I use a webcam in my Raspberry setup to send transactions to the online computer. For your offline linux machine, I'd recommend qtQR, easy to use and powerful. I've tried the audio interface previously with another Bitcoin client, but I failed to set it up and eventually gave up on the idea.
I like the idea of that sort of thing, but I wonder how it could be perceived by others.
All types of visitors to your household (friends and ostensible friends alike) could interpret the sophisticated physical setup to mean "Is it possible he/she might have large stacks of BTC?". Keeping the hardware somewhere secure (and hence out of sight) is probably not so OTT a measure also, but it's not going to be possible to keep it thoroughly secret.
And I suppose the absolute worst case scenario is that you don't actually have that many bitcoins anyway, say holding onto 100,000 uBTC on the off chance that worldwide adoption takes off. Getting targeted then could be a little hairy, depending on how "worst case" your luck is.
Not wishing this on anyone! But I thought I'd throw it out there, perhaps there's a better way to keep the hardware unseen/unrecognised. Could your setup convincingly be construed as a pair of video calling devices in the same house? Without internet connections?
It can be perceived by others if you're using the system in front of others, which I don't. And the awareness of Bitcoin is really low here, if someone, somehow intercepted all the physical parts needed to run my system they probably wouldn't know how to use it, and if they did, they would need my password to do so (system + Armory password)
And things are indeed hidden out of sight for visitors and the people I live with, but there are ways to hide things "in plain sight" by making them look irrelevant/not the sort of thing that can hold cash
As I said, the system is a Raspberry. As an example, I have the machine in one place and the SD card in another.
One also has to consider methods to make sure others can retrieve the funds in case something happens to you, but that's a story for another topic.
I don't think it's relevant to have a video calling device system without internet connection... I mean, I only use the storage for a few minutes, when I really need to retrieve funds. I usually have enough online for urgent needs. It's not really cumbersome to setup the system when you need to use it and keep it hidden meanwhile, all in name of privacy. The system has far more funds coming in than out.
For the amount of coins usually on my storage, I think the physical and virtual security measures I have in practice are good enough, and as I've said, they're a good compromise between security and safety. One thing is for sure, it's not the best system. But it's one I'm happy with.
Here's a suggestion to the OP: drop the SD's and the USB's on your offline system and use a webcam (easy way) or the audio interface (hard way). I use a webcam in my Raspberry setup to send transactions to the online computer. For your offline linux machine, I'd recommend qtQR, easy to use and powerful. I've tried the audio interface previously with another Bitcoin client, but I failed to set it up and eventually gave up on the idea.
Well, somehow I have to boot from Ubuntu and install it on my low-end laptop. So far I've been using USB. It doesn't have a CD drive. I guess I could use my external USB CD drive, but then we're back on USB again.
And no, SD connecting through the USB only gives and illusion of safety.
But why? Doesn't the device itself have to support the hackery in the first place? An SD card is an SD card, not a USB key. Different device, even if it's connecting to a USB on the computer internally. The malware we've been talking about regarding USB keys, can malware of the same kind also be placed on an SD card?
Any USB device comes with a firmware and class negotiation, even a plain hub. These manufacturers don't come up with their own MCUs, they buy generic MCUs from large manufacturers like TI, Microchip and FTDI. They upload a USB firmware in the MCU's NVRAM and this the vulnerability root kits target to survive a power cycle.
There you go, pf
The SD card cannot contain the malware. But you've got to make sure that, as an example, your laptop's SD card reader isn't just a simple USB reader to be safe with SD's
