Sen. Wyden is trying to get to the bottom of this:

In a statement, Apple said that Wyden’s letter gave them the opening they needed to share more details with the public about how governments monitored push notifications.

“In this case, the federal government prohibited us from sharing any information,” the company said in a statement. “Now that this method has become public we are updating our transparency reporting to detail these kinds of requests.”...

One of the hazards lies in the fact that, for some of the programs, downstream crop reversals are possible for viewers or readers of the document, not just the file’s creators or editors. Official instruction manuals, help pages, and promotional materials may mention that cropping is reversible, but this documentation at times fails to note that these operations are reversible by any viewers of a given image or document.

[…]

Uncropped versions of images can be preserved not just in Office apps, but also in a file’s own metadata. A photograph taken with a modern digital camera contains all types of metadata. Many image files record text-based metadata such as the camera make and model or the GPS coordinates at which the image was captured. Some photos also include binary data such as a thumbnail version of the original photo that may persist in the file’s metadata even after the photo has been edited in an image editor...

WhatsApp uses the Signal encryption protocol to provide encryption for its messages. That was absolutely a visionary choice that Brian and his team led back in the day ­- and big props to them for doing that. But you can’t just look at that and then stop at message protection. WhatsApp does not protect metadata the way that Signal does. Signal knows nothing about who you are. It doesn’t have your profile information and it has introduced group encryption protections. We don’t know who you are talking to or who is in the membership of a group. It has gone above and beyond to minimize the collection of metadata...

Most security schemes facilitate the coercive processes of an attacker because they disclose metadata about the secret data, such as the name and size of encrypted files. This allows specific and enforceable demands to be made: “Give us the passwords for these three encrypted files with names A, B and C, or else…”. In other words, security often focuses on protecting the confidentiality of data, but lacks deniability.

A scheme with deniability would make even the existence of secret files difficult to prove. This makes it difficult for an attacker to formulate a coherent demand: “There’s no evidence of undisclosed data. Should we even bother to make threats?” A lack of evidence makes it more difficult to make specific and enforceable demands...

I don’t see a lot of surprises in the document. Lots of apps leak all sorts of metadata: iMessage and WhatsApp seem to be the worst. Signal protects the most metadata. End-to-end encrypted message content can be available if the user uploads it to an unencrypted backup server.

EDITED TO ADD (12/13): Here’s a more legible copy of the text.