Add handling of Linked Data Signatures in payloads (#4687)

* Add handling of Linked Data Signatures in payloads

* Add a way to sign JSON, fix canonicalization of signature options

* Fix signatureValue encoding, send out signed JSON when distributing

* Add missing security context

app/helpers/jsonld_helper.rb

@@ -17,6 +17,11 @@ module JsonLdHelper
     !json.nil? && equals_or_includes?(json['@context'], ActivityPub::TagManager::CONTEXT)
   end
 
+  def canonicalize(json)
+    graph = RDF::Graph.new << JSON::LD::API.toRdf(json)
+    graph.dump(:normalize)
+  end
+
   def fetch_resource(uri)
     response = build_request(uri).perform
     return if response.code != 200
@@ -29,6 +34,14 @@ module JsonLdHelper
     nil
   end
 
+  def merge_context(context, new_context)
+    if context.is_a?(Array)
+      context << new_context
+    else
+      [context, new_context]
+    end
+  end
+
   private
 
   def build_request(uri)