from Hacker News

Testing Phone-Sized Faraday Bags

by jor-el on 12/2/21, 8:28 AM with 201 comments

by SamuelAdams on 12/2/21, 1:49 PM
This is a super cool blog post. However I think it alludes to a much larger problem. For many products today, it’s very difficult or impossible for most people to verify the product works.
For a network switch, there are free and open source tools like iPerf to test and verify speeds are as advertised. For a faraday box, you have to go through all these steps (and knowledge learned) just to be able to test these. What about for vitamins, or pet food, or any other “durable” products that are supposed to last for X years?
In an age of snake oil salesmen, paid for reviewers, and fraudulent products on Amazon, there is a real opportunity for creating systems that enable individuals to verify products do what the sellers claim they do.
by pdkl95 on 12/2/21, 11:19 AM
I really like Snowden's recommended Faraday cage for phones or other devices of similar size: two drink-shaker cups[1].
Drink shakers are cheap and widely available. The average hotel room in any medium/large hotel chain probably includes a shaker as a standard item as part of the minibar. The simple two-cup style (as seen in [1]) is preferred over the fancier "strainer top" style because you can press the two cups together; this should cause the inner cup to slightly cut into and/or deform the outer cup along the circle where they join together. The seal between the cups should work sort of like the knife-edge seals used in vacuum chambers.
A metal box or conductive bag is only a Faraday cage if it is fully closed/sealed. Any imperfection in the seal or hole[2] might allow the radio signal to leak out. Most improvised items (freezer, random metal box, etc) have poor seals. Making a high quality cage that actually block a modern phone can be done without much trouble, but the drink shaker method is the only method I know of that will do the job using widely available (free) or very cheap ($10-ish?) parts.
[1] https://ae01.alicdn.com/kf/HLB1yRyhXZTxK1Rjy0Fgq6yovpXaZ/Win...
[2] Holes of sufficient size. How big are the waves you are trying to block?
by bborud on 12/2/21, 10:35 AM
At my former job we had a rather large faraday cage for testing telco equipment. Two things surprised me: 1) It is more fiddly to ensure a room-sized faraday cage doesn't leak RF than I thought. It is really easy to make mistakes that compromises experiments. 2) There was a bath-towel sized piece of cloth that came with one of the expensive Rohde & Schwarz instruments that was astonishingly effective at blocking signal.
So I mainly used the room because that's where the magic blanket was - which was easier to use than ensuring the room was set up correctly.
by scottdupoy on 12/2/21, 10:36 AM
> While the cookie tin performed much better than the mylar bags, and could provide a modicum of useful attenuation under some circumstances, it was not sufficient to provide meaningful assurance of signal isolation at any frequency. However, it was unique among the containers tested in providing tasty snacks during measurements
Back in the day working for Symbian I used to regularly have to run a load of automated Bluetooth tests. Unfortunately there were 200+ engineers in the close vicinity of my test setup and they all had smartphones and all had Bluetooth turned on, causing loads of the test to timeout. We were very pleased with ourselves when we thought of using a biscuit tin as a Faraday cage to try and improve things. Didn't work as well as we hoped, but got the tests running within their timeouts. I always thought it'd be interesting to try using an old microwave.
Best thing was that we got to eat all the biscuits first :)
by parkertomatoes on 12/2/21, 2:32 PM
I was hoping they would test chip bags, because it was a plot device in the most recent Terminator movie.
After searching, I found that Kaspersky has a blog post on the topic: https://usa.kaspersky.com/blog/terminator-dark-fate-chips/18...
It surprised me that chip bags were more effective in their test than cookie tins, but only when they double-bagged it
by janekm on 12/2/21, 12:36 PM
The pink bag shown is actually a static dissipative bag, not a static shielding bag, so would not be expected to work. Though the article mentions trying other bags so no doubt some of those where static shielding (metallic appearance) bags.
