Faraday bags: what they block and when you need one
A Faraday bag cuts a phone or key fob off from every radio signal, as long as it is built and sealed properly.
A Faraday bag is a pouch lined with conductive metal fabric that stops radio signals from reaching or leaving whatever is inside: cellular, Wi-Fi, Bluetooth, GPS, NFC, and key fob signals. Use one when you want a phone that cannot be tracked or contacted without powering it off, or to stop thieves relaying your car key's signal. Test any bag you buy by sealing your phone inside and calling it. If it rings, the bag is not doing its job.
What a Faraday bag does
Radio waves cannot pass through a closed shell of conductive material. A Faraday bag uses that by lining a pouch with metal-coated fabric on every side. Close it properly and the device inside cannot hear a cell tower, join Wi-Fi, pair over Bluetooth, get a GPS fix, or answer an NFC or RFID reader. Nothing it sends gets out either.
The seal is the weak point: a corner left open or a fold done wrong lets signal through. Mission Darkness bags, for example, close by rolling the top down twice before you press a full-width Velcro strip shut.
When you need one
Most people never need one. A bag makes sense in these cases:
- Your phone, when you want it unreachable but still able to switch on in seconds. A powered-off phone is not always silent: Apple says an iPhone with Find My turned on may still be found through the Find My network for up to 24 hours after it is powered off.
- Your car key fob, if your car has keyless entry and push-button start. Thieves use relay devices that pick up the fob's signal from a distance and pass it to the car. In 2016 the National Insurance Crime Bureau bought one and found it unlocked many cars and trucks with keyless entry and push-button start, and could start the majority of them.
- Contactless cards and passports, if you want to stop them answering an RFID or NFC reader you did not choose.
- Travel, protests, and meetings where you want no record of where your phone was.
What to look for when you buy one
A 2016 study from Auckland University of Technology tested several phone Faraday bags sold online against cellular, Wi-Fi, and Bluetooth, and found that most did not perform as their vendors promised. Judge a bag by the details its seller commits to, and test it yourself before you rely on it.
Mission Darkness, from MOS Equipment, sells to forensic investigators as well as the public, and says agencies order its basic phone bag in bulk. Its Non-Window Faraday Bag for Phones costs $23, uses two layers of its TitanRF fabric on all interior sides, closes with a double roll and Velcro, and lists shielding from low MHz up to 40 GHz. The company says the fabric is lab tested and certified to MIL-STD 188-125 and IEEE 299-2006. A magnetic-closure version costs $48 and a version with a see-through shielded window costs $52.
| Feature | Why it matters |
|---|---|
| Two or more layers of shielding fabric | More shielding between the device and the outside, and a backup if one layer is damaged. |
| Roll-top closure folded at least twice | The closure is where signal leaks. A single flap is the weakest design. |
| Room for your phone in its case | A bag you have to force shut will not seal. |
| Stated frequency range and test standard | A seller that names a range and a standard has given you something to check. "Blocks all signals" alone tells you nothing. |
| Window (optional) | Lets you see the screen while sealed. Handy, but it costs more. |
How to test a Faraday bag at home
- 1Turn on your phone with cellular, Wi-Fi, and Bluetooth all enabled.
- 2Put it in the bag and seal it exactly as the instructions say, including every fold.
- 3Wait a minute, then call the phone from another phone. It should go straight to voicemail.
- 4Send it a message on Wi-Fi only, such as an iMessage or Signal message from a device on the same network. It should not arrive until you take the phone out.
- 5For a key fob, seal the fob in the bag, stand next to your car, and try the door handle. It should stay locked.
- 6Repeat the test every few months. You cannot see damage to the shielding, so a test is the only way to know the bag still works.
A call that does not ring proves the cellular signal was blocked. It says nothing about Wi-Fi or Bluetooth, which is why you also send the Wi-Fi message.
Can you make a DIY Faraday bag?
Yes, in a pinch. The same 2016 study tested tin foil and a tin can alongside the bags and concluded that common materials at hand may do the job in an urgent situation. Any improvised shield has to enclose the phone with no gaps. Foil tears and creases the first time you use it, and you cannot tell it has failed without testing it each time.
For a key fob that lives by your front door, a closed metal box works and you can test it with the door-handle trick above. For a phone you carry, buy a proper bag. At $23 for a well-built one, the DIY savings are small.
Do you need a Faraday bag for your hardware wallet?
No. A hardware wallet protects your bitcoin by keeping the seed phrase off networked devices, and shielding the signing device adds nothing to that. A metal seed backup is a passive plate with no electronics for a pulse or radio to affect. If you want a signer that never uses a radio at all, look at air-gapped wallets, which talk to your computer through QR codes or microSD cards.
For a bitcoiner, the phone is where a bag can help. If you worry about someone tracking where you go, such as to and from a Bitcoin meetup, a phone sealed in a bag is not reporting its location to anyone. Keeping your home address off data broker sites matters more, and my personal information removal guide covers that.
Frequently asked questions
What does a Faraday bag do?
It blocks radio signals from reaching or leaving the device inside. A phone in a sealed Faraday bag cannot connect to cell towers, Wi-Fi, Bluetooth, or GPS, and a key fob inside cannot be read or relayed.
Do Faraday bags work for key fobs?
Yes, if the bag seals properly. Relay attacks work on cars with keyless entry and push-button start by capturing the fob's signal from nearby. A sealed bag stops the fob from answering. Test it by standing next to the car and pulling the door handle with the fob sealed inside.
How do I know if my Faraday bag works?
Seal your phone inside and call it. It should go straight to voicemail. Then send it a Wi-Fi message from a device on the same network, which should not arrive until you open the bag. Retest every few months, because a test is the only way to spot damage to the shielding.
How do you make a Faraday bag?
Wrap the device in several layers of aluminum foil with no gaps, or seal it in a metal tin with a tight lid. A 2016 study that tested tin foil and a tin can concluded improvised materials may work in an urgent situation, but foil tears easily, so test it with a phone call every time.
Is a Faraday bag better than turning my phone off?
Usually, if tracking is your worry. Apple says an iPhone with Find My on may still be found through the Find My network for up to 24 hours after it is powered off. A sealed bag stops the signal whether the phone is on or off.