Apple Change Causes Scramble Among Private Messaging App Makers
This article doesn&#x27;t quite explain what&#x27;s going on as much as I would like, so maybe I can add some more detail.<p>The key problem from an app developer&#x27;s point of view is that the iOS push notification API doesn&#x27;t just let a server send an encrypted blob of data to a user&#x27;s app, and have the app handle it locally, deciding whether it needs to display a notification to the user.<p>What the iOS push notification API <i>does</i> let you do is send an encrypted notification to a user&#x27;s app, and then if the user is going to see this notification, have the app locally intercept that, and decide what exactly to display. So for an encrypted messaging service, you can send the encrypted message, and have the app locally decrypt it.<p>The difference is that the push notification API doesn&#x27;t let you send push notification data that won&#x27;t be visible to the end user. So to use this API, your server needs to know that a user just received a message. It can&#x27;t send out something like an encrypted blob that could be a received-message or could be a different sort of encrypted message like a read receipt, and let the app decide locally. This isn&#x27;t optimal for a private messaging app - ideally the server wouldn&#x27;t have to be able to distinguish your received messages from other metadata. This is similar to phone call metadata leaking, where even if someone can&#x27;t eavesdrop on the content of your phone calls, you still don&#x27;t want them knowing precisely when you made a phone call.<p>So one weird thing about iOS is that the VOIP APIs did give you ways to silently send an encrypted blob of data from a server to an app. This functionality would be logical in the push notification API, but the push notification API didn&#x27;t support it and the VOIP API did, so developers started using the VOIP API for it.<p>It makes sense that Apple wouldn&#x27;t want the VOIP API used in this way. But secure messaging apps have a legitimate reason to silently transfer encrypted data from server to application. Making this impossible will weaken the privacy that apps like Signal can offer, not improve it, because Signal will have to know a little bit more about your messaging behavior in order to send you push notifications.