Metadata is the message

Encrypting the body was the easy part. We solved it twice: once badly, in the nineties, and once well, in the last decade. What we did not solve is the envelope.

An envelope tells you who wrote to whom, at what hour, how large the letter was, and how quickly the reply came. From a year of envelopes you can reconstruct a life without ever reading a word of it.

The shape of a conversation

Two people who message each other at 2am every night for a month are telling you something. Two people whose exchange stops abruptly on a Tuesday are telling you something else. Neither of them typed it.

This is why the interesting attacks are no longer on ciphers. They are on timing, on volume, on the fact that your phone talks to one server and not another.

The defenses exist — padding, batching, cover traffic, mixnets — and every one of them costs latency or bandwidth, which is to say it costs the thing users actually notice. That trade is the whole design space.