I feel I should probably start out with a disclaimer that these opinions stated here are my own and may not be universally applicable, etc. I'm not an expert in this field or a lawyer. I hope this post is helpful, but please take into consideration that I am just one person writing opinions on the internet.

If you attend a protest in the USA, it is probably in your best interest to hide your identity at the protest (e.g., wear a mask to resist facial recognition, wear nondescript clothing, cover any identifying marks or tattoos) and conceal the fact that you were there. After the Ferguson protests, multiple activists were killed or (according to police) died from "suicide" (content warnings for deaths, including the methods of death). The FBI has a long history of treating political dissent as terrorism, and the DEA has been authorized to "conduct covert surveillance" on people participating in protests over the murder of George Floyd (onion mirror). So let's assume you don't want the people in charge to know you were at a protest.

I've tried to rank your options from best to worst here, and provide additional considerations while threat modeling. This issue is not "one size fits all" because different people have different circumstances.

1. Don't Carry Your Cell Phone

From a strictly privacy perspective, this is your best option. All cell phones (not just smart phones) uniquely identify themselves to nearby cell towers, allowing the cell company (or anyone who sets up an IMSI-catcher) to track who goes where or identify the members of a crowd.

Smart phones have a host of other privacy issues on top of this. (See Protecting Your Mobile Privacy down below.) If you can safely do without your phone, that's your best option. Ideally leave the phone on but at home. This way it will seem like you're home, not at the protest. If your phone turns off right before the protest and turns on after it, this metadata could be used to infer what you were doing. It's best to make it seem like nothing is out of the ordinary.

Some possible alternatives:

Carrying a Burner Phone

If you need a cell phone at a protest (for emergencies or to coordinate with a group if you get separated maybe) but don't want to carry your own, a burner phone might be an option for you. But you need to understand the implications of a burner phone.

The point of a burner phone is that it's not associated with you. So if you're taking this route, you need to be sure not to associate it with yourself. Here are some steps to take:

• Buy the phone and a card for phone service anonymously.

Pay with cash in-person. If you want to be really careful you could take steps like not carrying any tracking devices (e.g., another phone) with you when you go to the store. Try to cover visually identifying characteristics when you go so you can't be identified by surveillance cameras. You could even have someone else buy it for you to further obfuscate the trail.

• Don't turn the burner phone on near your house or your other phone. Ever.

You're trying to avoid the burner phone being associated with you. It doesn't matter if the phone was purchased anonymously if it spends 90% of its time on at your home. Remember, phone companies know where your phone goes. If you can, take the battery out of the phone when it's off in case it doesn't really turn off when you hold the power button.

• Don't publicly associate the phone number with yourself.

Again, the point is for this phone to not be associated with you. You might need some people to know the number for emergency purposes. Tell those people in-person if possible. Use end-to-end encryption if you must transmit it digitally. Don't use insecure communications to transmit it. Don't post it online.

Walkie-Talkies

If you would use your phone to communicate with a group at a protest, a handheld transciever might work for you instead. A major benefit of walkie-talkies over cell phones is that walkie-talkies don't leave records of where they are and when.

Remember...

• The actual communication is insecure.

Remember that anyone on the same signal can listen in, so don't say anything sensitive, use code names, etc. (Establish these code names securely. In general just don't plan protests using insecure communications.)

• They only work in short range.

Walkie-talkies won't work for you if you anticipate getting too far away from your group.

Camera

If you're using your phone to record the protest, first please make sure you're doing it responsibly. Don't out your fellow protesters. Don't record people's faces. If you do, censor them (as completely as possible... a black square over someone's face is a lot harder to undo than a gradient blur). Don't give those photos to Facebook or other surveillance machines.

That said, if you have a good reason to bring a camera (recording police is an excellent reason, for instance), a digital or film camera that doesn't connect to any networks is going to be a lot more private than a phone.

Tablet

If you need a smart phone for something like the secure messenger Briar which can be used short-range even without internet access, you might be better off with a tablet that can't connect to cell towers rather than a cell phone. See Protecting Your Mobile Privacy below as well if you take this route.

