Update follow activity

lib/nulla/activitypub.ex

@@ -349,15 +349,4 @@ defmodule Nulla.ActivityPub do
         )
     )
   end
-
-  @spec follow_accept(Activity.t()) :: Jason.OrderedObject.t()
-  def follow_accept(activity) do
-    Jason.OrderedObject.new(
-      "@context": "https://www.w3.org/ns/activitystreams",
-      id: activity.ap_id,
-      type: activity.type,
-      actor: activity.actor,
-      object: activity.object
-    )
-  end
 end

lib/nulla_web/controllers/inbox_controller.ex

@@ -22,14 +22,14 @@ defmodule NullaWeb.InboxController do
              |> Map.put("domain", URI.parse(remote_actor_json["id"]).host)
            ),
          follow_activity <-
-           Activity.create_activity(%{
+           Activity.activity(%{
              ap_id: follow_id,
              type: "Follow",
              actor: remote_actor.id,
              object: target_uri
            }),
          accept_activity <-
-           Activity.create_activity(%{
+           Activity.activity(%{
              type: "Accept",
              actor: local_actor.ap_id,
              object: follow_activity
@@ -40,12 +40,57 @@ defmodule NullaWeb.InboxController do
              local_actor_id: local_actor.id,
              remote_actor_id: remote_actor.id
            }) do
+      body = Jason.encode!(ActivityPub.follow_accept(accept_activity))
+
+      digest =
+        :crypto.hash(:sha256, body)
+        |> Base.encode64()
+        |> then(&("SHA-256=" <> &1))
+
+      date =
+        DateTime.utc_now()
+        |> Calendar.strftime("%a, %d %b %Y %H:%M:%S GMT")
+
+      host = URI.parse(actor_uri).host
+      request_target = "post /inbox"
+
+      signature_string =
+        """
+        (request-target): #{request_target}
+        host: #{host}
+        date: #{date}
+        digest: #{digest}
+        """
+
+      user = User.get_user(id: local_actor.id)
+      privateKeyPem = user.privateKey["privateKeyPem"]
+
+      private_key =
+        :public_key.pem_decode(local_actor.private_key_pem)
+        |> hd()
+        |> :public_key.pem_entry_decode()
+
+      signature =
+        :public_key.sign(signature_string, :sha256, private_key)
+        |> Base.encode64()
+
+      signature_header =
+        """
+        keyId="#{local_actor.publicKey["id"]}",
+        algorithm="rsa-sha256",
+        headers="(request-target) host date digest content-type",
+        signature="#{signature}"
+        """
+        |> String.replace("\n", "")
+        |> String.trim()
+
       conn
       |> put_resp_content_type("application/activity+json")
-      |> send_resp(
-        200,
-        Jason.encode!(ActivityPub.follow_accept(accept_activity))
-      )
+      |> put_resp_header("host", host)
+      |> put_resp_header("date", date)
+      |> put_resp_header("signature", signature_header)
+      |> put_resp_header("digest", digest)
+      |> send_resp(200, body)
     else
       error ->
         IO.inspect(error, label: "Follow error")