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/workers/activitypub/distribution_worker.rb

@@ -12,7 +12,7 @@ class ActivityPub::DistributionWorker
     return if skip_distribution?
 
     ActivityPub::DeliveryWorker.push_bulk(inboxes) do |inbox_url|
-      [payload, @account.id, inbox_url]
+      [signed_payload, @account.id, inbox_url]
     end
   rescue ActiveRecord::RecordNotFound
     true
@@ -28,11 +28,15 @@ class ActivityPub::DistributionWorker
     @inboxes ||= @account.followers.inboxes
   end
 
+  def signed_payload
+    @signed_payload ||= Oj.dump(ActivityPub::LinkedDataSignature.new(payload).sign!(@account))
+  end
+
   def payload
     @payload ||= ActiveModelSerializers::SerializableResource.new(
       @status,
       serializer: ActivityPub::ActivitySerializer,
       adapter: ActivityPub::Adapter
-    ).to_json
+    ).as_json
   end
 end