Why Snowden Asked Visitors in Hong Kong to Refrigerate Their Phones

Before a dinner of pizza and fried chicken late Sunday in Hong Kong, Edward J. Snowden insisted that a group of lawyers advising him in the Chinese territory “hide their cellphones in the refrigerator of the home where he was staying, to block any eavesdropping,” as my colleague Keith Bradsher reported.

Why a refrigerator? The answer does not, as some might assume, have anything to do with temperature. In fact, it does not matter particularly if the refrigerator was plugged in. It is the materials that make up refrigerator walls that could potentially turn them into anti-eavesdropping devices.

“What you want to do is block the radio signals which could be used to transmit voice data, and block the audio altogether,” Adam Harvey, a designer specializing in countersurveillance products explained. Refrigerators made fro m metal with thick insulation could potentially do both, he says, regardless of whether it is mild or icy within.

On the data-transmission front, thick metal walls can create a sort of electromagnetic barrier, which enables the device to function as something known as a Faraday cage. A true Faraday cage is a space where radio waves cannot pass and therefore data cannot be transmitted. Although all fridges don't function this way, those constructed with more metal have the potential to serve this purpose.

Peter DaSilva for The New York Times A Faraday cage is a metal shield that protects anything inside from electrical charges. This means a person wearing a Faraday suit, as pictured here, is protected from the high-voltage arcs of a Tesla coil.

Another household object that functions similarly, Mr. Harvey has learned through his research into cellphone data transmission, is a stainless steel martini shaker.

“It's a perfect Faraday cage â€" it will block all radio signals unless you decide you need to pour yourself a martini,” he said. Although this sounds like a plot point in a James Bond movie, Mr. Harvey has actually done extensive tests on the shaker in the process of developing a surveillance-blocking cellphone case called the OFF Pocket.

Blocking data transmission, of course, is a different issue from muffling audio. Al though a thick refrigerator door is good at masking sound (as anyone who has lost a cat inside one knows), soundproofing is not necessarily integral to its design. An ideal refrigerator for a person on the run would be one that functioned as an acoustic anechoic chamber - a sort of Faraday cage for sound - meaning that not one hint of a syllable could make it from the Pepsi-laden kitchen table to the phone in the veggie crisper. Given that refrigerators' insulation levels vary, however, from an audio perspective, burying the phone in a pile of clothes one room over, Mr. Harvey suggested, might be a more reliable solution for someone seeking to subvert prying ears.

Those new to these issues are most likely asking the question â€" why not just ask everyone to turn off his phone and remove the batteries? Beyond the fact that many phones these days do not easily enable battery removal, identifying a pure off is complicated.

“A lot of modern devices (not just phones ) do have states that are somewhere in between fully on and fully off, where some circuits
are powered up and others are powered down,” Seth Schoen, senior staff technologist at the Electronic Frontier Foundation, a civil liberties group that focuses on rights in the online world, explained by e-mail. (Snowden appears to be a supporter of the organization, as he was photographed with an E.F.F. sticker on his laptop.) “These modes often allow the device to wake up autonomously if certain conditions are met, such as pressing a certain key or even receiving certain data over the Internet on a wired Ethernet connection (known as ‘wake-on-LAN').”

Battery removal can be equally deceptive. Even once one figures out how to extract the primary battery, there may be additi onal power sources within the apparatus. “Some phones use an additional battery for memory management; it's unclear whether this battery could be used by logging and/or tracking systems such as Carrier IQ,” Mr. Harvey explained, referring to software that monitors mobile phone users.