I've had a lot of fun in the past trying to shield Bluetooth devices for testing... metal filing cabinet? Practically no attenuation. Biscuit tin? Minimal attenuation like in the article.
Modern radio receivers are ridiculously sensitive.
by tomxor on 12/2/21, 12:49 PM
> "Wrap it in tin foil" is perhaps the most common advice [...] Unfortunately the results were extremely inconsistent [...] 90 dB attenuation in one test would produce only 50 dB the next time [...] You would have no way to tell whether you've managed to fold and seal it adequately well
On the other hand, aluminium foil is incredibly commonly available, if not in a kitchen draw, it will be at the nearest shop. Although unreliable, the _potentially_ high attenuation the author shared is pretty good (we don't know his worst reading), better than all other improvised solutions tested, and similar to commercial products. I wonder if a more reliable construction method could be found.
I'm not sure what movie scene scenario I'm envisioning, but in the case you needed a Faraday cage and don't constantly carry one around, perhaps simply taking an entire kitchen roll of foil and wrapping the device into a giant unsightly ball of it would be a reliable enough process for an "emergency" (if not pocket sized). i.e rather than trying to make a neatly folded minimal version, just resort to sheer number of layers of material - unless RF doesn't care and even 1000 layers with tiny gaps is no good?
[edit] Similarly, I wonder how well (or poorly) common household appliances work as a Faraday cage, e.g a fridge, microwave-oven - From what the author described, it seems they would all have too many gaps, however they are also constructed from higher gauge metals... i'm particularly interested in a microwave-oven which is specifically designed to reflect and retain microwave frequency fields.
by ncmncm on 12/2/21, 12:08 PM
I wish it covered (so to speak) aluminum foil more thoroughly.
In particular, foil is cheap, so rolling up in several layers (3? 5? 10?) and twisting the ends cracker-style ought to be more reliable and practical than trying to do it with one layer and fiddly seams. A suggested minimum number of layers and number of twists would be useful.
by intrasight on 12/2/21, 2:43 PM
I built something in college that needed UL testing for RF emissions, and visited a facility in Long Island with a TEMPEST room about the size of my house. Was very memorable.
I also read an article this year by a guy that made his house radio-wave proof, because, he claimed, he was very sensitive to radio waves. It was WAY more involved than I would have guessed - for the same reasons discussed in this article and in this HN discussion. Nails and screws are a problem. Windows are a big problem. But still - how cool to be cut off from the onslaught of EM radiation.
by lettergram on 12/2/21, 2:57 PM
I actually have a little cookie box, which is lined with heavy aluminum foil, which I put in a steel lunch box.
Perhaps it sounds like overkill, but it’s the only way I seem to be able to fully isolate the devices.
That said, I’d probably just buy a bag... but I’ve been trying to keep it with what I have lying around the house.
by dsign on 12/2/21, 10:09 AM
Cheap science fiction book idea: imagine having to choose between living without your smartphone or living with every company and government agency willy-nilly spying on you. Oh, wait...
by jpl56 on 12/2/21, 12:55 PM
Please reduce the image size, they seem to be 3 Mb each. Some people (especially if their phone is in a pouch) won't be able to get to the interesting parts of your article!
by JohnJamesRambo on 12/2/21, 9:25 AM
That was really interesting. I can’t believe how poorly the metal tin box did compared to real faraday pouches.
by f38zf5vdt on 12/2/21, 2:41 PM
If all phones were mandated to have a physically isolating means to disable their wireless chips, we could avoid this problem all together.
by noduerme on 12/2/21, 11:28 AM
Where I live, all I have to do to block 4G is to stop standing in the far corner of the yard.
by tgb on 12/2/21, 11:24 AM
Interesting results. The article states that Faraday cages are no directional. My physics 101 understanding of them was otherwise. You show they work by integrating over the surface of the cage and saying that's equal to the charge within the surface. But you don't say it's equal to the charge outside. Indeed it couldn't possible, since that charge could be light-years away and steady state would never be achieved. Now that was for electrostatic which isn't really what we care about, if I remember correctly.
