activitypub: verify remote http signature digests by recomputing the digest and replacing the digest header

lib/pleroma/plugs/digest.ex

@@ -0,0 +1,10 @@
+defmodule Pleroma.Web.Plugs.DigestPlug do
+  alias Plug.Conn
+  require Logger
+
+  def read_body(conn, opts) do
+    {:ok, body, conn} = Conn.read_body(conn, opts)
+    digest = "SHA-256=" <> (:crypto.hash(:sha256, body) |> Base.encode64())
+    {:ok, body, Conn.assign(conn, :digest, digest)}
+  end
+end

lib/pleroma/plugs/http_signature.ex

@@ -19,6 +19,8 @@ defmodule Pleroma.Web.Plugs.HTTPSignaturePlug do
 
     cond do
       signature && String.contains?(signature, user) ->
+        # set (request-target) header to the appropriate value
+        # we also replace the digest header with the one we computed
         conn =
           conn
           |> put_req_header(
@@ -26,6 +28,14 @@ defmodule Pleroma.Web.Plugs.HTTPSignaturePlug do
             String.downcase("#{conn.method}") <> " #{conn.request_path}"
           )
 
+        conn =
+          if conn.assigns[:digest] do
+            conn
+            |> put_req_header("digest", conn.assigns[:digest])
+          else
+            conn
+          end
+
         assign(conn, :valid_signature, HTTPSignatures.validate_conn(conn))
 
       signature ->