pleroma/signature.ex b/lib/pleroma/signature.ex
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+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2022 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Signature do
+  @behaviour HTTPSignatures.Adapter
+
+  alias Pleroma.EctoType.ActivityPub.ObjectValidators
+  alias Pleroma.Keys
+  alias Pleroma.User
+  alias Pleroma.Web.ActivityPub.ActivityPub
+
+  @known_suffixes ["/publickey", "/main-key"]
+
+  def key_id_to_actor_id(key_id) do
+    uri =
+      key_id
+      |> URI.parse()
+      |> Map.put(:fragment, nil)
+      |> remove_suffix(@known_suffixes)
+
+    maybe_ap_id = URI.to_string(uri)
+
+    case ObjectValidators.ObjectID.cast(maybe_ap_id) do
+      {:ok, ap_id} ->
+        {:ok, ap_id}
+
+      _ ->
+        case Pleroma.Web.WebFinger.finger(maybe_ap_id) do
+          %{"ap_id" => ap_id} -> {:ok, ap_id}
+          _ -> {:error, maybe_ap_id}
+        end
+    end
+  end
+
+  defp remove_suffix(uri, [test | rest]) do
+    if not is_nil(uri.path) and String.ends_with?(uri.path, test) do
+      Map.put(uri, :path, String.replace(uri.path, test, ""))
+    else
+      remove_suffix(uri, rest)
+    end
+  end
+
+  defp remove_suffix(uri, []), do: uri
+
+  def fetch_public_key(conn) do
+    with %{"keyId" => kid} <- HTTPSignatures.signature_for_conn(conn),
+         {:ok, actor_id} <- key_id_to_actor_id(kid),
+         {:ok, public_key} <- User.get_public_key_for_ap_id(actor_id) do
+      {:ok, public_key}
+    else
+      e ->
+        {:error, e}
+    end
+  end
+
+  def refetch_public_key(conn) do
+    with %{"keyId" => kid} <- HTTPSignatures.signature_for_conn(conn),
+         {:ok, actor_id} <- key_id_to_actor_id(kid),
+         {:ok, _user} <- ActivityPub.make_user_from_ap_id(actor_id),
+         {:ok, public_key} <- User.get_public_key_for_ap_id(actor_id) do
+      {:ok, public_key}
+    else
+      e ->
+        {:error, e}
+    end
+  end
+
+  def sign(%User{keys: keys} = user, headers) do
+    with {:ok, private_key, _} <- Keys.keys_from_pem(keys) do
+      HTTPSignatures.sign(private_key, user.ap_id <> "#main-key", headers)
+    end
+  end
+
+  def signed_date, do: signed_date(NaiveDateTime.utc_now())
+
+  def signed_date(%NaiveDateTime{} = date) do
+    Timex.format!(date, "{WDshort}, {0D} {Mshort} {YYYY} {h24}:{m}:{s} GMT")
+  end
+end