Fix relayable signatures for messages with invalid XML characters

Sometimes messages contain characters that are invalid for XML, but they
are filteres out before creating the XML, otherwise the property would
be empty in the XML.

But for relayables the value is also used for creating the signatures,
so we need to filter the invalid characters earlier, before calculating
the signature.

lib/diaspora_federation/entity.rb

@@ -277,7 +277,7 @@ module DiasporaFederation
 
       case self.class.class_props[name]
       when :string
-        value.to_s
+        value.to_s.gsub(INVALID_XML_REGEX, "\uFFFD")
       when :timestamp
         value.nil? ? "" : value.utc.iso8601
       else
@@ -310,7 +310,7 @@ module DiasporaFederation
     # Create simple node, fill it with text and append to root
     def simple_node(doc, name, value)
       Nokogiri::XML::Element.new(name.to_s, doc).tap do |node|
-        node.content = value.gsub(INVALID_XML_REGEX, "\uFFFD") unless value.empty?
+        node.content = value unless value.empty?
       end
     end
 

spec/lib/diaspora_federation/entities/relayable_spec.rb

@@ -208,6 +208,22 @@ XML
         expect(verify_signature(parent_pkey, parent_author_signature, signature_data)).to be_truthy
       end
 
+      it "computes correct signatures for the entity with invalid XML characters" do
+        expect_callback(:fetch_private_key, author).and_return(author_pkey)
+        expect_callback(:fetch_private_key, local_parent.author).and_return(parent_pkey)
+
+        invalid_property = "asdfasdf asdf💩asdf\nasdf"
+        signature_data_with_fixed_property = "#{author};#{guid};#{parent_guid};asdf<64>asdf asdf💩asdf\nasdf"
+
+        xml = Entities::SomeRelayable.new(hash.merge(property: invalid_property)).to_xml
+
+        author_signature = xml.at_xpath("author_signature").text
+        parent_author_signature = xml.at_xpath("parent_author_signature").text
+
+        expect(verify_signature(author_pkey, author_signature, signature_data_with_fixed_property)).to be_truthy
+        expect(verify_signature(parent_pkey, parent_author_signature, signature_data_with_fixed_property)).to be_truthy
+      end
+
       it "computes correct signatures for the entity when the parent is a relayable itself" do
         intermediate_author = Fabricate.sequence(:diaspora_id)
         parent = Fabricate(:related_entity, author: intermediate_author, local: true, parent: local_parent)