I think IPFS is an excellent foundation for such a system. Indeed, a few friends and I are working on it: [1], [2]. It's too early to call it robust yet and it

I think IPFS is an excellent foundation for such a system. Indeed, a few friends and I are working on it: [1], [2]. It's too early to call it robust yet and it needs an independent security audit, but it has all your bonus features: * all storage nodes can't decrypt anything * you can grant read and write access independently to individual files or folders * you could easily store all previous versions of data, by never unpinning anything in IPFS, or having another server listening for updates and pinning them [1] https://github.com/Peergos/Peergos https://github.com/Peergos/Peergos [2] https://peergos.github.io/book https://peergos.github.io/book