I'd just given a talk at a conference and was having a nice chat with a young man who was doing similar work and wanted to stay in touch.

"Great, just give me your Signal number," he said.

I hesitated. I've been using Signal for several years, since it was TextSecure. As the premier end-to-end encrypted messaging app, it's by the far the most trusted app of its kind in my circles, and although it's been slow to catch up to WhatsApp and other tools when it comes to fancy features, I use just as much among friends.

But Signal—as well as WhatsApp and Viber—require you to register with and use your phone number as an identifier. What this means practically is that when I meet someone with whom I wish to connect on one of these apps, I have to give them my phone number for them to be able to message me.