Currently, the NSA can tap into a broad range of communications, but have no means to compel communications to be in a form they can monitor. This is likely to change; after all, they will need to be able to hunt down those involved in, or providing support to, terrorist groups like Black Lives Matter and Friends Of The Earth, not to mention the President's extensive list of enemies. As such, it is quite likely that, at some point during Trump's first year, end-to-end encrypted messaging systems will be required to provide real-time plaintext to the security services. (Things have already been moving slowly in this direction, and will only accelerate under a president who has expressed admiration for autocrats and a brutishly Hobbesian view of how power works.)
Similar laws are already in force in more established autocracies such as Russia and Turkey. The difference is that American companies, subject to American law, provide many of the communications systems used worldwide, such as Apple iMessage, WhatsApp and Signal. These are likely to be compelled to provide the US homeland-security authorities with the plaintext of all messages coming through them, in real time, and to make whatever changes are necessary to their architecture to achieve this.
With iMessage, this would be theoretically easy to do. iMessage messages are encrypted from end to end, so Apple have no means of reading them, but each message is encrypted several times with the public keys of each of the recipients' devices (i.e., if you're sending one to someone with an iPhone and an iPad, your iMessage client will encrypt it with the public keys of both of their devices). Once they are legally compelled to do so, Apple could just quietly add an extra key, whose private key is held by the NSA iMessage ingestion gateway. Given that the entire iMessage system is closed-source and completely under Apple's control, Apple could push this to all users, without worrying about rogue clients that feed the NSA junk.
WhatsApp, Facebook Messenger Google Allo and so on are also proprietary systems, and could be made compliant in a similar fashion. Granted, WhatsApp and Messenger use the open-source Signal protocol for end-to-end-encrypted messages, but this algorithm sits entirely embedded within the app; there is no guarantee that the app actually uses it, or that it doesn't send a carbon copy of the message to a machine in Utah, in compliance with the law. The fine print could be amended on the website to not actually promise that your message is secret from everyone, including the authorities.
The Signal app itself appears to be a somewhat tougher nut to crack in practice; it's open-source and publicly documented, to the point where any third party could download the source code, examine it minutely, and then, once satisfied, build their own client and use that to communicate securely. However, the creator, Open Whisper Systems is a US company, subject to US laws. Legally, Giuliani or Arpaio or whoever ends up in charge of Homeland Security could billet a team of NSA engineers at their office, with the authority to dictate changes to code and architecture, all covered by a blanket gag order. The question now is how they could go about this:
Were Open Whisper Systems to preemptively move abroad to a more privacy-friendly jurisdiction (and Germany is a good one, for obvious reasons) before Trump's inauguration, it may complicate things more. Forcing an established app with a large user-base out of the App Store would be a lot harder than forcing an underground fork of an app out. This would involve all officers involved in running the company moving out of US jurisdiction, and potentially avoiding flights going to the US, UK or Russia.
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