Some Thoughts on Faraday Bags and Operational Security

I recently took a trip to Moscow to interview National Security Agency whistblower Edward Snowden about operational security. In the article I published on The Intercept, I mentioned that I used a faraday bag.

Our first meeting would be in the hotel lobby, and I arrived with all my important electronic gear in tow. I had powered down my smartphone and placed it in a “faraday bag” designed to block all radio emissions.

Since I published my interview, many people have asked me for more information about this faraday bag — which product did I get, what does it protect against, how does it work? So here are some quick thoughts on the topic.

What are faraday bags?

Faraday bags, or more generally, faraday cages, are named after the brilliant scientist Michael Faraday. If you’re a nerd, I recommend watching the 10th episode of Neil DeGrasse Tyson’s Cosmos to learn more about him.

They’re made of a material that blocks electric fields from passing through it. Smartphones have a ton of different built-in radios: GSM, CDMA, 2G, 3G, 4G, wifi, bluetooth, NFC, GPS, and others. Each of these communicates wirelessly by sending and receiving information through the air in the form of radio emissions on different frequencies. If you put your smartphone inside of a faraday bag, it might be listening for incoming radio emissions, but none will reach it, and it might be attempting to communicate to the outside, but all of its messages will fail to penetrate the bag.

Which one should I buy?

I bought my faraday bag on Amazon. I didn’t have a particular product in mind; I basically just read reviews and got a nice one that was a little on the bigger side so that I could fit multiple phones, and also my passport (which has an RFID chip, which also communicates using radio emissions).

It doesn’t particularly matter which product you choose, but make sure that you test it after you get it to confirm that it works. Testing it is easy enough. Put your phone inside the bag, and then use another phone to try calling it. If your phone rings, it doesn’t work. You can also test data in a similar fashion — try sending yourself a notification over data (like a Facebook message) and see if your phone receives it while it’s inside the bag.

Why might this be useful for operational security?

First, assume that your smartphone is hacked and that the attacker can utilize all of its radios and sensors. Also assume that your attacker can get information from third party companies, such as your cell phone carrier, which will know your location and be in a position to eavesdrop on much of your phone usage.

Smartphones are crazy useful, so it sucks to not use them just because you can’t trust them. Instead, you can use faraday bags to selectively keep certain information away from your phone, despite the fact that you can’t trust it.

If you carry your phone around with you, your attacker gets to learn your location. From when you turn on your phone after your airplane lands, to taking public transit or a taxi to your hotel, to going out for dinner or drinks, or anything else — the attacker can see your exact location.

So if you want to have a meeting without revealing to your attacker where this meeting is taking place (your attacker can likely infer who you’re meeting with based on what other phones are in the same location), but you don’t want to leave your phone in your hotel room, you can safely bring it with you inside a faraday bag, because your phone itself won’t be able to determine its location. It may try to, but those signals won’t penetrate the material of the bag.

Your phone might be spying on you in other ways, too. It has a microphone, so it could be listening to your conversations and streaming them back to your attacker over the internet, or using some other wireless technology like wifi or bluetooth. Keeping your phone in a faraday bag will prevent your phone from communicating at all.

But here’s an important caveat: Your phone could be listening to your conversations and storing them on disk, waiting for an internet connection. As soon as you take your phone out of your faraday bag, it can use the internet to upload recorded audio to your attacker.

Faraday bags block electric fields, but they don’t block sound. If you don’t want your phone to overhear a conversation, just putting it in a faraday bag isn’t enough. You also need to put it out of earshot. Put it in a different room, muffle it under some pillows, or put it in the refrigerator.

20 thoughts on “Some Thoughts on Faraday Bags and Operational Security

  1. NR

    (Also a fan of Zero History, btw)

    Whilst I get the Faraday bag / pouch re. RFID chips in a passport… is there an assumption here that – for a compromised phone – “off” is not necessarily “OFF”?

    I.e. Why not just power down? Or is this a tech double-bagging for surety?

    1. Micah Lee Post author

      Yeah. While it’s very unlikely, it is in fact possible that when you power your phone off some malware displays a video for you that looks exactly like you’re powering it off, but is still able to use your sensors. Removing the battery is also another way to be sure.

  2. neddy

    Please, please, please learn the difference between “it’s” and “its” — because not knowing (or caring) makes you look illiterate.

  3. gaiko

    Hi, this is marginally related to compromised phones, I noticed you are a Qubes fan, have you taken a look at “Secure Spaces” the isolated android rom? Would really be curious to hear your thoughts.

  4. Tschernobyl

    Scanning my wlan earlier I’ve noticed that the microwave does not disturb it. Inspired by this post, I unplugged my microwave and put one of my phones in it. and yes, a modern microwave oven is a faraday cage. I also suspect that it has similar muffling capabilities as the fridge, though I have not tested this myself.

  5. Josh B.

    Our Faraday bags and sleeves have double the shielding material and are guaranteed to block ALL signals to and from your wireless devices.

    We have had them tested at two different, independent labs, and there are positive reviews of our products all over the internet.

    Free shipping to US customers and greatly discounted international shipping as well.

    1. Babe Brown

      Josh B., but you don’t say what company you are from, so that if we are interested, we could check it out.

  6. Michael

    Just as a note regarding the cons of a Faraday bag, there is an interesting example within one the Tom Clancy Campus Novels. The Agency is collecting all the meta data on their employee’s mobile phones, they notice that it went to a certain area within the city and then stopped moving, at which time a new sim was activated. I believe, without doing the research wasn’t this also mentioned in Snowden’s stuff that it isn’t just the tracking of mobiles but also the mobiles that have been turned off (or in this case placed in a F/Bag), so if two mobiles are turned off at the same times some one is up to something.


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