These are orthogonal to one another. For read-protection, encryption is a viable solution: specific paths into your data structure (e.g. certain keys in a map)

These are orthogonal to one another. For read-protection, encryption is a viable solution: specific paths into your data structure (e.g. certain keys in a map) can be encrypted with a key that isn't provided to all participants. For write-protection, the solution is the same as with any syncing service: when receiving an update, if you don't authorize the sender of the update, you don't accept it.