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Car theft using relay attacks has become one of the most common vehicle crimes in northern Europe. Two people with EUR 50 to 100 of off-the-shelf equipment can amplify the signal from a key fob sitting on your hall table, fool your car into thinking the key is present, and drive away in under a minute. No window broken, no alarm triggered, no evidence left behind. The equipment is cheap, widely available, and requires no technical knowledge to use.
A Faraday bag stops this cleanly. You put the key fob inside, the bag blocks the signal, and the relay attack has nothing to amplify. That is the clearest, most documented use case for a Faraday bag. Understanding what it actually does, and what it cannot do, determines whether it belongs in your kit.
This piece covers the physics, the real threat model, how to verify a bag works, and what to look for when buying. Product recommendations are at the end.
What a Faraday bag actually does
A Faraday cage is a conductive enclosure that redirects electromagnetic energy around its contents rather than allowing it to pass through. Michael Faraday described the principle in 1836. The physics has not changed.
A Faraday bag is the same principle in flexible form. The lining is a conductive metallic fabric, usually multiple layers of metallised material woven or laminated together. When a device sits inside the sealed bag, radio signals cannot penetrate the conductive barrier. The device is isolated from the electromagnetic spectrum.
The signals blocked are cellular (2G through 5G), GPS, Wi-Fi at 2.4 and 5 GHz, Bluetooth, and NFC. All of them. The bag does not distinguish between signals you want blocked and ones you would rather leave open. It blocks everything in the frequencies it covers.
A phone in a properly sealed Faraday bag cannot receive calls or messages, cannot be tracked by its carrier, cannot connect to Wi-Fi, and cannot be remotely activated. A key fob inside the bag cannot respond to a relay signal.
Why the attenuation number matters
Signal attenuation is measured in decibels, on a logarithmic scale. Twenty decibels represents a tenfold reduction in signal strength. Forty decibels is a hundredfold reduction. At eighty decibels, the signal reaching your device is ten thousand times weaker than without shielding.
For practical isolation, you need at least 60 to 80 dB across the full frequency range you care about. Below that, signal leakage remains.
This is where cheap bags fail. A Mylar bag or an ESD pouch from an electronics supplier gives you roughly 7 to 9 dB, close to nothing. A metal biscuit tin gives you 28 to 43 dB, inconsistent and dependent on how well the lid seals. A quality Faraday bag made from proper metallised fabric gives you 80 to 110 dB.
At 7 dB, your phone can still receive a tracking ping. At 100 dB, it cannot. The difference is not a matter of degree, it is a matter of whether the bag works at all.
Independent testing by security researcher Matt Blaze, using calibrated RF equipment across the 1 to 6 GHz range, found that many cheap commercial Faraday bags failed at frequencies above 3 GHz. Some failed completely across the range. The bags that held up in his measurements were from a small number of manufacturers who publish their attenuation specifications and have had those specs independently verified.
Who actually needs one
The car key case is straightforward. Relay attacks on keyless-entry vehicles are well documented, growing in frequency, and require only inexpensive consumer equipment to execute. A key fob pouch costs under EUR 30. If you own a keyless-entry car and park where other people pass, the expenditure is practical.
Beyond car keys, the genuine use cases narrow considerably.
Lawyers and advisers who attend depositions or sensitive client meetings sometimes want to carry a phone without the possibility of remote activation during the session. A Faraday bag is faster than powering off and waiting for restart.
Journalists and researchers working with sources who face real risk treat device isolation as a standard precaution during meetings, not a paranoid one.
Anyone crossing a border with a device containing sensitive work product may want the additional layer. Powering the device off provides similar protection in most scenarios, but not all environments are the same.
The honest counterpoint: a Faraday bag blocks RF transmission. It does nothing about data that has already left your device. It does not address software already installed. If your concern is the historical record of your movements, that data has already been collected by your carrier, your operating system, and your apps. Blocking the signal on your commute changes nothing about what was uploaded this morning.
