Add plug to validate signed http requests.

lib/pleroma/plugs/http_signature.ex

+defmodule Pleroma.Web.Plugs.HTTPSignaturePlug do
+  alias Pleroma.Web.HTTPSignatures
+  import Plug.Conn
+
+  def init(options) do
+    options
+  end
+
+  def call(conn, opts) do
+    if get_req_header(conn, "signature") do
+      conn = conn
+      |> put_req_header("(request-target)", String.downcase("#{conn.method} #{conn.request_path}"))
+
+      assign(conn, :valid_signature, HTTPSignatures.validate_conn(conn))
+    else
+      conn
+    end
+  end
+end

lib/pleroma/user.ex

@@ -376,4 +376,14 @@ def delete (%User{} = user) do
 
     :ok
   end
+
+  def get_public_key_for_ap_id(ap_id) do
+    with %User{} = user <- get_cached_by_ap_id(ap_id),
+         %{info: %{"magic_key" => magic_key}} <- user,
+         public_key <- Pleroma.Web.Salmon.decode_key(magic_key) do
+      {:ok, public_key}
+    else
+      _ -> :error
+    end
+  end
 end

lib/pleroma/web/http_signatures/http_signatures.ex

 # https://tools.ietf.org/html/draft-cavage-http-signatures-08
 defmodule Pleroma.Web.HTTPSignatures do
+  alias Pleroma.User
+
   def split_signature(sig) do
     default = %{"headers" => "date"}
 
@@ -18,7 +20,18 @@ def split_signature(sig) do
   def validate(headers, signature, public_key) do
     sigstring = build_signing_string(headers, signature["headers"])
     {:ok, sig} = Base.decode64(signature["signature"])
-    verify = :public_key.verify(sigstring, :sha256, sig, public_key)
+    :public_key.verify(sigstring, :sha256, sig, public_key)
+  end
+
+  def validate_conn(conn) do
+    # TODO: How to get the right key and see if it is actually valid for that request.
+    # For now, fetch the key for the actor.
+    with actor_id <- conn.params["actor"],
+         {:ok, public_key} <- User.get_public_key_for_ap_id(actor_id) do
+      validate_conn(conn, public_key)
+    else
+      _ -> false
+    end
   end
 
   def validate_conn(conn, public_key) do