tl;dr - How do you secure the private key on publicly sold paper wallets (coins, cards, bills etc)?
Since there are a lot of smart people on these forums I figured I would bring this up here and see if anyone can come up with a solution to what I think would change the security of Bitcoin in a big way.
I am a huge fan of paper wallets. So much that I store all my Bitcoin on paper.. When I go out to eat I even tip the waiter/waitress in Bitcoin by pre printing out some piper paper wallets and leaving one as a tip with a note on how to use it.
Over the years I have watched people sell Bitcoin cards with holograms, Bitcoin bearer bonds, Bitcoin paper money etc and have seen these same wallets have the funds stolen from them because the creator knew the private key. There are close to 60,000 BTC in unopened Casascius coins out there. I know I would lose a bit of sleep knowing at if this 7 million in value turned into 700 million and someone decided to claim the coins because they had the private keys backed up somewhere.
What I would like to create is a Bitcoin card that you can buy with absolute confidence that only you know the private key. Then anyone can put Bitcoin in cold storage without any fear of it being hacked.
Imagine being able to buy an empty Bitcoin card and push any amount of Bitcoin to it and know with 100% certainty that this Bitcoin is secure as long as Bitcoin is (ie: proper random number generation, only inbound transactions etc, no brain wallet keys etc). Then you just send BTC to this address and watch it on the blockchain without ever losing any sleep.
To pull this off though there needs to be a way to put the private key on the card without actually having an opportunity for an employee or manufacturer or anyone for that matter to see it without breaking the tamper mechanism. Even more importantly that the end user knows that his key is secure and known only by him and that the key will actually work when he goes to access it.
One idea that might work would be a multi signature approach where you need both the private key on the card and the private key that you generate online to access the BTC then the creator can't steal the BTC.