One big issue with bitcoin is its not as safe as a bank in that a bank imposes limits on how much you can withdraw a day (usually around $300 or so). So regardless if you have a brain wallet, a set of encrypted keys with several backups, etc. all it takes to steal your entire fortune is for a robber to coerce you to give up your credentials and, then, bam!, he has your entire fortune. A bank has an advantage here in that a thief may be able to grab your ATM card and coerce you to hand over the pin, but he still can't get much money from doing that.
So, I was thinking what if a single address could have several keys with different rules attached to them:
* A low security key, with a withdrawal limit of XXX BTC within a certain period of time
* A master key, and a set of companion keys that can withdrawal more or all of the BTC associated with the address. The master key, along with a certain number of companion keys would be needed to unlock the funds and initiate a large transfer (for example, 3 out of 5 of the companion keys). In addition, using this combination of keys, one could create or revoke companion keys, and low security keys.
By taking the set of companion keys and handing them out to friends and neighbors (while keeping at least one for himself), a person can reasonably protect his BTC funds from coercion. If an individual doesn't have many trustworthy contacts, he could use some sort of institution to hold on to his keys (except for the one he personally has, obviously).
Since no one has the master key but himself, he prevents people from colluding to steal his funds. With the low priority keys, he now can use his wallet without worrying about carrying too much virtual BTC on his person. And no one has the power to confiscate his funds.
Does this sound like a good idea? Is there something planned in the coming versions of bitcoin which addresses this problem?