2. Keep Your Cell Phone Off

Maybe you need your cell phone in case of emergency. That's completely reasonable! But how can you maintain your privacy while carrying a tracking device with you? Keep it off! Here are three things you might be able to do (perhaps in some combination):

Put your phone in a Faraday shield.

A Faraday shield blocks electromagnetic fields, so it will prevent your phone from talking to the outside world (such as cell towers, Wi-Fi access points, Bluetooth devices). Some companies make small bags that act as Faraday shields for your phone, but you can also make your own.

Take out your phone's battery.

If you can take out the phone's battery, you can be more confident (not fully confident - it could have a hidden extra battery) that your phone is really, truly off.

Turn off your phone.

If you can't physically remove the battery, turning the phone off is the best you can do. Make sure it's off off, not just on but with a locked screen.

I think the best strategy is to do as many of these things as possible. Turn off your phone and make sure it's off by taking out the battery. Then put it in a Faraday shield, so that just in case it's still somehow on, it can't talk to the outside world.

Bear in mind as well that while on, your phone is always communicating with cell towers, so a change in behavior like turning it off before the protest and turning it on afterwards might draw suspicion. Also make sure you turn the phone off before you go near the site of the protest. If you go to the site of the protest then turn off your phone, it will still be known that you were there.

3. Protecting Your Mobile Privacy

Maybe you really do need your phone (or a tablet) on and out. There are still some things you can do to protect your privacy.

Turn on airplane mode if you can.

Note that airplane mode may not fully shut down cell tower connectivity. There's a reason this falls under #3 on the list, not #2. That said, it's definitely better than not trying.

Turn off Wi-Fi and Bluetooth if you don't need them.

Generally, you want to reduce connectivity as much as possible. Anything that can connect to your phone, or anything your phone tries to connect to might be a threat.

Turn off location services.

Turn off location services if you can. Also remove app-level location permissions from any apps that don't need it. ESPECIALLY any surveillance apps (like anything made by Google or Facebook).

Turn of Wi-Fi and Bluetooth scanning.

Some phones have settings that scan for Wi-Fi and Bluetooth networks even if Wi-Fi and Bluetooth are turned off. Check for these and turn them off.

Encrypt your phone.

Enable full-disk encryption on your phone and use a strong password. Here's a recent guide on how to encrypt smart phones. This way if you're arrested, law enforcement won't be able to access the data on your device. Note: Full-disk encryption only works for you if the device is off. (You have to decrypt it to use it.) So turn off your phone if you feel it's in particular danger of being confiscated.

Turn off biometrics.

If you use your fingerprint or scan your face to unlock your phone, turn this functionality off before the protest and switch to a strong passcode instead. Police can (legally or not) hold your phone up to your face or press your finger to it, and you may have more protections under the law to refuse to divulge a password than if you refuse to unlock your device with biometrics.

Remove unnecessary data and apps.

Back up and remove any data and apps you don't need on your device during the protest. You can add them back later, but this will minimize how much data can be extracted from your device if it's confiscated and reduce your attack surface.

More Resources

• PCMag - How to Lock Down Your Phone for a Protest
• EFF Surveillance Self-Defense - Attending a Protest
• EFF - A Quick and Dirty Guide to Cell Phone Surveillance at Protests
• CNet - How to maintain your digital privacy at protests

I don't necessarily agree with everything these sources say, but I think they are generally useful and will give you things to consider when threat modeling.

They may recommend apps like WhatsApp (which is owned by Facebook) and Signal (which has various issues including centralization, requirement to use a phone number, and use of Google services and Amazon infrastructure). I would recommend instead something like Briar, XMPP with OMEMO (Conversations is an excellent Android client available at no cost on F-Droid) or Matrix (see the Riot client). All of these protocols have clients available on F-Droid if you're an Android user.

Closing Words

Stay safe, and don't let Big Brother know what you're up to.