Wikipedia also states they are more effective at blocking incoming than outgoing signals. Can anyone clarify the situation?
by djmips on 12/2/21, 11:29 AM
If you are sufficiently concerned, why not mod the phone to add a physical battery disconnect? Would this not provide 100% protection?
by lmilcin on 12/3/21, 1:54 AM
That red plastic "mylar bag" isn't what should have been tested.
Red plastic == electrostatic dissipative. It is not good at conducting electricity -- it is still very good insulator. It just has ability to dissipate electric charge fast enough so that it will not accumulate in normal use.
What you have wanted to check is a clear bag with metalized layer (which is silver or gray looking). It should, per my understanding, act as much better faraday cage than an electrostatic dissipative bag. And I would be really interested in seeing that measurement.
by IncRnd on 12/2/21, 5:28 PM
This article was a fun read, and it really highlights how we trust the words on the tin to be accurate and not mistaken marketing gibberish.
However, the part about the cost ranging from $40 - $80 is simply not true. I know this, because I purchased for $20 yesterday a faraday box to hold my auto key fobs and my cell phone. [1] There are very many items for sale at this price point, fabric, pouches and boxes.
[1] https://smile.amazon.com/s?k=faraday+box
by Tronno on 12/2/21, 8:16 PM
What's the use case for these products?
Even if the radio is blocked, how does that stop a device from collecting data anyway, and exfiltrating it the moment it finds a network? Won't it look for a network the moment you take it out of the bag? And don't you have to remove it from the bag to use it practically?
Ultimately, if your device is compromised or untrustworthy to the extent that it has to be policed this way, is it not safer to just... get rid of it?
by comeonseriously on 12/2/21, 5:01 PM
I think they could have done a better job with the aluminum foil. It looks rather not well done. He could have made a pouch and folded the sides over a few times.
I made a pouch for my car's key fob with aluminum foil and metal tape. It isn't as pretty as the ones you can buy, but it wasn't $25 either and It has worked really well.
by motohagiography on 12/2/21, 2:01 PM
Glad to see these tested. I've been using the EDEC bags for probably a decade to have on hand for incident response/forensics in security consulting, but I never had the gear to test the bags, I just trusted their supplier. Good to know about millimeter wave / 5g though.
by seanhunter on 12/3/21, 7:22 AM
One thing that most people should use a Faraday pouch for (in my opinion) is storing your key fobs if your vehicle uses keyless ignition with a fob. This prevents a very common relay attack which is to stand with a laptop by someone's front door and relay the keyfob signal between the keys (somewhere inside the person's house) and the car. For people who live in cities where you park on the street (as I do) it's extra-convenient for thieves in that most cars will flash some lights to say the alarm is disabled so the thieves know which car they just unlocked.
by yosito on 12/2/21, 3:37 PM
I'd be curious if anyone finds it worthwhile to have a smartphone while being so paranoid that you have to hide it in a Faraday bag. I understand the problem. I just don't understand the practical use case: every time you use the phone, you have to take it out of the Faraday bag, and it can be used to spy on you or report information about you. Assuming you're keeping it in the Faraday bag all day, when and why would you find it useful and worth the risk to take it out? And with that level of use, wouldn't it be more practical to have a dumb phone or a laptop?
by jmnicolas on 12/2/21, 1:28 PM
Interesting experiment however his experience with tinfoil and metal boxes doesn't match mine.
I placed my phone wrapped in a towel inside a metal box: as soon as I close the lid I can't ping the phone on local WiFi and can't reach it with call (it goes direct to voicemail as if the phone was switched off). The GPS doesn't record any position while in the box. I didn't test the Bluetooth though.
Same thing for my car keys inside a paper towel and aluminum foil: impossible to open the car even at touching distance.
