diff --git a/config/test.exs b/config/test.exs
index fefdc2bbc..6446ad49f 100644
--- a/config/test.exs
+++ b/config/test.exs
@@ -170,6 +170,7 @@ config :pleroma, Pleroma.Upload.Filter.Mogrify, config_impl: Pleroma.StaticStubb
 config :pleroma, Pleroma.Upload.Filter.Mogrify, mogrify_impl: Pleroma.MogrifyMock
 
 config :pleroma, Pleroma.Signature, http_signatures_impl: Pleroma.StubbedHTTPSignaturesMock
+config :pleroma, Pleroma.Web.ActivityPub.Publisher, signature_impl: Pleroma.SignatureMock
 
 peer_module =
   if String.to_integer(System.otp_release()) >= 25 do
diff --git a/lib/pleroma/signature.ex b/lib/pleroma/signature.ex
index 195513478..47d9d46f6 100644
--- a/lib/pleroma/signature.ex
+++ b/lib/pleroma/signature.ex
@@ -3,6 +3,7 @@
 # SPDX-License-Identifier: AGPL-3.0-only
 
 defmodule Pleroma.Signature do
+  @behaviour Pleroma.Signature.API
   @behaviour HTTPSignatures.Adapter
 
   alias Pleroma.EctoType.ActivityPub.ObjectValidators
diff --git a/lib/pleroma/signature/api.ex b/lib/pleroma/signature/api.ex
new file mode 100644
index 000000000..8b4f0c2e4
--- /dev/null
+++ b/lib/pleroma/signature/api.ex
@@ -0,0 +1,14 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2022 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Signature.API do
+  @moduledoc """
+  Behaviour for signing requests and producing HTTP Date headers.
+
+  This is used to allow tests to replace the signing implementation with Mox.
+  """
+
+  @callback sign(user :: Pleroma.User.t(), headers :: map()) :: String.t()
+  @callback signed_date() :: String.t()
+end
diff --git a/lib/pleroma/web/activity_pub/publisher.ex b/lib/pleroma/web/activity_pub/publisher.ex
index 9b37712db..1d8896857 100644
--- a/lib/pleroma/web/activity_pub/publisher.ex
+++ b/lib/pleroma/web/activity_pub/publisher.ex
@@ -26,6 +26,12 @@ defmodule Pleroma.Web.ActivityPub.Publisher do
   ActivityPub outgoing federation module.
   """
 
+  @signature_impl Application.compile_env(
+                    :pleroma,
+                    [__MODULE__, :signature_impl],
+                    Pleroma.Signature
+                  )
+
   @doc """
   Enqueue publishing a single activity.
   """
@@ -125,10 +131,10 @@ defmodule Pleroma.Web.ActivityPub.Publisher do
 
     digest = "SHA-256=" <> (:crypto.hash(:sha256, json) |> Base.encode64())
 
-    date = Pleroma.Signature.signed_date()
+    date = @signature_impl.signed_date()
 
     signature =
-      Pleroma.Signature.sign(actor, %{
+      @signature_impl.sign(actor, %{
         "(request-target)": "post #{path}",
         host: signature_host(uri),
         "content-length": byte_size(json),
diff --git a/test/pleroma/web/activity_pub/publisher_test.exs b/test/pleroma/web/activity_pub/publisher_test.exs
index 8da9df23c..4721b2432 100644
--- a/test/pleroma/web/activity_pub/publisher_test.exs
+++ b/test/pleroma/web/activity_pub/publisher_test.exs
@@ -556,13 +556,14 @@ defmodule Pleroma.Web.ActivityPub.PublisherTest do
   end
 
   describe "prepare_one/1 with reporter anonymization" do
-    test_with_mock "signs with the anonymized actor keys when Transmogrifier changes actor",
-                   Pleroma.Signature,
-                   [:passthrough],
-                   sign: fn %Pleroma.User{} = user, headers ->
-                     send(self(), {:signed_as, user.ap_id})
-                     "TESTSIG"
-                   end do
+    test "signs with the anonymized actor keys when Transmogrifier changes actor" do
+      Pleroma.SignatureMock
+      |> Mox.stub(:signed_date, fn -> Pleroma.Signature.signed_date() end)
+      |> Mox.expect(:sign, fn %Pleroma.User{} = user, _headers ->
+        send(self(), {:signed_as, user.ap_id})
+        "TESTSIG"
+      end)
+
       placeholder = insert(:user)
       reporter = insert(:user)
       target_account = insert(:user)
diff --git a/test/support/data_case.ex b/test/support/data_case.ex
index 304bee5da..e33ab4b34 100644
--- a/test/support/data_case.ex
+++ b/test/support/data_case.ex
@@ -119,6 +119,7 @@ defmodule Pleroma.DataCase do
     Mox.stub_with(Pleroma.StubbedHTTPSignaturesMock, Pleroma.Test.HTTPSignaturesProxy)
 
     Mox.stub_with(Pleroma.DateTimeMock, Pleroma.DateTime.Impl)
+    Mox.stub_with(Pleroma.SignatureMock, Pleroma.Signature)
   end
 
   def ensure_local_uploader(context) do
diff --git a/test/support/mocks.ex b/test/support/mocks.ex
index b26834871..25bcb6472 100644
--- a/test/support/mocks.ex
+++ b/test/support/mocks.ex
@@ -40,3 +40,5 @@ Mox.defmock(Pleroma.Language.LanguageDetectorMock,
 
 Mox.defmock(Pleroma.DateTimeMock, for: Pleroma.DateTime)
 Mox.defmock(Pleroma.MogrifyMock, for: Pleroma.MogrifyBehaviour)
+
+Mox.defmock(Pleroma.SignatureMock, for: Pleroma.Signature.API)