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cnet.com says:

Even in Snowden's room, the group took precautions not to be overheard. Greenwald and Poitras would remove the batteries from their mobile phones and put them in refrigerator of Snowden's minibar (...)

Is there any security reason they would do that?

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11  
I'm starting to think they've all become one with the tinfoil. – Mark Buffalo Feb 5 at 14:53
17  
On Snowden-scale, this amount of paranoia might actually make sense. The question is whether it was for security at all. – d33tah Feb 5 at 14:54
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They put the batteries or the phones in the fridge? The batteries does not make any sense at all. Putting the phones would help if they were still powered up, as the fridge would act as an Faraday cage. But I agree they had become one with the tinfoil... – ThoriumBR Feb 5 at 14:56
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Mordern phones have so little spare space in them that integrating a bug with built-in power is extremely difficult. So I'd guess the batteries are primary targets for bug housing. Also, batteries can be swapped in a short period of time, but difficult to make a bugged phone to replicate the exact dents and scratches and data in the original! – billc.cn Feb 5 at 14:57
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This is an English language issue: "put them" is ambiguous and could be either "the batteries", "mobile phones" or both. The intended meaning is that they put the mobile phones in the refrigerator, not the batteries. – isanae Feb 5 at 22:06

With its insulated walls and rubber door seals, a refrigerator is the most soundproof box commonly found in ordinary living spaces. If it's running, even better, it provides white noise. If one was worried about listening devices, then a fridge would be a reasonable and available place to stash them. And Snowden has alleged the NSA can do lots of things even with a phone that's turned off.

Minor update: Out of curiosity, I put my phone in the fridge, and later in the freezer, with batteries still inserted. In both cases I was able to call it quite easily - the refrigerator did not act as a Faraday cage - but the ringtone volume was noticeably subdued. So I do think the fridge in the quoted story was a soundproofing issue, not a Faraday Cage as has been suggested.

I know it's just an experiment of one, but it was a fun experiment :)

Even more minor update: I was curious and checked... my microwave, which is a Faraday Cage, doesn't block cell signals either (good explanation here).

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14  
I often leave my phone in the freezer... its either that or leave the food behind.. 2 hand problems. – silverpenguin Feb 5 at 16:40
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"Snowden has alleged the NSA can do lots of things even with a phone that's turned off." It's only a trick to assume the phone is off. It's really on, and just listening. It just doesn't let you know it's on. Security through obscurity. :] – Mark Buffalo Feb 5 at 16:50
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@NeilSmithline, the article postulates that Snowden's fridge could act as a Faraday cage. But for a cell phone you need the cage to mesh around 1/4" - and on any fridge I've ever used, the rubber seal around the door easily exceeds that. Also there are holes in the metal for the vent ducting... I'm really not optimistic about the reality of the average fridge blocking cell signals. – gowenfawr Feb 5 at 17:34
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@gowenfawr continuing down the tinfoil hat rabbit hole, is it possible that a fridge would be the least suspicious place in a home to have modified to actually be a proper Faraday cage? Unless your day job is RF antenna design consulting, it would be somewhat suspicious to own a cage, and beefing up the door seals to also have a conductive liner (and "fixing" any other non-conductive openings) sounds like it would be easy if you are willing to take a small fridge apart. Hmm. Is there a market there selling into paranoia for bar-fridges that are cages? – RBerteig Feb 5 at 22:42
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@RBerteig I think the Microwave, which is exactly a Faraday cage, is the least suspicious way... but even my microwave isn't tight enough to block cell phone signals. – gowenfawr Feb 5 at 23:16

I suspect that the passage is intended to say that they took the battery out of their phone, and then put both the battery and the phone in the fridge.

Assuming that's right, most likely there are two threats this is defending against:

  1. Taking the battery out of the phone ensures that the phone is powered off. This defends against a malicious application or malicious code in the phone's firmware. (The concern is that someone might have attacked the software running on the phone and then installed malicious software or a malicious application that runs in the background and uses the microphone to record conversations. Taking the battery out of the phone powers off the phone's processor and prevents any such malicious code from running.)

  2. Putting the items in the fridge provides soundproofing. This defends against a bug that was surreptitiously added to the phone. (The concern here is that an attacker might have, at some point in the past, planted a bug inside their phone. Such a bug might have its own battery and its own circuitry and its own microphone, and be recording their conversations. Taking the battery out of the phone will not stop such a bug. However, putting the components into the fridge will provide sound isolation, because the fridge is relatively soundproofed and also because the fan in the fridge provides a bit of additional noise.)