Buy a key fob pouch if you own a keyless car. Consider a phone bag if your work involves real confidentiality obligations. If the appeal is primarily about feeling private rather than solving a specific problem, the bag will not do what you hope.
How to verify it works
The simplest test requires nothing beyond a second phone. Place your phone inside the sealed bag and call it. If the call rings through, the bag is failing. If it goes straight to voicemail, the shielding is working.
For a key fob: stand close to your car, place the fob inside the sealed bag, and try to unlock the door. No response means the bag is doing its job.
A slightly more rigorous test: connect your phone to Wi-Fi before sealing the bag, then check the router’s connected device list from another device. The phone should drop off within seconds.
For anything more serious, Apple’s AirTag provides a simple real-world verification method: place an AirTag inside the sealed bag and check whether an iPhone can detect it nearby. If the bag is working, the AirTag goes dark.
What to look for when buying
Attenuation specification: any manufacturer worth considering publishes this figure. Look for 60 dB minimum across the full frequency range, including above 3 GHz, where many cheaper bags begin to fail. A range figure (70 to 100 dB) is more transparent than a single number without qualification.
Closure: the closure is where most signal leakage occurs. A double-roll closure, where the bag is folded over itself twice before sealing, is the standard for professional-grade products. Single-zip closures and simple velcro strips allow signal bleed at the seam, particularly at higher frequencies.
Size: a bag that is too large for the device creates excess material that must be folded correctly on every use. A missed fold creates a gap. Match the bag to the device you are protecting.
Layers: better bags use two or more conductive layers rather than one. Multiple layers increase attenuation at high frequencies and provide redundancy if one layer is compromised.
What to buy
Mission Darkness makes the most widely deployed Faraday phone bags in professional settings. Standard pouches achieve 60 to 80 dB average RF attenuation; premium models including the Window Pouch and FreeRoam average 90 dB. The TitanRF fabric used in Mission Darkness products is certified to MIL STD 188-125 and IEEE 299-2006. Their basic non-window phone pouch is available in Europe through authorised distributors at around EUR 25 to 30. The Window Pouch variant, which adds a transparent panel for screen visibility, recorded 96 to 110 dB in independent testing by security researcher Matt Blaze across the full 1 to 6 GHz range. Mission Darkness products are used by law enforcement forensics teams in Europe and the US.
Disklabs is a UK-based manufacturer that has built Faraday products for digital forensics since 2004. Their Phone Shield range (models PS1 to PS3) carries the UK Police Approved Secured by Design certification. EU distribution is available through their authorised partner network.
EDEC makes the OffGrid Pouch, which recorded 89 to 111 dB in Blaze’s study, the highest consistent result across the 1 to 6 GHz range in his measurements. The brand ships globally from offgrid.co at USD 32 to 40 (approximately EUR 29 to 37 before shipping). No EU-based distributor is currently listed; for EU buyers the Mission Darkness Window Pouch offers comparable independently verified performance at confirmed EU pricing.
For key fobs specifically, any Mission Darkness key fob pouch or equivalent from Disklabs does the job. The threat model for a car key is simpler than for a phone, and the basic pouches are built exactly for this use.
If you are buying for a keyless car and nothing else, the EUR 25 Mission Darkness basic pouch is sufficient. If you need reliable phone isolation for professional or travel use, spend the extra and get a model with a published, independently verified attenuation specification.
The broader point
A Faraday bag is a lined pouch. It has no software, no firmware, no subscription, and no manufacturer update that can change what it does. It works exactly the same in ten years as it does today. The threat it addresses most clearly, relay attacks on keyless vehicles, is growing as keyless entry becomes the default in the new car market. The equipment to execute those attacks is getting cheaper.
This is a category of tool with no digital dependency and no ongoing cost after purchase. It does one thing, it does it well, and it requires nothing from you to keep working.
The broader question of how much wireless connectivity you want in your daily carry is covered in our guide to dumbphone alternatives. The case for owning physical tools that cannot be switched off remotely, updated into something different, or tied to a service contract is in The Subscription Trap.