[Cryptography] Rubber-hose resistance?

Jeremy Stanley fungi at yuggoth.org
Thu Dec 21 13:53:50 EST 2017


On 2017-12-21 10:37:16 -0600 (-0600), Nico Williams wrote:
> On Wed, Dec 20, 2017 at 06:31:24PM +0000, Jeremy Stanley wrote:
> > Also, when crossing some borders, your devices may leave your person
> > and be out of visual range for extended periods of time before you
> > get them back. In such circumstances do you consider them probably
> > compromised (perhaps even at the firmware or hardware level) and
> > quarantine or dispose of them accordingly?
> 
> If that happens you just dispose of the devices.
> 
> A simple defense against this sort of attack is to carry just a
> raspberry pi and an SD card with a minimal OS and content.  You can
> always buy a new SD card, download an image, and build yourself a remote
> access terminal.  It's pretty simple.  It's not likely that customs will
> have a hardware attach for every SBC out there, and you can always
> inspect it, as these computers are very small and their boards highly
> accessible.
> 
> If you stay at a hotel then chances are you can just display onto the
> room's TV with an HDMI cable.  Or you can carry a 14" portable display
> -- these are widely available and cheap.
> 
> You will have to carry a keyboard and mouse, but that's a plus.  I
> always do anyways for ergonomics reasons, and maybe so should you.
> 
> This approach compares well to a proper laptop if all you'll be needing
> is a terminal anyways.

That's fairly similar to how I've been handling things. When I
travel domestically I do so with homebrewed netbook-like devices
cobbled together from SBCs with commodity tablet-sized display
panels and USB mini-keyboards obtained from inexpensive tablet
cases. I'm a little uneasy trying to cross international borders
with homemade computers though, so I've resorted to using very cheap
"burner" mini-laptops that I won't be too upset if I have to ditch
(I tend to keep one in checked luggage and one in my carry-on in
case that happens). Most recently I've been using the Fusion5
lapbooks which run around us$160 and can run a fairly unmolested
Debian install with a mainline Linux kernel, but Chromebooks are
another popular choice for this among my colleagues who take similar
measures.

I've also invested in bulk packs of tamper-evident evidence bags
large enough to put my devices in, and rolls of tamper-evident
serial number labels to cover all ports on them. These are obviously
not foolproof, but they increase the amount of time an adversary
needs to muck with my hardware and still go undetected (I bring a
stack of bags, so that when I go out to dinner I can put my devices
in a fresh bag before putting that in the not-terribly-trustworthy
safe in my hotel room).

And as mentioned elsewhere in the thread, I too have determined that
a good, strong memorized password/passphrase is a safer choice to
bring across borders than an SSH key when it comes to having a way
to bootstrap my actual keys once I reach my destination. It's about
the only time I SSH with a password (been meaning to set up a second
sshd specifically for this with some sort of port-knocking scheme so
it's not easily discoverable by all the brute-forcing portscanners
out there, and then I can leave my normal sshd set for key-only
auth). I also tend to make short-lived keys I'll use while
travelling and yank their access as soon as I get home, just for
good hygiene.

Of course, travel into mainland China adds an extra layer of fun
here, but for the moment it's still possible to use a wireless modem
or phone tether with a SIM for a non-Chinese mobile provider on an
international roaming plan to get around the GFW block for SSH and
VPN protocols.
-- 
Jeremy Stanley
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