> the sort of messaging that's of use to terrorists

It seems increasingly apparent that recruitment and radicalisation are done in the open. After all, political speech is free speech.

Then people either self-radicalise (go on a murder spree without coordinating action with a larger group; e.g. many mass shooters), or they meet up in person. Lots of radicalised people go to the Middle East where they can't be surveilled by western security forces.

There's remarkably little evidence that terrorists are making routine use of strong encryption.