I wonder if he wrapped the device before putting it in the box or aluminum.
by _0w8t on 12/2/21, 9:06 PM
I visited at the end of eighties a research institute in a former USSR. They had a whole room with computers put in a Faraday cage to prevent any data leaks with supposedly secret information. The cage was a mesh made of thin metal sheets with 1mm holes. As I remember the whole thing was done seriously, not a fake to please some inspector. It had double doors, all cables were put into metal tubes, all lights were outside I suppose to minimize the cabling.
by gdelfino01 on 12/2/21, 4:36 PM
Electronic voting machines should be put inside Faraday cages too. Specially now that communication chips have been found hidden in some of their motherboards.
https://letsfixstuff.org/2021/04/modem-chips-embedded-in-vot...
by pseudolus on 12/2/21, 2:15 PM
Bit of an aside but the ineffectiveness of tin foil hats to prevent government intrusion into our minds has been known for some time :)
http://web.archive.org/web/20100708230258/http://people.csai...
by bflesch on 12/2/21, 10:58 AM
Perfect timing, I was just looking up this topic in the last week.
But how come these are all US-exclusive. It seems both winners don't ship to EU :/
by whiw on 12/2/21, 12:00 PM
> Faraday cages are non-directional.
Per-se, yes, agreed, but I was wondering whether the transmitter and receiver antenna couple into the surrounding screen differently. The degree of coupling may de-tune them differently. If so then the setup which most accurately represents the intended use would be with the receiver inside the screen.
by aaroninsf on 12/2/21, 6:51 PM
I am happy to see EDEC performed well.
Idle comment: we use their smaller keychain bags for car fobs, after hearing with dismay first- and second- hand accounts of relay attacks being used to steal or ransack cars in our neighborhood (94110). We now keep our fobs in them except in use.
by wolverine876 on 12/2/21, 7:59 PM
What is the use case for disabling wireless communication for your phone? How will you use it? How will anyone call you? If you don't want to use wireless functions, buy a tablet that lacks all the radio hardware.
by BongoMcCat on 12/2/21, 3:34 PM
> 20 dB represents a change of a factor of 10 in amplitude (volts), while 40 dB is a factor of 100, 60 dB a factor of 1000, and so on.
I always thought it was 10dB, or one B that was a change of factor 10. Have I missed something?
by WalterBright on 12/2/21, 10:43 PM
> The question is, then, how well do they actually work?
I tested some by putting the phone in the Faraday container, and then calling its number to see if it rang.
by fsflover on 12/2/21, 1:50 PM
I prefer smartphones with kill switches, Librem 5 and Pinephone.
by docflabby on 12/2/21, 6:16 PM
Tinfoil hat on - has anyone considered how suspicious that only the commercial products worked, maybe the post is just a very clever shill for those products?
by ziggus on 12/2/21, 8:26 PM
Solution: Wrap device in aluminum foil, place in Mylar bag, place bag in cookie tin.
by pluc on 12/2/21, 1:53 PM
Surprised slnt.com isn't mentioned. Happy with their product.
by zanybear on 12/3/21, 5:33 PM
I can see a 99 dollars Faraday Cloth in the future
by Ntrails on 12/3/21, 10:51 AM
Doesn't even try a crisp packet? Madness.
by Wistar on 12/2/21, 7:30 PM
Matt Blaze is a treasure.
by jimmySixDOF on 12/2/21, 10:59 AM
My understanding is that a Faraday Cage, of any size, relies on a proper grounding system to work at all so these boxes and rfid blocking pockets etc all seem to be tinfoil hats parading as space age metamaterials.
by 0xdeadb00f on 12/2/21, 10:10 AM
Wrapping your phone in aluminum foil also works.
I've made a few DIY Faraday bags that way. They're not perfect of course, but less expensive than these bags most of the time.
Edit: Ah, I see this was covered in the article. I didn't read it before posting this comment.