Re: Blockchain backup?
It has been fairly obvious for a while that this sort of thing is an obvious use (maybe even the only killer application for) a blockchain.
For many years I have indulged myself by thinking up all sorts of places to put data which someone needs to be able to easily access over the very long term, update when necessary but be able to deny the existence of. Purely theoretical, of course: I have no data like that but it is fun to think about, and I imagine it could be useful for activists/journalists in totalitarian countries, for example (as well as James Bond, etc).
There are many, many public places to hide a small amount of data. Some are very easy to use but are not anonymous (domain names are an obvious one) - that might be good enough if the activist has friends outside the country who will maintain the data. There are also a number of other less well known but still well maintained "registries" of various forms (for example, public key lists), some of which are much more anonymous.
However, very, very few of them are resilient against deletion or modification of the data if your adversary knows where you put it (for example, DNS names can be removed or their information changed). Blockchain, with its visible history, seems to be the ultimate in this. Even if your adversary knows where you have stored it, they can't remove it. If they try to update/replace it, the old values are still visible in the blockchain and, as long as you make sure the data is signed with your key, they can't appear to make a valid update.
I'm no expert on blockchain, encryption or malware - so if it was obvious to me, presumably malware writers have been using it for ages. And the problem is nothing to do with bitcoin specifically - any blockchain will do, as long as there is a way to include a large enough block of user-specified data (a single DNS name, encrypted and then signed would be enough).