Good old secret associations and secret meetings. The Italians did it quite well 200 years ago [1]:
> Cardinals Ercole Consalvi and Bartolomeo Pacca issued an edict forbidding all secret societies, to become members of these secret associations, to attend their meetings, or to furnish a meeting-place for such, under severe penalties
It also f.cks with the head with those in power, having secret associations and secret meetings, that is. Having it all out in the open gives them (the authorities) the illusion of control, and, I'd argue, actual control over any significant protest movement. Moving it in the "shadows" is a good tactic because it helps make things blurry and it starts "attaching" doubts to said authorities' monopoly of violence. The authorities will ask themselves: "whom should I punish? where are the revolutionaries? is everyone a revolutionary?", and, as such, that will decrease their legitimacy and their hold on actual power.
Even if you could get that to work someone would take a cellphone to the secret meeting and share a geotagged selfie, people just can't help themselves. Hello secret police!
Or, they'd somehow manage to control themselves, but would only turn off their cellphone just before they got there. It would be easy to figure out the pattern of a bunch of likely instigators heading to the same location.
Or, people would somehow learn to leave their cellphone at home. If anyone's then stopped by the police not having a cellphone in your pocket would be extremely suspicious in a totalitarian dictatorship.
Even then it's not Italy in the 1800s. There's surveillance cameras on every corner, and you're right back to the problem of no signal being the signal.
Nobody in Naples was filing multiple notices in their local papers before they had a normal meeting with their friends on a Tuesday, but that's what everyone's doing via social media. You can tease out suspicious meetings of any non-trivial size as being those without such a signal.
The only way this is going to work for any amount of time is if it's organized like modern terrorist cells, but that's assuming a lot of steps between angry students staging demonstrations and covert operatives.
Yeah, that's a big no-no, I'm starting to realise it more and more. I honestly don't know if there's a solution to it.
I remember leaving some piece of advice about 10 years ago on this forum about how best to attend an anti-government protest without getting into potential trouble afterwards (in the context of these protests [1]). My advice back then was to take a tram/bus to about 2 or 3 stations away from the location of the protests themselves, and then to walk to said location, that way one would have avoided the security cameras located at the metro stations or in some buses back then.
Nowadays, unfortunately, that piece of advice is null because there are cameras actually everywhere (including at the location of the protests), and the government, if it so desires, has access to them almost at will.
Not to mention the advances in machine learning and integrated systems to piece the picture together.
Now you probably won't need to follow the breadcrumb trail of some protestor trying to evade surveillance through the methods you describe, facial recognition will be enough.
And if it isn't completing the puzzle might only take computer time, and not someone in the police forces trying to manually track people down.
The CCP is apparently giddy at the opportunity to help Iran build out these systems, so they're likely state of the art.
Western companies would probably be keen to compete, with the only thing stopping them being the US sanctions. We'll only sell these sort of systems to regimes that respect human rights, like The Netherlands, Romania, and Saudi Arabia.
> Even if you could get that to work someone would take a cellphone to the secret meeting and share a geotagged selfie, people just can't help themselves. Hello secret police!
The book Deep Green Resistance goes into various organizing techniques (agree with them or not, it's a fascinating read on the history of leftist movements with quite a bit in the way of practical techniques), but this is one of the reasons why you need multiple different layers within a group - some core trusted layers, and then what amount to messengers who handle communication with that sort of person, with only what's needed.
And "the silence is the signal" is a reason one really needs to start cultivating these habits in ones social circles of not using social media, not posting everything online, etc. It's long past time to reject the draw of social media as intermediating everything, and either start hosting your own stuff at small scale, or simply not using consumer/networked tech for anything important.
Landlines?