Faraday Bag

A Faraday bag is a portable, flexible version of a Faraday cage: a pouch lined with conductive material that blocks electromagnetic and radio-frequency signals from reaching the device sealed inside. The principle dates to Michael Faraday's 1836 demonstration that a conductive enclosure redistributes external charge across its surface, leaving the interior shielded. In a bag, layered conductive fabrics — typically copper, nickel, or aluminum composites — reflect and absorb incoming RF so that a phone, key fob, or hardware device inside can neither transmit nor receive.

What it isolates

A properly constructed bag suppresses cellular, Wi-Fi, Bluetooth, GPS, and RFID/NFC signals. That has several practical uses: stopping a smartphone from being tracked or remotely accessed, blocking relay attacks against keyless-entry car fobs, and isolating a device suspected of compromise so it cannot phone home. No bag is perfect — shielding reduces signal to the point that communication fails rather than guaranteeing zero leakage — and effectiveness depends on the seal and the materials.

Relevance to self-custody

For Bitcoiners, a Faraday bag is a modest but real OPSEC tool. A signing phone or air-gapped device can be transported without leaking location data, and an everyday phone placed in a bag during a sensitive transaction cannot be used to geolocate you or exfiltrate data over a wireless channel. It is not a substitute for an air gap — a truly offline signing device has no radios to shield — but it complements one by preventing accidental or covert wireless activation.

Think of a Faraday bag as one item in a layered defense; its place is best decided within your Threat Model and your broader OPSEC practice.