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