In other words, the first is about malicious software; the second is about malicious hardware. If you were only concerned about malicious software, it would be enough to take the battery out of the phone without putting it into the fridge -- but putting both of them into the fridge covers both bases. In their setting, this kind of paranoia makes sense.

Note that it wouldn't be enough to just put the battery into the fridge, but not the phone. The problem with that is that it wouldn't stop a bug that was surreptitiously planted inside their phone. That's why I suspect the quote meant to say that they took the battery out of their phone and then put both the battery and the phone into their fridge. As it stands, the wording of the quote is a bit ambiguous, so I realize I'm making a particular interpretation of the quote -- but given the two kinds of threats, I think my interpretation makes sense.

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2  
fans do not make white noise. but it's still might be more that enough to bury other sounds, of course. – Sarge Borsch Feb 6 at 9:50
    
I think the worry on part 2 is not so much electronics being tampered with through home invasions but rather in transit as they were shipped when bought; this practice by the NSA has been documented (see e.g. here) from the Snowden cache, so he'd have been aware of it, and at least it's a powerful competing method to the home invasion. – E.P. Feb 6 at 15:04

Because they thought the refrigerator would act as a Faraday cage. Completely powering off their phones and placing them in a thick metal box gave them the peace of mind that those devices were not transmitting anything they were saying, either through the stock phone, or a bug placed by a government agency on the battery pack or the phone.

Stainless steel is quite effective at blocking cell tower signals, and many other frequencies used to transmit data across distances. Heather Murphy writes:

It is the materials that make up refrigerator walls that could potentially turn them into anti-eavesdropping devices...Refrigerators made from metal with thick insulation could potentially [block noise and cell transmissions].

Source: http://thelede.blogs.nytimes.com/2013/06/25/why-snowdens-visitors-put-their-phones-in-the-fridge/?smid=fb-share&_r=2

On a less serious note: Placing your battery in the fridge helps it retain a charge better because it reduces the chemical reactions that cause passive drain. Could they have been environmentally minded? :-)

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2  
Most refrigerators are plastic and not metal. Only a few made would block any signal these days. – GdD Feb 5 at 15:06
    
I think the referenced NYTimes article is a better link. You may want to add a quote from that to your answer so it gets more attention. – Neil Smithline Feb 5 at 16:47
    
What about the door? Are fridge doors metal on metal against the rest of the fridge? – user1886419 Feb 5 at 22:04
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Blocking the radio signals with a Faraday cage doesn't seem that useful, anyway. You'd think any sanely designed listening device, if it couldn't connect to its remote server to transmit its data, would simply store it and transmit later. So assuming you are eventually going to take the device out of the Faraday cage, that in itself doesn't gain you anything except a little bit of time. – Nate Eldredge Feb 6 at 16:47
    
A faraday cage is only as good as the longest dimension of any hole penetrating it. In this case, the door rubbers (rubber/plastic is as good as air there!) form a very long hole, significantly longer than what would be needed (a fraction of the wavelength of the signal you want to stop escaping). – rackandboneman Feb 6 at 17:38

Its not just malware. It has been claimed that mobile telephones are subject to an attack in which they can be remotely activated from the cell tower to relay audio back to the tower but does not ring or change the appearance of the phone.

Reference: https://web.archive.org/web/20140112133850/https://www.schneier.com/blog/archives/2006/12/remotely_eavesd_1.html

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Their concern was that their phones may have been infected with malware which would record sound and/or video of their conversations. Fridges are insulated and have ambient noise from the machinery running, so would effectively block sound from conversations being recorded.

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But the phones were still outside the fridge, it was just the batteries that were put in the fridge. Allegedly – RoraΖ Feb 5 at 14:58
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@RoraZ from that quote there, it's ambiguous whether "put them" refers to the batteries or the cell phones. It could mean the batteries, the cell phones, or the pile of batteries and (disconnected) cell phones. I strongly suspect the cell phones ended up in the fridge. – gowenfawr Feb 5 at 14:59
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They put both the phones and the batteries in the fridge, not the batteries alone. – GdD Feb 5 at 15:00

I can't think of anything security-related with using fridge as battery storage, however, I do this myself. The reason for this is that Li-ion batteries live longer when they are kept reasonably cold:

Table 3: Estimated recoverable capacity when storing Li-ion for one year at various temperatures
Temperature    40% charge    100% charge
  0°C            98%           94%
  25°C           96%           80%
  40°C           85%           65%
  60°C           75%           60% (after 3 months)

Of course, this only makes sense if the battery is unused for a long period of time (a month or more), storing the battery in the fridge overnight only to use it again in the morning will do more harm than good due to moisture.

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